Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: bry593 on February 23, 2021, 04:30:51 PM
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Edelbrock 1903 to be used on HT383, 9.1:1 static, 196/206 .431/.451 109, 10 initial + 22 mech @3k.
Calibration as purchased used, but never ran on this engine. Note this carb was all black inside. Was running rich!
Hot Air Choke
Needle Seat .130
Float 7/16
IT .033
UIAB .070 (in body)
DCR .055
LIAB .064
IDP .084
IABP .052
SMAB .052 (in horn)
Jet .073
Primary Rod 50M
APT Spring Silver
Acl Well .040
2ndy Rod DR
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8
Second calibration per Cliff. I could not get this to idle down until I plugged the IABP. However, it still ran on even on 91 octane. Pull off vac hose and idle goes up. Zero response to APT tip-in procedure. No hesitation off-idle or thru 2ndy. Engine tries to die when let off throttle to complete stop. Electric choke works fine, but loads up.
Electric Choke
Needle Seat .156
Float 7/16
IT .036
UIAB .070 (in body)
DCR .055
LIAB .064
IDP .089
IABP Plugged
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8
3rd Calibration. Leaned it out, trying to kill the run-on and overly rich condition. Bushed primary shaft. Idle screws 4.0 turns out and turning in kills engine, bad off-idle stumble in cold weather (have to flutter acl pump), no engine run-on, pulling vac hose increases idle, zero response to APT tip-in procedure, choke works
New Primary Shaft Bushings
New Needle & Seat .135 <-Found tool chatter in the larger seat
New Float <- Old and new both weighed 6.5g, but new had heel farther away from fulcrum
Float 7/16
IT .031
UIAB .041 (moved to horn 6-32 setscrew, plugged old with sinker lead)
DCR .047 (8-32 setscrew)
LIAB .064
IDP .089
IABP plugged
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8
4th Calibration. Opened IABP since idle seemed to want air when removing vac hose. Idle screws 3.5 turns out and turning in kills engine, bad off-idle stumble in cold weather (have to flutter acl pump), pulling vac hose increases idle, zero response to APT tip-in procedure, choke runs rough
Seat .135
Float 7/16
IT .031
UIAB .041 (moved to horn 6-32 setscrew, plugged old with sinker lead)
DCR .047 (8-32 setscrew)
LIAB .064
IDP .089
IABP .052
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8
5th Calibration
Pulled the 1903 off and replaced with a junk 70 series from a 350 truck. Runs fine, but no choke (I have vortec heads). Surprised how much quieter the exhaust is with this carb. No run-on, no stumble, no dying when quickly decelerating to a stop. Pretty good for an old junk carb.
Got to say, I've spent way too much time and money on this thing. Pretty frustrated. Can anyone see where I've gone wrong? Never had this much "fun" with a quad before.
I'm scared to go back to a bigger idle tube, since it already seems rich regardless of what I do. Setting the float to 9/32" doesn't seem like it would help. Why is this not responding to the APT Tip-in?
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The main issue is the single main airbleeds size. Less air equals more fuel from the same jetting.
As the tip-in is done at over 2000 rpm´s the idle circuit is out of the equation.
.052" SMAB were used in 1977-78 Truck Q-jets with small (.064") jets and rods (38B-40B) and 1979 and later Truck Q-jets with 350HD or 454 engines.
All other 1979 and later Truck Q-jets uses .087" single main airbleeds.
I would use the 3rd recipe, open the SMAB to .087", the DCR´s to .055" and the IBA to .080"-.087".
HTH
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Lets go back to basics here as you missed something along the way.
First post says 383CID, 9 to 1 compression and tiny 196/206/109 camshaft.
The 1903 wouldn't have needed much help to be flawless on that engine.
Did you block the vacuum supply to choke housing when converting to electric?
How much timing is being added by the vacuum advance?
Manifold or ported vacuum to the VA?
What intake manifold is being used and does it have an exhaust crossover in it?
Lots of SMAB later carburetors will have smaller MAB's and since they are indirect they will not have a huge impact on the jets/metering rods required.
Edelbrock patterned their later SMAB units after 17080213 units so they will have small lower IAB's and smaller MAB's in the airhorn.
They don't need a lot of idle tube for what you are doing and stock .055" DCR's would have been adequate with the stock upper and lower IAB sizes.
I suspect the issues early on were due to leaking at the N/S assembly or some fundamental issue which started you on the path of adding fuel to the idle system and moving the upper IAB's to the airhorn. That move would NOT have been needed and sent you into a rich condition.
Keep in mind here that the main system is fed thru the transfer slots so it does impact transition from the idle to the main system and will add some fuel at cruise at very light throttle openings.
The Edelbrock 1910 is basically the same carb you are using with a rich idle calibration and the secondary airflap stop ground off some to provide a little more CFM. They used several different calibrations in the 1910's the most common is nearly identical to the 1903 with only larger idle tubes and a little more bypass air. They are known for idling very well on engines with pretty "hefty" cams in them and lower vacuum at idle speed.
I've build hundreds of these carburetors and they are very nice units, but I have found numerous fundamental issues with them over the years which we can discuss later after you answer the basic questions above.
In any case it should have been pretty close right off the bench with just a few minor enhancements which would have included good rebuild parts and slightly larger idle tubes and possibly just a tad more bypass air......Cliff
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Hey thanks guys. Appreciate you guys listening to my woes. This thing is driving me crazy and it's a short trip...
Q. Did you block the vacuum supply to choke housing when converting to electric?
A. Yes, per your recommendation I blocked everything except the timed, aux and PCV ports with 3/16, 1/4 and 3/8 Al bar driven in and red Loc-Tite. Zero vac leaks from those ports (did a smoke check).
Q. How much timing is being added by the vacuum advance?
A. I had the factory ZZ4 dizzy advance, but the curve started at too low of Hg. After discussing with you, I bought your higher Hg advance unit. Don't have my notes at the moment, but IIRC is 14 degrees. Cured my transition ping from cruise to light throttle.
Q. Manifold or ported vacuum to the VA?
A. At first I was running ported to lower the rpms and help prevent the ignition off run-on. After the seat replacment and throttle shaft bush, I switched over to manifold
Q. What intake manifold is being used and does it have an exhaust crossover in it?
A. GMPP Vortec, no crossover in Vortec
Lots of SMAB later carburetors will have smaller MAB's and since they are indirect they will not have a huge impact on the jets/metering rods required.
Yes, seemed small but worked well with Calibration 1. Only issues were engine run-on and could not get APT to go lean.
They don't need a lot of idle tube for what you are doing and stock .055" DCR's would have been adequate with the stock upper and lower IAB sizes.
Are you sure stock DCR was .055? I thought someone had drilled these out?
I suspect the issues early on were due to leaking at the N/S which started you on the path of adding idle fuel and moving upper IAB's to the airhorn.
I reduced idle circuit from IT .036 to .031, but still unable to get APT to go lean. At 2000 rpm tip-in engine slowed even with APT screw turned all the way down. Why?
Keep in mind here that the main system is fed thru the transfer slots so it does impact transition from the idle to the main system and will add some fuel at cruise at very light throttle openings.
Yes, IT .036 to .031 killed my transition fuel. Bad hesitation especially in cold weather.
I've found numerous issues with them over the years we can discuss later.
I've checked piston arms for level, blown air thru channels, checked well plugs, etc. So far everything checks out.
It should have been pretty close right off the bench with good rebuild parts and larger idle tubes and a tad more bypass.
It's all good stuff from you except the second needle and seat (Hygrade). I soldered up your .036 idle tubes to .031, but can easily drill them .033. Are you sure IDCR .055" is original? I thought it was supposed to be .047? What size should I make the UIAB now that I've moved it to the horn? Should I also reinstall the 50M rods and 73 jets? I was thinking .086" IABP.
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Moving the upper idle bleeds to the airhorn made it go rich at idle and didn't help part throttle either.
I save that move for heavily modified engines with hefty cams in them and very little vacuum at idle speed.
If you do move them to the airhorn in a later SMAB carb a .052" UIAB, .070" lower, and .034 idle tube with .046" DCR's would be enough idle fuel.
Adding timing at idle speed typically raises vacuum some and lowers the throttle angle. I would doubt if your engine combo would need much beyond 10-12 degrees initial timing to be happy, although I'm not familiar with that particular camshaft on a 109 LSA so don't know exactly how it would behave in a 383 build with some compression in it.
If it helps some the toughest engines to tune I've ran into over the years were larger CID, moderate compression and tight LSA camshafts with early intake closing. I've had a few pulling my hair out to get them to work well everyplace, and it's a good thing I'm nearly bald because I would have been by the time I got finished with them!
As far as "fundamental" issues with Edelbrock q-jets I've ran into a few that had one or both jet holes machined at different depths in the casting or too deep in the casting making tuning difficult if not near impossible. It's not uncommon for them to miss drilling passages by either not connecting them/missing the intended target. Most of the time that problem was with secondary POE which wouldn't really impact anything you are doing.
In any case the cold intake isn't helping things and all tuning needs to be done when it's fully heat soaked, so give it plenty of warm up time before making idle adjustments and changes to the idle system.
The 74 jets and 50C rods are fine and not making it rich. I've used that combo in quite a few of those carbs and had to get the APT up to 3.5-4.5 turns for most of them before they had adequate fuel at light load/part throttle.
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Thanks Cliff, I'll give that a try.
For sure the direct horn UIAB is much more sensitive to tiny changes in the circuit. Also a lot easier to get plugged up than the larger and down pointed body bleeds.
I have another question which assumes a return to original 1903 calibration. Is it possible to tap the main body UIAB #4-40, insert a brass SS and drill the center to .070? Or will this not work the same as the original .070 drilled hole? I didn't see this technique mentioned in your book, only the Al tig rod and resize. I have both 3/32 and 1/8 rod on hand, but it seems like it would be easier to drill a setscrew.
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About the biggest you can drill a #4-40 will be .059" if you stay on center.
The stock idle airbleed sizes in the 1903 are fine for what you are doing.
The 1910's showed up with several different calibrations as it seems like Edelbrock was playing around some there but they are the exact same unit. The most common 1910's have .038" idle tubes but I've seen them with .040's.
They used the same smaller MAB's and 74 jets with 50M rods, pretty sure the 1903's got 73 jets.
Looking at your engine combo it has a very small cam in it for the CID and compression ratio. Not sure why they ground it on a 109LSA as that would close the intake pretty early with such short seat timing and may make your engine octane sensitive, or at least you may experience some pinging at light throttle if you add too much vacuum advance and heavy/full throttle as well if the mechanical comes on too quick or too much of it.
Have you taken a vacuum reading at idle speed, 700-750 rpms with around 10-12 initial timing and no additional timing from the advance?
Anytime I've tuned larger engines with tiny cams in them they have not wanted, liked, or responded well to a lot of timing at idle speed and adding timing has made them difficult to tune as the throttle plates were pretty well closed at idle speed.
As mentioned previously the only thing I'm not sure of is the small cam on such a tight LSA. I did work on a 355 last year with a 194/204/108 cam in it. The owner brought it here for idle issues. I did the carb for it about 10 years ago for the stock 305 engine and this new combo wasn't happy about it.
I went in and opened up the idle tubes .002" and DCR's to .055" and that's all it needed. Even so I was not impressed with the engine anyplace. It had a "quirky" idle and even though the engine builder dubbed that cam a "torque monster" in a 350 build, I thought is was "weak" compared to 350's I've done with the 194/204/112 cam in them......FWIW......Cliff
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Haven't checked the vac since last year, but it seems like it was around 18" Hg.
Yes, you know how GM is with crate motors, they just throw whatever they have lying around at it. The cam is left over from the Ram-Jet 350. This HT383 also has cold piston slap due to the short 880 block bore combined with a stock 5.7 rod. I wouldn't recommend it.
For drilling setscrews, I made a jig to hold the screws. It's just a piece of 20 gauge sheet with a 4-40, 6-32 and 8-32 nut epoxied to the sheet. You simply thread the setscrew into the nut with the cup point up. The cup point acts as a centering pilot for your pin drills. Maybe not the best method, but works okay for me.
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I usually drill them in place but have a piece of flat aluminum about 3/16" thick tapped part way with 4-40, 6-32, 8-32 and 10-32. I just seat the brass set screws and drill the desired hole size in them, not really fussy and only take a few seconds.
Will it make that much vacuum without a lot of timing and down around 700-750rpms?
I've been building SBC's for over 40 years and have never done or wanted to do a "383" build.
I have several excellent "recipes" for 355's and stick with them. They make great power with improved rod length to stroke and bore to stroke parameters.
I actually prefer a 327 or using the shorter 3.25" stroke cranks, but they didn't make a lot of large journal 307's and 327's so they are difficult to obtain.
Compression is also your friend with SBC engines and they are fine on pump gas WAY beyond the proverbial "brick wall" of 9.5 to 1 we see regurgitated on every Forum you log onto by folks who probably haven't built an engine that made chit for power or even have an SBC powered car or truck that moves!.......LOL....
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Thanks for all the advice Cliff! The off-idle hesitation problem is fixed.
The power piston spring had tilted and slightly slipped into the vacuum channel and those coils to bind. This had the effect of a shorter spring and would not lift the primary needles out of the jets at light throttle (too much vacuum with this tiny cam). Note that I'm using the stiffest (DrkBlu) piston spring.
To reduce the vacuum channel diameter, I dropped a #8 brass washer into the piston bore. The OD was too big too fit, so I "turned" it down using the following method. The washer was sandwiched between the head and nut on a screw, chucked in my drill and sanded with a die grinder while the drill was activated. Deburred by flat sanding on a piece of 400 grit.
Retained the lean idle calibration (.031 IT, .041 UIAB, .064 LIAB), and added .052 IABP. Also installed a 195F thermostat to improve efficiency per your recommendation. With the spring fixed, throttle is instant and seamless through the primaries with only a slight delay in the 2ndy.
Will run this calibration a few weeks to get a feel for it. I'm hoping the smaller idle tubes will help achieve a target of 18mpg @ 75mph (1971 3/4 Chevy, 383, 4L80E, 4.10 HO52).
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Good news.
APT and vacuum advance tuning will get you there.
Make sure the power piston hanger arms are exactly even, and make very small changes. It may take several tanks of fuel to find the ideal APT setting. You can also use the vacuum advance to fine tune for fuel economy.
Most engines with higher compression, tight quench and smaller cams will not need much, but you still need to look there for best efficiency in the "normal" driving range....
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Apparently, I can't leave well enough alone....
Although the lean idle ran well, it was insufficient to support the choke. During choke operation, there was not enough fuel being pulled to cope with freezing ambient temps. It sort of worked, but I like my choke to be: pump gas once, turn key, engine fires immediately and idles at around 1000 rpm. With this lean setup, I had to feather the throttle a little before the high idle would stabilize. Just not enough idle fuel.
I considered adding idle fuel by drilling the DCR from .047 to .052 (would this have worked?). Instead, the idle tubes were increased from .031 to .033 so they will not plug up as easily. I also drilled the horn UIAB to .052, but left the DCR .047 and LIAB .064. This morning was 32F and it fired immediately and with a stable high idle. Seems to have lost a little pep on part throttle, but I suspect I can lower the APT a half turn and regain the lost cruise power.
Out of curiosity, what is the easiest was to verify the APT arms are even?
This HT383 has a poor quench. The pistons are .040" in the hole, then add the gasket. Not a great combination to prevent detonation. Not sure why GM did it this way considering the pistons were custom made by Speed-Pro for GM (383 with a 3.800, not 3.750 stroke).
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probably rebuilder pistons that are shorter
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I would proceed adjusting the choke pull-off to open the choke blade a little less, about .160", measured between upper end of blade and rear wall of choke housing and increase fast idle speed some to keep it running.
For off-idle respond at least .052" idle DCR´s will help.
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We do NOT add fuel to the idle system to improve cold weather starts or choke operation.
The pull-off unload angle if FULLY adjustable on those carburetors via a screw. Move quickly on a cold start and simply adjust the flap so the engine doesn't stall out from being too lean and doesn't "blubber" from being too rich. It's a simple and easy procedure but you have to move quickly and I'd also lower the fast idle speed (cam on the highest step) down some to about 1400-1600rpm's at the same time.
99 percent of the power piston hanger arms on carbs I get in here at bent up, down, and not even. Spend some time there to get the arms LEVEL and EVEN. Just pull the tips together underneath the center of the power piston and gently "adjust" the arms till the tips are even.
It's not difficult or fussy but REQUIRED if you want a good end result with these things.......
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I would proceed adjusting the choke pull-off to open the choke blade a little less, about .160", measured between upper end of blade and rear wall of choke housing and increase fast idle speed some to keep it running.
For off-idle respond at least .052" idle DCR´s will help.
I did consider closing the flap a bit more and that was probably the right thing to do. I had previously set up the choke flap with a 1/4" bit per typical Quad specs (and an 1/8" inside the coil housing). But, opening the IT from .031 to .033 did do the trick. But now, wouldn't you know it, got a new problem.
Now, when the engine is warm and I stop at a light, the idle is fine for about 10 seconds. Then suddenly, like someone is flipping a switch, the idle stumbles and tries to die. I can pump it a few times, bring the rpm to 2000 and it clears out. But then once it returns to idle, it runs great for about 10 seconds, then like a switch tries to die. My first thought is that the air horn gasket has become fuzzy around the power piston from the many times I've had to remove/reinstall. Maybe some of this fuzz has circled the drain (the jets) and is plugging up the feed to the idle tubes? I can't think of any other reason it would act like a switch (unless my needle has gotten damaged, but I don't think so).
Tonight, I'll pull the jets and idle tubes and blow out the channel. Might as well solder the tubes back to .031, reinstall my .041 UIAB, and bend the tang to put the flap @ .160. I'll check the gasket and replace if it looks fuzzed up.
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Are you setting the mixture screws with hot engine, in gear at 700 rpm´s.
How far out are the screws at best lean idle setting?
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Yes, I take it for a drive, get the stat open, then adjust the screws for highest rpm but on edge of lean. When I had IT .031, UIAB .041, screws were out 3.0-3.5. Now that IT .033, UIAB .052, screws are 4.0-4.5. I thought maybe it would go the opposite direction with larger orifices, but the screws are farther out now.
After adjustment, I set idle screw to about 750 in park and it drops about 100 when put in gear. Nothing odd about that.
On a side note, this carb has some casting irregularities down near the jets. No obvious porosity, just some lumpy looking areas. I might see if I can lower my air pressure as low as possible, fill the bowl with water and blow thru the idle tube channel with my finger over the jet hole. Just to help retain my sanity a bit....
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I would have stayed with .031" IT and .041" UIAB.
Enlarging the UIAB´s to .052" and IT´s only to .033" will make the blend leaner than the previous. You may need at least .035" IT´s for .052" UIAB´s.
Also, .052" or thereabout DCR´s will make for better control of mixture screws and cure any off-idle issues.
JMHO
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I measured my fuel pressure and found it to flutter per pulse, between 5 psi (must be a checkball in the pump?) and 9 psi. I then noticed the airhorn gasket was soaked with fuel. It was flooding at idle and was the cause of my on-off switch at a stop light.
The carb was removed and I went to work figuring out what was going on with the float. First off, the air regulator was set to 9 psi dead head. Note that I double checked the pressure using the same gauge I used to measure the fuel pump pressure. Then the bowl was filled with gas up to about 1/4" from the top of the casting. Air was applied to the fuel port and bubbles came out under the needle. This was with the float set to 9/32".
The needle was removed and the seat peened with a checkball. The needle and float were reassembled and 9 psi applied. Still bubbled. I put my finger over the needle and it stopped bubbling. Apparently, not enough force on the needle.
I then removed the float and started bending and reassembling until it quit bubbling at 9 psi and the fuel level was about 3/8" from top. I thought maybe I should add a little safety factor, so I bumped the pressure to 10 psi. It quit bubbling at about 5/16" from the top. This float level ended up being 3/8".
Edelbrock says the 1910 should use a .300" setting. I looked at all the fuel pumps on RockAuto and most say pump pressure is 5-9psi. So, unless you have a regulator, I'd recommend a lower float level, around 3/8".
To speed the process of removing and installing the float for test purposes, I removed the little wire clip from the needle. Turns out, you don't need it. Just an extra part. Works fine without it and the needle ain't going anywhere so long as the float is installed.
The idle tubes were removed, the collars tapped back in place and then the tips soldered and drilled .031. I reinstalled the previous UIAB setscrews, which actually measured about .042.
The APT did have about .015" difference in the arms. I measured this using the depth side of my dial calipers. To do this, I stood the APT on the piston end on a steel plate. Then I put the caliper tail on top of the arm and measured down to the plate. Bent them until they were within a few thousandths using my crude method. For now, I've set the APT at 3.5 turns out and will lower a bit at a time until I get lean surge or off idle hesitation. Not today though...
What do you know, there is a screw on the 170 series choke pulloff. I applied vacuum to it and stuck a .140" drill bit between the front edge of the choke flap and airhorn. Then I turned the screw until the flap just contacted the bit. I still need to fine tune it in the morning when it is cold.
Reinstalled the carb and started the engine. Turned the idle mix screws in until the engine tried to die then backed out a half-turn. Let it warm up and then readjusted for verge of lean. Idle is smooth, very little engine vibration. At this point, I put a vacuum gauge on it and found this cam is pulling 19.5 in Hg!
Took it for a several mile cruise. No problems dying at the stop light. No run-on when I turn off the engine. Restart is instant, no having to fiddle with the pedal.
I'll fine tune the choke tomorrow morning when it is cold.
Plan to drive it like this for a few weeks to see how I like it.
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Carb Assy Tip - Spray your gaskets with Aerosol Silicone Lubricant. This will help prevent sticking and ripping when disassembling for multiple tuning changes.
I have disassembled this carb at least 10 times and still using all three gaskets: airhorn, throttle plate and manifold.
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I breezed back over the thread and noticed a few common denominators with not having a big enough camshaft in an engine based on the CID and static compression ratio, or the LSA is a little too tight, etc.
Some run-on after shut down.
Pinging in certain conditions without a lot of timing.
Making a LOT of vacuum at idle.
Difficulty tuning the idle system.
Not a deal breaker but for sure this engine would have loved about 10-15 degrees more seat timing, duration @ .050" and pushed out of a wider LSA.....IMHO
I tune couple of Saturdays a month and ran into a very similar scenario with a 454 "LS5" engine last summer.
Long story but it follows yours pretty close. The short version is that I did the OEM carb for it based on the owner supplying me specs that the engine was built to "stock".
This was a "high end" restoration so the carb was restored and built to stock specs with the only change being the addition of an external APT screw and slightly smaller primary metering rods.
The engine didn't run for chit from day one and of course the carb guy was getting ALL the blame from the engine builder. After at least a year of correspondence and trying to help out I simply had the owner bring the car here so I could see what was going on.
Immediately when he fired it up and backed it off the trailer I knew the engine was off someplace. I very quickly found out that the timing was retarded pretty far, it had pretty high cranking pressure, and idling very smooth and making a butt-ton of vacuum despite not much initial timing.
Any attempts to increase initial timing resulted in "bucking" the starter pretty hard when it was hot and ping right off idle and at heavy/WOT.
I tried few things with the distributor and timing and it didn't like any of it, but taking away timing everyplace helped some.
So I went to the owner and asked him once again what the engine specs were? I told him that there was no way this was a stock or even close to stock cam in the engine. So he contacted the engine builder via his cell phone and had him send a copy of the cam card.
Turns out the engine "builder" and "guru" had Howards supply the cam. It was supposed to be a "modern" version of the original LS5 camshaft but turns out it had about 30-40 degrees LESS seat timing and ground on a much tighter LSA. The .050" numbers were close but even slightly less than the factory camshaft.
We were pretty much done at that point. I recommended removing the cam and tossing it down over the hill and making a better selection since the engine had pretty close to the stock compression ratio up near 10.75 to 1.
Of course that went over like a fart in Church and the owner ended up leaving with the car and not overly happy about it. I did "de-tune" the carb to help out some as it had WAY too much idle fuel since the engine was making up near 20" vacuum if you put any timing in it.
So more or less a "crutch" fix for a fundamental issue not really related to the carburetor or distributor.
As for your set-up I think it can be effectively tuned to correct all the issues. I personally believe that the real fix is to install a bigger camshaft in it on a wider LSA.......FWIW......Cliff
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I have unintentionally installed an A/F alarm to the truck. What is that you ask? Well, I had my mufflers changed from short to long last Friday, and one has a loose baffle. Any slight backfire will vibrate like a snare drum.
This morning I fired it up with one pump on the throttle. At first I heard some snare drum, but it faded within a few seconds and smoothed out at 1400 rpm. The next step on the choke, same thing with a bit of drum followed by quiet. By the time it hit warm idle, no backfires and smooth.
A slight drum during transition, but is gone instantly.
Cruise seems good, no drum noises from the muffler.
Acceleration gets a bit rattly at higher rpms, but I suppose that's just due to a more powerful exhaust pulse.
Tomorrow, a replacement muffler will be installed and I plan to run it through all four barrels. Right now it is just too embarrassing sounding with the loose baffle to open her up.
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it's interesting how the exhaust note can change as the tune gets better. i built a Qjet for a friend of mine for his 80 firebird a few years ago. he had (at the time) owned the car for 12+ years, never really ran right, but did run. One of cliff's kits, some major straightening of the air horn & main body, major tune up and fixing an exhaust leak it had had for sometime. the note totally changed from idle to part throttle and wot.
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Yes, I noticed a substantial decrease in exhaust noise going from the 800cfm 170 to a 750cfm 70 series. However I assume it was due to the calibration. The 70 series was calibrated to a '71 350 and had the IABP plugged to work on a 305.
How did you straighten the main body (zinc die casting) of a quadrajet? The two front corners are typically mashed due to an overzealous mechanic. I've heard that you can bolt the body solidly to a thick piece of steel plate, bake it in the oven at ?F for awhile, loosen and re-tighten the screws, then repeat the process. After ? cycles, let it cool while bolted to the plate and voila, the corners magically return to almost flat. Not sure how this works exactly, but that's what I heard.
Of course I also heard that you might as well toss that mashed quad body in the scrap pile.
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I would have stayed with .031" IT and .041" UIAB.
Enlarging the UIAB´s to .052" and IT´s only to .033" will make the blend leaner than the previous. You may need at least .035" IT´s for .052" UIAB´s.
I get what you are saying and for grins, calculated the ratio of IT to UIAB areas and assumed a constant .047 DCR.
IT .031, UIAB .042 -> UIAB/IT = 1.8356
IT .033, UIAB .052 -> UIAB/IT = 2.4830, This means the air bleed area is 2.5x larger than the idle tube. So it will pull more air under the same vacuum, but isn't it pulling a greater volume of fuel through the bigger idle tube? I think it results in more fuel and air, but the mix is leaner.
IT .036, UIAB .050 -> UIAB/IT = 1.929, This is Cliff's book recipe and is close to the ratio of .031/.042. Surely this has to pull a larger quantity of both fuel and air through a larger hole (even with the DCR constant). This would suggest idle fuel is greater than .031/.042.
I could be wrong, I have been before....
Carb tip: To set your power piston arms even:
1. Assemble your pp with needles into the carb, but leave out the power piston spring.
2. Set your dial caliper tail on the top of the needle where it bend and enters the arm. Then measure down to the top of the jet.
3. Measure the other side and bend the arm so it matches the previous arm dimension.
This should be darn accurate since it is measuring distance from the needle to the jet.
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Staightening the main body the way you descibed is the way I do it. I built 2 plates that bolt through the carb. Heat till plates are 300ish degrees, snug, cool down, repeat if necessary. Only reason I did was because it was a numbers original carb. Have used it on others and really helped seal things up. I also do the airhorn. Always requires some flat filing to get gooder. The plates I made are machined so I dont have to remove any tubes or parts from the airhorn. Of the few I have done, seems the warp is concentrated at the point where the divider?? plate is in the secondary airvalve bores are. Possibly because the main casting is 'thinest' there.
Doug.
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Use of .031" idle tubes with .042" upper idle air bleeds OR .038" idle tubes with .052" upper idle air bleeds, the A/F mixture will be about the same. Your choice.
.033"-.035" with .052" upper idle air bleeds will obviously be leaner.
What counts for you now is to get this idle A/F mixture down to the lower air bleeds and mixture screw needles though the idle down channel restrictions in proper amounts, the A/F blend is fixed.
Larger lower idle air bleeds provides larger amounts of idle/low speed A/F mixture that cures off-idle and low speed issues.
And, even if you measure and bend the arms on power piston there is no guarantee the drilled hole for the rods are at the same spot. Most times they´re not.
Only way to know the rods hangs the same distance is to mount them on the power piston and bend the rod ends towards eachother until they meet at center of power piston end and then adjust the arms if required.
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I thought it would be best to take the hole in the piston arm completely out of the equation. That's why I suggested measuring from top of the needle to top of the jet. Only two items in this measurement, needle and jet. Being that we are trying to control needle relative to jet, woudn't this be accurate? Maybe I'm missing something?
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I spend a LOT of time on every carb that I build here to insure that the metering rod hanger arms are level and metering rods are EXACTLY even. I do it like Kenth suggests above, pull them together under the bottom of the piston and make sure the tips are even.
Another good bit of advice is here is to make sure the rods are the same length and that they are the correct length. Not uncommon at all to find the WRONG rods in a carburetor as most have been out there 40-50 years and who knows what has been done to them.
I also recommend putting new jets in them during a rebuild. I find jets all the time that have been drilled out at some point and WAY bigger than they should be.......
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Alright, that's two for tips under the piston method. I'll try it next time.
Do you ever see jets that are obround due to the needle rubbing?
I would assume this a common issue due to spring tension of the primary needle retaining clip.
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I see jets every single day that are worn or have been drilled so I replace them with new ones.
I get carburetors in here from every "builder" out there and see at least a half dozen fundamental problems with them and why they weren't working for the customer.
Not too long ago I got in a fully restored early Buick 430 carburetor that was done by a shop in NJ.
The owner had it fully restored and it was running really rich. A couple of trips back to the "builder" made no improvement so it end up here.
I took it apart, and aside from using a small N/S assembly, clipped POS power piston spring and crappy accl pump (soft blue seal already swelled up some) I couldn't see why it was so rich. It had the correct 71 main jets and 45B metering rods in it. Hanger was bent slightly down not up so not contributing to the rich condition.
I took a closer look at the main jets and they looked pretty big for 71's so I removed them. They ended up being .078"! Most likely at some point the owners friend or one of his beer drinking buddy's thought he help him out while the carb was apart and give it some more fuel.
Moral of the story is to be meticulous and leave no stones unturned with this sort of thing, plus use new parts and high quality parts if you want the carb to be reliable in long term service......FWIW.....
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Cliff and Kenth,
Thanks guys for stickin' with me through this.
I couldn't leave well enough alone. Engine seemed like it wanted a bit more fuel, so I tried to figure out why it was running well at:
IT .031
UIAB .042
DCR .047
IABP .052
Jets .074
Rods 50C
First off, remember that this engine has a roller cam and short piston skirts. It also has a crazy tight 109 center cam (19.5 in Hg), vortec heads and moderate compression (9.1:1). Put this all together, and it is more efficient than most.
If Cliff will recall from our phone convo a year ago, I originally used the .036 jets, but couldn't get it to idle down until I blocked the IABP. This is when I was using the bore UIAB at .070. I also had high sensitivity to engine run-on after the key was turned off. Engine run-on typically means too much fuel in the motor after shut-off. That's why I killed it back with a .031" idle tube.
I ran some calculations on fuel through an orifice and found vacuum of 19.5 in Hg through a .031 idle tube, pulls the same as 16 in Hg through a .033 tube.
I then looked at the ratio of air to fuel between the UIAB and IT with my current IT .031, UIAB .042. Keeping the ratio the same, I extrapolated the following table:
1.75-1.85
IT (in) UIAB Rec UIAB Rec (mm) Input Actual UIAB Rec UIAB/IT Actual UIAB/IT
0.031 0.042 1.07 0.042 1.84 1.84
0.032 0.043 1.10 0.043 1.84 1.81
0.033 0.045 1.14 0.043 1.84 1.70
0.034 0.046 1.17 0.047 1.84 1.91
0.035 0.047 1.20 0.047 1.84 1.80
0.036 0.049 1.24 0.047 1.84 1.70
0.037 0.050 1.27 0.052 1.84 1.98
0.038 0.051 1.31 0.052 1.84 1.87
Thing is, you only have a few sizes in a numbered drill set to achieve the orifices. So, you gotta choose what is closest and hope for the best. That is what "actual" indicates. The actual drill bit you have in your index.
Being that the idle screw was just barely off the stop at IABP .052, I decided to choke out some bypass air to reduce my idle and allow better use of the idle screw. I choked it back to the next drill bit, .047".
I also thought it wouldn't hurt to run the float a bit lower than my tested and true 3/8. So I put it at 7/16. With a .135 needle seat orifice, I should never have a float issue. Obviously, I don't plan to go racing. 4500 rpm is my shift point.
So my new calibration is:
IT .032 to .0325
UIAB .043
DCR .047
IABP .047
Jets .074
Rods 50C
APT 2.5 (still does not go lean with a quick hand over the airhorn)
Float 7/16
Note that idle mix screws are 3.0 turns out. It actually works at 2.5, but I can eliminate the pip, pip, pip form the exhaust with another 1/2 turn.
I think your single taper 50C rods will optimize gas mileage. But, I do like the double taper 50M because once it hits the second taper, it comes alive well before full throttle. However, if you are pulling a trailer and the piston rises to this level, you will surely consume more gas than with the single taper.
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Keep in mind when doing all of this that changing the float level also changes the fuel level in the bowl. This has a DIRECT impact on all of your results.
Same thing with the size of the fuel inlet seat and fuel pressure.
I can set the fast idle on my engine at 2000rpm's with the engine fully warmed up. Starting at 4psi, for example I can use the fuel pressure regulator and start increasing the fuel pressure (raises the fuel level in the bowl). You can hear the engine speed increase as you increase pressure as it's raising the fuel level.
So every "recipe" you come up with is impacted by the fuel level, which is controlled by fuel pressure, fuel inlet seat diameter, float level (and size of the float).
Thru all of this I'm still not sure why you chose to move the upper IAB's to the airhorn instead of leaving them in the main casting. The indirect bleeds in the 1903 are fine at the stock size and location. If you move them to the airhorn they become a direct bleed and have a much greater impact on idle fuel delivery typically requiring much smaller idle tubes and DCR's to keep the carb from being way too rich at idle speed.
I may have mentioned it before but will throw it out there again. The tiny 109LSA cam is NOT helping the scenario or making the engine easy to tune. I've never once in all the years of doing this been the first bit impressed with cams like that and they have thrown me some pretty big curves with tuning. Pinging was among them, and "quirky" idle quality.
Not saying here that you can't make one work OK. Just keep in mind that small camshafts on tight LSA's do several things. They close the intake valve EARLY, pull power DOWN in the RPM range, and NARROW up the power curve. This very quickly takes a lower compression engine and makes it behave like it has higher compression. Peak VE occurs earlier, so with the good cylinder filling going on at lower RPM's we get a pretty high "spike" in cylinder pressure and more times than not the engine will ping at both light throttle and heavy/full throttle if we try to put very much timing in it.
I think the "quirky" idle simply comes from closing the intake early but still having a decent amount of overlap from the tight LSA. That's just a guess on my part but all of the engines I've had in here to custom tune showed me those characteristics when outfitted with similar camshafts.....FWIW.......Cliff
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I also prefer a bore type UIAB. It's just a better place to put it. The large passage, typically .065-.070, does not protrude into the bore and the upward angle all work together to virtually eliminate clogging.
Here is my assumption on why the airhorn bleed must be smaller than the equivalent bore bleed. The airhorn bleed enters the fuel channel at 90 degrees, while the bore bleed enters at a reverse angle. The bore bleeds cause the air to crash nearly head-on into the oncoming fuel. This has the effect of reducing air flow. Hence, the horn bleeds introduce more air and are necessarily smaller. At least that's the way I see it...
The reason I moved the UIAB to the airhorn was to allow an IT .031. Being that the bore UIAB was .070", I couldn't really take it any smaller. How would you make this smaller? I don't think I can fit an 8-32 without taking out part of the bowl top surface. And there is not much sealing surface around the upper idle channel. So, it seemed logical to move it to the airhorn. There I can easily change air bleed size.
I agree, fuel pressure is definitely something to consider when setting float levels. I observed that typical, over the counter pumps are pushing 9psi. So work best with a lower float level, especially for a driver.
As for the cam, I do have a ZZ4 roller squirreled away somewhere. It is 112 208/221 .474/.510. Would definitely wake this engine up. Also easy to tune with a .036 IT, .050 UIAB, float 3/8, static timing 12. But, I'd have to change out the springs, retainer, pull the condenser, radiator, accessories, intake, etc. Probably should have considering how much time I have in this quirky tune.
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I have found it´s far easier to add fuel to the equation than to relocate the UIAB´s.
If the engine lacks fuel in a certain circuit, add fuel, and the engine will be happy.
Different applications WILL have different sized idle tubes despite having the same sized airbleeds.
And, lots of EM4ME ECM Q-jets has .031"-.032" idle tube using .034" upper IAB´s in bowl with .077" lower IAB´s.
You will be hard pressed using .031" idle tubes with .070" UIAB´s and .070"-.077" LIAB´s for a performance built engine. You will need at least .036" to .038" idle tubes for result.
Then add proper amounts of idle bypass air to get the throttle blades in a proper angle at idle to reduce risk of nozzle drip and engine run-on at shut off.
FWIW
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Kenth is correct about the later carburetors and man use a larger Lower IAB and uppers at or close to .070". On an engine making a lot of vacuum at idle a .034" idle tube will be plenty. You can install larger idle tubes if the engine gets a cam that lowers vacuum some, but it works better to reduce the lower IAB and increase the idle tube at the same time.
A large Lower IAB vents off the signal and reduces "pull" to the DCR's and on to the idle tubes. Adding fuel with a larger idle tubes works, but you may notice a slight "skip" or "quirky" idle quality with the larger Lower IAB vs making it smaller.
The larger Lower IAB also leans out transition fuel as well. Fine for really mild engines with tiny cams, may cause issues with engines having larger cams in them and less signal or vacuum at idle and right off idle.
Early, non-emission calibrated and Marine Q-jets used really tiny Upper IAB's and would bring down PLENTY of idle fuel with tiny little idle tubes and very small DCR's. As emissions tightened up we saw those tiny little Upper IAB's become larger, then disappear and they went to indirect Upper IAB's in the main casting. Some carbs use pretty small Upper IAB's even though they were indirect, usually teamed up with a pretty large LIAB but not always.
For custom tuning the "recipes" in my work recommending .070" upper and lower IAB's work very well. I'm custom tuned many thousands of carburetors using those recipes with perfect results.
Even so I still experiment with Idle Airbleeds to this day, and may go that direction if I get a carb that just needs a tad more idle/off idle fuel and I don't want to pull the idle tubes or drill the DCR's larger.....Cliff
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I did leave the LIAB at .064 to better match the smaller idle tubes.
Drove to work this morning, seems to like the additional idle fuel. Working okay except for a slight bog after shifting at normal speeds. I suspect my APT needs to be a bit higher.
Now that I've choked back the IABP, I notice my throttle does not settle unless I blip it. Just slowly letting off the throttle will end up a few hundred rpm high and causes run-on at engine off. Might be caused by a non-worn portion of the throttle shaft (still has green plating) hanging in the new bushing. Possibly polishing next to the throttle linkage with a strip of 400 might free it up. I did try a stiffer return spring, but did not like the feel.
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I did leave the LIAB at .064 to better match the smaller idle tubes.
Drove to work this morning, seems to like the additional idle fuel. Working okay except for a slight bog after shifting at normal speeds. I suspect my APT needs to be a bit higher.
Just checking you now have .032"-.033" idle tubes and .043" upper idle airbleeds located in airhorn? Not too bad.
1. The LIAB´s switches to idle mixture discharge holes at greater than off-idle/low speed throttle blade angle.
Too small LIAB´s for the application equals to small amounts of added mixture and makes the hesitation at throttle movements like you are experiencing.
2. Also, too small IDCR`s will restrict the amounts of idle mixture needed for proper throttle respond past off-idle/lowspeed.
I have found it´s hard to go wrong with .051"-.055" IDCR´s and .070" LIAB´s for applications like yours.
After LIAB´s and IDCR´r are set you can look at main jetting corresponding to main airbleeds and APT setting using the "tip-in" with a slow hand, not quick hand method.
FWIW
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The LIAB´s switches to idle mixture discharge holes at greater than off-idle/low speed throttle blade angle.
Yes Sir, I see the LIAB as an inefficient nozzle when the throttle blades are open. At that point, all idle fuel and air is metered only by the DCR. This is why you will get better gas mileage from small idle tubes than large tubes. It is true, the engine will pull only as much fuel as it needs. But, if it pulls thru the primary boosters it is better atomized and more efficient than fuel dribbling out the LIAB hole.....
Too small LIAB´s for the application equals to small amounts of added mixture and makes the hesitation at throttle movements like you are experiencing.
I don't have any off-idle hesitation. The time I did was caused by the power piston spring getting xxxxed and bound up in the vacuum channel. I dropped a brass washer in the bore to keep this from happening.
The issue I have right now is after it shifts and the rpm drops, while the throttle is held constant, there is a slight hesitation. I think this is power piston related. Probably need to raise it a 1/2 turn or so.
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Forgot to mention, I did try out the needles crossed under the piston method for adjusting the piston arms. I like it. Seems accurate enough and is a whole lot easier than trying to measure things.
I also tried the tip-in procedure again at 2000rpm. No dice. Unresponsive. Never goes lean.
Current calibration: IT .032-.0325, UIAB .043, DCR .047, LIAB .064, IABP .047. The "channel mix" is rich, but the overall calibration (fuel to the engine) is lean. This engine doesn't need much to spin it at an idle. And since I'm not getting off idle hesitation, seems to be enough for the transition as well.
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The engine ran great last night on the way home from work. Ambient was about 70F and there was no hesitation as the auto-transmission shifted and the rpm dropped. This suggested my idea about raising the APT might be correct, but that the current setting was pretty close.
I raised the APT 1/4 turn. Drove to work this morning in 45F temp and the hesitation was gone. This setting is on the edge of lean and should provide highest mpg. At this point, I think the calibration is dialed-in.
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Couple of things.
"I don't have any off-idle hesitation. The time I did was caused by the power piston spring getting xxxxed and bound up in the vacuum channel. I dropped a brass washer in the bore to keep this from happening."
The power piston spring is NOT a player for off idle transition unless you go very quickly to heavy/full throttle and vacuum falls off.
For all light throttle or "normal" driving vacuum never gets anywhere close to allowing the power piston spring to raise the power piston.
The distributor vacuum advance is a big player in the same range. Idle to off idle at light throttle openings applies vacuum advance timing no matter which place you source it, ported or manifold. So it is also a tuning tool in your arsenal to correct issues in that area.
The Q-jet is far more sensitive to very light throttle openings than other designs so will bring fuel in early from the boosters. This simply means that you don't need to add a LOT of fuel from the idle system, but all the fuel you do add effects the overall calibration at very light throttle openings and light engine load.
So it's basically a "recipe" to find the right settings. For most carbs I use .070" Lower IAB's and conservative with everything else.
It's also a standard bit size in the 1-60 set and a LOT of factory Q-jets will already be there.
The next choice is DCR, Upper IAB and idle tube size. You will have either "direct" Upper IAB's in the airhorn or "indirect" in the main casting just above the DCR's.
Direct Upper IAB's don't need to be very big and they have a pretty direct and dramatic effect on fuel delivery to the mixture screws and transfer slots. So be conservative if you go that direction or your carb uses them.
Very early carburetors had tiny Upper IAB's when they were in the airhorn as do Marine units (many were .040" or even smaller). They do NOT need very big idle tubes or DCR's to put PLENTY of fuel to the engine when they are in play. A .040" Upper IAB, teamed up with a .070" Lower IAB, for example, will put down a LOT of idle fuel with idle tubes in the .028-.032" range and DCR's from .040-.046". So keep that in mind when you come up with a "recipe".
I'd add here as a disclaimer that there are other items in the mix that effect your specific results, like length and location of the transfer slots, bypass air, the size of the primaries (large or small), type of booster (there were several different variety used over the years), etc.
So if you want to be a custom "tuner" you'll have to do pretty much what we are seeing here, experimenting with different combinations, making small changes, more testing, etc.
To further complicate your efforts don't forget about timing. The mechanical and vacuum advance are players as well. I'd also recommend making ONE change, and if it is initially favorable drive the vehicle for several tanks of fuel to get a feel for how it's going to work under all conditions and fuel economy as well.
continued......
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Keep in mind the entire time that being "rich" typically gives good results as you may not initially notice any problems with throttle response, power in any range, etc, but you can end up with horrible fuel economy. This is why aftermarket carburetors initially seem like an improvement over stock parts, they are set up without any emissions in mind, almost always very generous for fuel delivery everyplace, and the companies making them know they will get them right back IF they are lean anyplace.
The last thing I'll add here and this will blow all your tuning out of the water is that folks forget that a carburetor is a load-sensing device. It will add MORE fuel simply based on throttle angle, engine vacuum, pressure differential above/below the venturi and how much air is flowing thru it. So don't get all hung up and calculating metering area and mathematical formulas based on a "static" mind-set.
I can set a q-jet up to run straight off the jets and not even use a power piston and primary metering rods and it will be FLAWLESS everyplace. You may not even notice much difference in fuel economy either, if the right "recipe" is used when doing this conversion.......Cliff
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"I don't have any off-idle hesitation. The time I did was caused by the power piston spring getting xxxxed and bound up in the vacuum channel. I dropped a brass washer in the bore to keep this from happening."
The power piston spring is NOT a player for off idle transition unless you go very quickly to heavy/full throttle and vacuum falls off.
I should have more carefully worded my description of the hesitation. As you pointed out, just sitting there idling and flicking the throttle did not result in a hesitation. Obviously, I would have noticed this type of hesitation after reassembling the carb and adjusting the idle mix screws. The hesitation occurred on a below freezing morning, as I pulled into traffic from a side street and then when leaving each stop light thereafter. The evening temps were higher and the drive home from work had much less hesitation.
Does that make more sense?
Anyway, truck is running great. Will take it on a few hundred mile trip this weekend to and from the swap meet. Hope to see 18 mpg for this 1971 3/4 ton Chevy, 383, 4L80E with 4.10 gearing.
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Not really. IF the hesitation occurs "easing" into the throttle raise the APT slightly.
I also don't remember if you are using an exhaust crossover in the intake or not?
Most later set-ups were designed for FI and the intake is cold.
I didn't read back over the thread but what heads and intake are you using?......
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No crossover in these Vortec heads. Using 195F thermostat for now.
Vortec requires an external coolant bypass similar to big block Chevy. Current restriction is 3/8, but this takes too long to warm up. Plan to reduce the flow area by about 2/3, which is .217" or a #3 drill bit. The new restriction will be made from an 1/8 NPT brass plug threaded into the 3/8 NPT x 5/8 HSB intake manifold fitting.
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Cliff,
What are the specs on the vac advance I bought from you? You said you typically use only two cans, and I bought the one that starts at a higher vacuum. Seems like you said it was 9 distributor degrees (18 crank), but I don't recall the start and all-in vacuum signals.
I ask because I need to start tuning in my cruise curve. Looking to have around 50 total (10 initial + 22 centrifugal + 18 vac) at cruise (2450rpm @ 75mph). At least that is my initial target. Not sure if the vortec heads will like having the full centrifugal in by 2500 rpm.
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According to GM, my ZZ4 distributor has a centrifugal advance curve:
16 @ 2400
22 @ 4600
initial + mechanical @2400 + Vac @ 18 in Hg = 52 max
10 + 16 + X = 52 max
X = 26 crank max @ 18 in Hg => 13 distributor
Possible vac advance candidates:
VC700 starts 8-10 in Hg, 11.5 @ 19-21, $40 @ Napa
VC1755 starts 8-10 in Hg, 12.5 @ 18-20.5, $18 @ Napa
For VC700 -> 52 - 23 - 10 = 19 mechanical.
Need to determine what rpm makes centrifugal = 19 degrees by interpolation:
16 2400
19 X
22 4600
(19 - 16)/(22-16) = (X-2400)/(4600-2400)
X = 3500 rpm which is well above any cruise rpm. So, this will never exceed 52 total degrees. Provides a little safety factor for non-linearity in mechanical advance.
For VC1755 -> 52 - 25 - 10 = 17 mechanical.
Need to determine what rpm makes centrifugal = 17 degrees by interpolation:
16 2400
17 X
22 4600
(17 - 16)/(22-16) = (X-2400)/(4600-2400)
X = 2767 rpm which is slightly above cruise rpm. So, this would be very close to 52 total degrees but might exceed 52 if the torque converter unlocks on a grade.
The VC1755 might be a little much, probably ought to use the VC700.
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My bad, the vacuum advance listed above are for 1974 and older, points distributors.
Translating the above findings into HEI results in a couple of candidates:
VC1831 / AR7 / VC217 6-8, 12@14-16
VC1828 / AR4 6-9, 10@12-14
My goal is to keep the start of curve high, so advance drops out quickly to avoid transition ping.
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If I supplied the advance can you woln't need another one. You do NOT need a really early starting point unless you plan on running manifold vacuum to it and your engine isn't making much vacuum at idle speed. I consider that move more of a "crutch" anyhow, vs correctly tuning the carbs idle system and ideal initial timing in it right to start with.
You can "adjust" the amount added by changing the distance between the rod and the stop, or making the window it rides in longer or shorter.
Stock it's around 14-15 degrees, or should be close to that. Use a timing light to determine exactly how much it adds, hook up and unhook manifold vacuum to it or use a vacuum pump and watch the timing with a timing light.
Use drill bits as measuring tools when making changes.
The cold intake is part of your issues. They require complete "heat soak" before things will work well and why you saw minor issues on cold mornings.
A wet flow system needs a hot intake and why dating clear back to as far as I can remember EVERYTHING I've ever worked on, old cars, tractors, etc used exhaust heat to warm the intake for improved thermal efficiency and able to run leaner mixtures, etc.
Not really a deal breaker, just be aware that ALL tuning needs to be done with the engine fully warmed up and intake fully heat soaked. Things will also get better in hot weather.
You may read some conflicting information on that subject as there as still folks out there on various Forums recommending to block heat crossovers and run cold intakes, etc. Heat is your friend with these things, and at most you might see a very slight improvement if drag racing and running a dead cold intake but you will need to "flood" the engine with fuel and pour the timing to it to see any improvement. Even then it is difficult if not near IMPOSSIBLE to continue to run (race) that way IF you win and get into later rounds as the staff at the track will be trying to "hot-lap" you back to staging so they can get finished up and home at a decent hour. I've lost count of how many times I've beat up on much faster and really well prepared "race" cars (I race my car fully warmed up and heat soaked every round) with my daily driver in final rounds even though they had all their friends and beer drinking buddies pushing the car in staging, dry ice and even bug sprayers out trying to cool the engine back down to where it was on their last pass.....hint, hint.
continued....
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Anyhow, for street driven vehicles a working heat crossover is your friend, unless you live on the equator and it seldom drops much below 90 degrees outside....FWIW.
For timing I have found over the years that about 8-12 initial, 10-12 from the mechanical advance (20-24 at the crank) and about 10-15 from the vacuum unit are good numbers to work with.
Of course IF you have made poor choices for CID vs COMPRESSION vs CAM SPECS you may need to add MORE timing in a lot of places to make things happy. For well thought out street and street/strip engine builds the numbers above will typically get you there.
I do NOT like, use or advocate super-quick mechanical advance curves and a good engine build woln't need them anyhow. The advance curve (mechanical) should start right off idle, or about 900-1000rpm's and steadily increase to full advance around 2800-3200 rpms or so. No need to bring it in any sooner and ALWAYS use the vacuum advance. It is a load sensing device and required to improve engine efficiency, burn leaner mixtures, improve throttle response, fuel economy etc in the "normal" driving range.
Anyone who tells you to ditch the VA and just install a POS spring weight kit to get all the timing in right off idle needs to keep their day jobs.....IMHO.......Cliff
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Rigged a vacuum gauge into the cab before I went on my trip this morning. Cruising down the turnpike at 75mph, here are my vac readings in in Hg:
22 - This is higher than idle (19.5), but only happens coasting or going downhill....
18-22 - It has to be pretty flat and not much wind. Very little time is actually spent in this range.
14-18 - It stays about 16 unless the converter clutch drops out. This is where the majority of cruise happens. Need the vac advance to be in at this range.
10-14 - Converter clutch dropout, hills, headwinds and slight accelerations . This is probably a 1/3 of cruise time.
6-10 - Leaving stop lights, medium throttle
Based on the above, a vac advnace that starts around 8-10 and is all in by 14-16 is probably about right.
From my previous two selections:
VC1831 / AR7 6-8, 12@14-16
VC1828 / AR4 6-9, 10@12-14
Both look pretty good for this application. But first, I'll check the stamping on Cliff's advance and see if it is close.
Edit: I went ahead and ordered a VC1831 / VC217.
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I didn't get a chance to check one mine today in the shop but it's going to start around 8-9" and all in around 11-12 if memory serves me correctly.
If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
That's the way it's supposed to work, and what I've seen with all the street engines I've had in my cars over the years.
The "moderate" accl from a stoplight will vary the vacuum readings a LOT depending on how heavy right foot is. For "normal" driving those vacuum numbers should be pretty high......
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I put the pedal to the metal at a rolling 30mph and had ping in the primaries. It did not do this when the float was flooding and excessively rich. Probably needs more fuel and current calibration is 74 jets with 50C rods. Which way should I go, 75 jet or 49 rod? Probably easiest to find a 75 jet.
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For those reading this thread, be aware changing a jet changes both cruise and WOT response. On the other hand, rod changes on a 170, do not change WOT. The end of the rod, when the power piston has risen against the airhorn gasket, is still the same size..... So, apparently I need a 75 jet change to fix ping in the primaries.
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No need to change the metering rod that would have ZERO effect on any heavy/full throttle.
To test PRIMARY size jetting do NOT use the secondaries.
Climb a steep grade in a higher gear and lug the engine down while heavy on the primary side and not into the secondaries. If it pulls hard without any lag or feeling "flat" on power the jet size is adequate.
Most likely it just needs a little timing pulled out of it.
Keep in mine that with a tiny cam on a tight LSA and 383cid with decent compression cylinder pressure will be spike pretty high in the mid-range and early. I'll bet the hardest pinging will occur around 3000-3200 rpms at heavy/full throttle.
You may also find some light pinging trying to add much timing from the VA as well......
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Did you disconnect the secondaries during this last test?
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I did not block the secondaries. I simply mashed it to the floor and it started pinging immediately, so got out of it.
Would AX secondary rods prevent a lean condition in the secondary? The AX have long, thin tips and are about as rich as you can get.
Have not yet mapped out Cliff's advance. Will wait until the new advance arrives and check both the same evening.
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If I do find that the primaries need richened, there are still rods available with a smaller (.026 vs .036) power tip.
Here's a table listing % change in fuel relative to rod and jet:
Rod Jet Cruise Rod WOT Rod Cruise Area WOT Area Cruise % WOT %
50C 0.074 0.050 0.036 0.0023 0.0033
50C 0.075 0.050 0.036 0.0025 0.0034 5.01% 3.56%
50C 0.076 0.050 0.036 0.0026 0.0035 10.08% 7.18%
47K 0.072 0.047 0.026 0.0023 0.0035 -0.03% 7.85%
48K 0.073 0.048 0.026 0.0024 0.0037 1.65% 11.32%
49K 0.074 0.049 0.026 0.0024 0.0038 3.33% 14.83%
If I were to make a change, it looks like 48K rod with a 73 jet is a safe bet.
EDIT: Forgot to mention, I got close to 18mpg on the highway last weekend. So, I do like my lean calibration and it seems to be working. Just need to fatten the primary WOT I think....
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This truck is my daily driver and I have put some miles on it since the beginning of this thread. Noticed this morning my sticky throttle seems to be wearing in. One less thing, eh?
New vac advance should arrive in a couple of days.
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"K" series rods are next to useless for tuning with the APT system.
We offer a full tapered 45C instead, which tapers from .045" all the way to .026".
"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.
When tuning the primary side determine the jet size FIRST. Do this by climbing a hill or steep grade in a higher gear w/o the secondaries. Once you nail down the correct jet size then use the APT for light load/part throttle tuning.
For the later SMAB APT carbs with very few exceptions .036" tipped primary rods will be the best choice. I have tuned a few with .026" rods and dropped the main jet size down about 4-5 numbers to sort of split the difference between using .036" and .026" tipped rods, with good results.
Even with that said I only took that approach before we had the 50C rods made and I didn't have a decent set of "M" series rods available for that particular build......
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Thanks Cliff. I will take that into consideration if my vacuum advance mods are insufficient to cure the ping. I know that with the pedal to the metal, the vac should be all out, but I'm hoping for a miracle :)
Tonight I checked my initial timing and checked your vacuum advance curve. Turns out, I must have done a parking lot adjustment because my initial was 12 degrees and was supposed to be 10.
Here is your curve as tested using a Mighty Vac and dial-back timing light:
4 in Hg = 0
6 in Hg = 5
8 in Hg = 10
10 in Hg = 18
12 in Hg = 18
14 in Hg = 18
So basically, you have a 9 degree that starts at 6 in Hg and is all in by 10. I need the advance to be all in closer to 14 since my engine produces 14-22 at typical cruise loads. Actually, a better way of looking at it is that I need the vac advance to start falling out at 12-14. Don't want that advance under load. This cam and compression is prone to detonation. My new advance should arrive in a couple days and I'll run the same test on it.
Just for posterity, my current total cruise (2450ish rpm @ 75 mph) is 12 + 18 + 16 = 46 crank degrees.
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Edit: The Mighty Vac might read low by 1 in Hg. I calibrated against my vacuum gauge and there was 1 unit difference. Even so, that would mean your pod starts at 7 in Hg and is all in by 11.
If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.[\quote]
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Edit: The Mighty Vac might read low by 1 in Hg. I calibrated against my vacuum gauge and there was 1 unit difference. Even so, that would mean your pod starts at 7 in Hg and is all in by 11.
If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
Yes, this engine makes great vacuum. But, it is a '71 3/4 ton truck with 76lb wheel assemblies and a 4.10 differential (locker, 10-1/8 ring gear). It likes a bit of primary taking off. Not unusual to drop to 10 in Hg. I have the stiffest power piston spring + a brass shim.
"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.
Yes, similar to the 50M rods that come stock in the Edelbrock are 2-step. If you hit the 6-10 in Hg range, my truck would suddenly come alive well before the 2ndys. It is obvious when it reaches the step down and continuous length of .036". Problem is that it is max primary flow range, and pre-WOT vacuum signal. If you are towing and reduce vacuum enough to lift the needles to the step down, gas mileage is over...
Please remember my application is not a hot rod. It is a daily driver, 1971 truck that I can tow a tractor with when needed.
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My gauge shows them just a tad higher as well, but I've dropped it enough times over the last 40 years it may not be dead nuts on the money either.
In any case I've been using them for decades on thousands of applications and never once had to switch to something else to cure any type of "tuning issue" related to ping.
Your problems are fundamental more than tuning, so efforts to split hairs with these things are more a "crutch" fix than anything else.
Not trying to be critical or put a big black cloud over the engine build, believe me I've been EXACTLY where you are before.
There are "camps" on the NET that advocate tight LSA and early closing intake, and some of those folks have names that are household words when it comes to engine building and parts selection.
I don't give a chit about all that, but can tell you from doing this for a living for over 20 years full time and another 20 something years part time I have NEVER once been overly impressed with a relatively "small" camshaft on a tight LSA in a well thought out engine build.
Overlap is NOT your friend with these things, nor is early closing intake.
Most of this "trend" started quite a while back when some very popular folks started advocating LOWERING compression to some sort of proverbial "brick wall" of 9.5 to 1. At the same time they would recommend a short seat timing cam on a tight LSA to bring back the lost power. I just about want to puke every time someone calls up telling me that's exactly what they've done with their "new" engine build and they are asking me to crawl thru the phone to fix it....not overly impressive for power, pings, etc, etc.
Yes, when you LOWER compression you LOOSE power (torque) at every RPM, all else being equal. Problem is that tight LSA narrows up the power curve and makes higher peak power and it occurs earlier in the RPM range. That move INCREASES dynamic compression with much higher "spike" in cylinder pressure down where the engine is much better at cylinder filling.
So when we go that direction we INCREASE octane requirements as peak VE it higher and sooner. Peak VE is the point in the RPM range where detonation is most likely to occur. What screws the pooch here is that IF you are evaluating engine power by the "seat of your pants" those builds superficially "feel" pretty strong, responsive, etc. All you are feeling is throwing a LOT of power at you right off idle and in a narrow RPM range. So that little "shot of nitrous" feeling fools the driver into thinking they have improved things and the engine is now making more power. A higher compression build with tight quench, larger cam, wider LSA, later intake closing, etc will have a broad/flat/smooth power curve more like a locomotive. Since there is not quick "spike" or rush of power anyplace it will be evaluated as not as good, when in reality the engine is making excellent average power over a broad RPOM range. So the butt-meter tells the driver it's just not as good.
If we simply go to a larger cam and wider LSA, we flatten up the power curve and push peak power higher. This does two things, lowers cylinder pressure over the RPM range and lowers octane requirements at the same time. Since we've pushed VE higher, past that in a N/A engine it simply can't fill the cylinders better (more cycles per seconds) so no worry about ping past that point.
.......continued
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As it relates to what you are doing here a recent customer called me for carb parts for his "new" engine build. 454 from a truck or motorhome, compression raised to a bit over 9 to 1. Application is a HD truck mostly heavy work, towing, etc.
I asked about cam choice and he said that they had a custom cam ground on a really tight LSA, like 106 if memory serves me correctly, with the ICL at something like 100 or 102. I said to myself WTF??? Being a nice guy and not wanting to be critical of others I didn't say much but knew it wouldn't be a good end result. I did ask a little about it and got some long winded response about how articles in the "high performance" magazines from a "guru" with these things goes that direction and those engine make great power, walk on water, leap tall buildings better, blah, blah, blah.
Well, guess what......that customer is having all sorts of tuning issues and I've had to spend a good portion of my life trying to crawl thru the phone to fix them in recent weeks. It's really not a bad thing for me, I get to sell MORE parts to help "crutch" tune a poor engine combination!.......FWIW......Cliff
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Cliff,
Your 45"Custom" rod might work okay considering I lifted the 50C rods a bit to cure my cold weather lean hesitation. In other words, I wasn't cruising at .050" diameter.
%Change in fuel from 50C to 45C rods:
Rod Jet Cruise Rod WOT Rod Cruise Area WOT Area Cruise % WOT %
50C 0.074 0.050 0.036 0.0023 0.0033
45C 0.072 0.045 0.026 0.0025 0.0035 6.15% 7.85%
45C 0.073 0.045 0.026 0.0026 0.0037 11.02% 11.32%
I assume your 45C rod is a repop of the 45J?
Most J rods are not available anymore. At least not in the size I am considering.
45J 7043545 SMOOTH SINGLE TAPER TO .026
46J 7043546 SMOOTH SINGLE TAPER TO .026
49J 7043549 SMOOTH SINGLE TAPER TO .026
Some K rods are still available, but the step to a continuous .026 presents efficiency issues:
45K 17051345 SINGLE TAPER (0.045 - 0.039) & STEP TO .026
46K 17051346 SINGLE TAPER (0.046 - 0.040) & STEP TO .026
47K 17051347 SINGLE TAPER (0.047 - 0.041) & STEP TO .026
48K 17051348 SINGLE TAPER (0.048 - 0.042) & STEP TO .026
49K 17051349 SINGLE TAPER (0.049 - 0.043) & STEP TO .026
50K 17051350 SINGLE TAPER (0.050 - 0.044) & STEP TO .026
I was thinking I could probably keep the needles from rising to the step in typical day to day driving by going to a slightly less stiff power piston spring. Maybe target half throttle load or something like that.
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The 45C really aren't a "re-pop" of anything. I had them made to taper much like "J" rods continuous from .045" down to .026" tips.
They have been selling very well.
As mentioned earlier "K" series rods are pretty much useless for APT tuning. Right to start with the very top portion of the tapered section is in the actual orifice of the jet. You really only get a few thou of metering area to work with using them.
I have many hundreds of used "K" rods here, never use them for any reason and just toss them in a drawer......
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Swapped in the VC-217/VC1831/AR7 and measured the advance curve. Have to say, it looks almost like your advance at the beginning, just seems to have a bit more travel for more total advance.
4 in Hg = 0
7 in Hg = 2
9 in Hg = 10
11 in Hg = 17
14 in Hg = 23 Max crank (11.5 distributor)
This fairly matches advertised spec: starts @ 6-8 in-Hg, 12 @ 14-16. It is Standard Motor brand.
Your vacuum advance was marked VC-234/DV1862/AR31, and had the following curve (corrected by 1" Hg from previous post).
4 in Hg = 0
7 in Hg = 5
9 in Hg = 10
11 in Hg = 18 Max crank (9 distributor)
This VC-234 does not seem to follow advertised spec: starts @ 2-4 in-Hg, 8 @ 6-8.
Good news! No more ping in the primaries. Bad news, I have detonation on the 2ndys. Oddly, it seems my 2ndys are delayed about 2-3 secs. I don't recall a delay like this previously. 2ndy ping with rich AX rods, not helped by the Edelbrock "S" hanger. That thing rides low.
Seems to want more primary WOT. Maybe that is the cause of the seemed delay. Not running as strong as it was when it was flooding out. At that time, it ran through the secondaries fairly seamlessly. Don't remember which needles I had in it at that time, maybe the 50M step needles.
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Basic tuning guidelines and I've covered this on here and other Forums a few times.
Start out with known "recipes" for N/A engine combo's. What we need to do first is to get close so the vehicle drives well without any detonation anyplace. Detonation, especially at heavy or full load will damage or destroy the best engine build out there.
Tune JET size on the primary side first. This assumes your "recipe" for idle/off idle fuel is where it needs to be.
Do this by climbing some steep grades/hills in higher gears w/o downshifting and without the secondaries.
Once you determine the most ideal jet size leave it alone, at least till much later in the equation and most likely you won't need to make any changes.
If you are also doing some distributor tuning tune TOTAL TIMING first. Make sure the engine does not ping under any type of heavy load as this can damage the rod bearings, piston rings, piston ring lands, etc.
I like to start out a bit "fat" on the secondary side but may lean that up later. Being a bit rich is always better than lean when it comes to heavy and WOT operation.
Anyhow, tune the APT system next. Initially you want to start out a bit lean or rods pretty low in the jets then sneak up on the most ideal setting. This part of the tuning may take several tanks of fuel and driving in various conditions to nail things down. I may do some vacuum advance tuning here as well in later stages, but initially get the best results that you can.
Realize when tuning in the "normal" driving range that superficially a lean setting may show a slight improvement in MPG's, but overall I've found being lean on the primary side of the carb requires more throttle angle in many driving scenarios than it would if things were a little richer and overall everything else suffers. Remember that the basic laws of physics apply here and it takes a given amount of energy to accomplish a specific task.
For WOT testing it is difficult if not near impossible to do it "by the seat of your pants". It's even worse with some vehicles if they have poor traction as spinning tires flaws all testing. It really helps here to use a dyno (engine initially) then the drag strip or some sort of timed/distance thing to real nail down the best tune. Weather, head wind, air quality (DA), slight uphill/downhill grade and even how much weight is in the vehicle effect this testing pretty dramatically so once you get close it may take some time to nail down the best settings.
Continued.....
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For the distributor you need to nail down initial and total timing. Some engines will not tolerate a lot of initial timing or they will "buck" the starter on a hot restart. So after you determine best total timing you may have to "adjust" the mechanical spark curve some if the initial ends up high enough it causes restart problems.
For spark curve I have never found to date that a super-quick mechanical curve getting ALL the timing in early works well for a well thought out engine build. Engines with optimum compression for pump gas with well chosen cams and good flowing heads typically don't like, want, need or respond well to a lot of timing right to start with, nor will they like a lot of it early in the RPM range.
For vacuum advance, with a well thought out efficient engine build I can't remember ever going past about 14 degrees VA timing and most are fine with a bit less. If you have a "low" compression engine and put a "hefty" cam in it forget the last statements, it's going to need a lot of timing everyplace to make up for poor dynamic compression and poor combustion efficiency.
Anyhow, this particular engine combo has a small cam in it and tight LSA. This will spike cylinder pressure pretty high and early in the RPM range. Unlikely you'll need to throw a lot of timing at it anyplace, even at light engine load.
For "basic" distributor numbers I usually start out with around 9-11 degrees of mechanical advance 18-22 at the crank. A good starting point for initial timing is usually around 10-12 degrees so we'll be in the 28-34 total range right off the bench. I usually set the VA up for around 10-14 and work from there. The mechanical advance needs to start right off idle, but NOTHING in at idle speed or idle tuning will be difficult if not near impossible.
I'll add here that most of these "new" shiny billet offshore HEI's are HORRIBLE for that sort of thing to a point where I refuse to work on them here in the shop. Many have low quality center cams/weights and the relationship between those parts has been "adjusted" to a point where by the time I put enough spring tension in them to keep some timing out at idle they delay too far in the RPM range for start and stop points with the advance curve. Not tying to put a big black cloud over all of those parts, but quite a few I've seen here are JUNK and if you have good luck with them I'm happy for you. I'll also note here that we don't know what old car bumpers the Chinese melted down any particular day to cast the gears from and I've seen enough of them chewed to nothing in 200 miles or so to NEVER want one in any of my engines!
One to better things. So some good basic numbers for N/A engines will be 10-14 initial timing. 18-22 more mechanical, and 10-14 from the vacuum advance. This puts total timing around 28-34 degrees and around 40-48 at cruise, or potentially that much depending on your exact cruise RPM and when the mechanical curve reaches full advance. I like to see 2800-3200rpms for full mechanical advance with most of these engines.
Continued......
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The above numbers have served me very well for decades. I'd add here that I tune for a living, and still open up a couple Saturdays a month for custom tuning. I've had vehicle brought here from as far as 2800 miles one way, but most are 200 or less. It's ALWAYS the same thing, and fixing the issues are to go into the carb and distributor, usually the distributor first. I yank out the POS MrGasket or other crappy spring/weight kit (EVERYONE buys that junk for some reason), get the distributor up to par then move on to the carb. I'm about 98 percent successful to date, the few I couldn't fix I had to "crutch" but still got them to work OK. The ones I couldn't nail down, guess what? They had short seat timing cams in them on pretty tight LSA's and to much static compression for the CID and cam choice.......FWIW.......
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Correct, Chinese distributors are junk. The weights have no stop and it will advance to the moon. I only use GM distributors and this one has the nice #41 weights.
Agreed, aftermarket curve kits are crap. Not sure why they didn't simply copy the original GM design including the correct size bushings. Anyway, I steer clear of aftermarket everything when possible.
My total current cruise timing is: 10 + 16 + 23 = 49. It can go a little higher since my mechanical is not "all-in" at 2400.
If I do encounter kick-back during hot start, a switch from manifold to ported vacuum would resolve it.
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There isn't enough vacuum cranking to overcome the spring tension in any VA I've ever seen, so NOT part of the tuning for "base" timing.
As for "aftermarket" distributors the readers should know that there is NOTHING waiting for you with any of them, aside from not having to replace points every 20,000 miles or so.
I've dyno'd MSD billet distributors back to back against my stock HEI with a factory "990" module and coil and they gain NOTHING anyplace. Matter of fact the runs showed a very slight advantage to the factory HEI. Most likely because of the very PRESISE fine points on the reluctor vs the wide (and usually all rusted and corroded up because they didn't plate them) ones on the MSD stuff.
I'd add that I hooked up a 6AL box on the MSD for those pulls, still NOTHING over a stock HEI.
At the drag strip on back to back runs we swapped in an MSD billet distributor to replace a stock points distributor I had done for a good friend about 30 years back. It was "locked out" and plate brazed down, we just used the points as a trigger.
To my surprise the car slowed down about .03-.04 seconds and LOST about .5 mph....hum?
An "old timer" later told me that the points provide a "natural" timing-retard at high RPM's and why we went a little quicker with them vs the MSD billet.'
Learn something new all the time.......
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Since my last post, I've wired the secondaries shut and have been driving it back and forth to work. No issues although it has a little lag, under part throttle with the choke still on. Once warm, runs like a 2-barrel. No sag or surge, but noticeably less power when pulling into traffic. I have also tried WOT and found nothing out of the ordinary.
Having the 2ndys wired shut lowered warm idle a few hundred rpm. I assume this is because the wire is holding the throttle plates more closely shut. This results in less air getting into the engine.
As for detonation, there is some slight ping from the vacuum advance while in transition mode. This morning, I plugged the Vac advance and will run some additional tests to see if the ping is solely due to vac advance.
Ordered some colder spark plugs. Changing from R44LTS, to MR43LTS. The "M" means marine, so has a stainless steel body. Will see what affect these have on ping. If not enough, might retard the timing back to 8 degrees BTDC.
Here are the rods and jets I am considering and % fuel change relative to current setup:
Rod Jet Cruise WOT C Area WOT Area Cruise % WOT %
50M 0.073 0.050 0.036 0.0022 0.0032 -4.94% -3.52% <<Stock 1903
50C 0.074 0.050 0.036 0.0023 0.0033 0.00% 0.00% <<Current
48K 0.073 0.048 0.026 0.0024 0.0037 1.65% 11.32% <<.006 Taper, Step to .026
45C 0.072 0.045 0.026 0.0025 0.0035 6.15% 7.85% <<Single Taper
45C 0.073 0.045 0.026 0.0026 0.0037 11.02% 11.32%
50P 0.074 0.050 0.026 0.0023 0.0038 0.00% 14.83% <<.012 Taper, Step to .026
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LOL, figured out why my idle was a couple hundred rpm lower. Remember when I said I disconnected the vacuum advance this morning? Ha, I'm an idiot.... :D
I plan to modify the advance stop window so there is absolutely no advance until 10 in Hg. I can do that by applying the mighty vac, pumping it to 10 in Hg, and then welding a sheet metal stop over the gap between the pull rod and end of window. I might even see if I can lengthen the other end of the window so it stops at 2 in Hg below my idle vacuum (19.5 - 2 = 17.5 or 16-17ish). Hopefully there is enough stroke to accommodate moving the window stop. Seems like this should work. Will let you know how it turns out.
I think most HEI advances are made to work with smogged, detuned engines of the 70's and 80's. That's why I can't find anything that actually matches my ideal: starts @ 10, and all in at 17, with 18-22 degrees at the crank.
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FWIW. Do a search for Lars Grimsrud HEI timing papers. He has a pretty extensive list of factory VA's.
Might be something in there that suits your needs.
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Thanks PAV, that is good advice. I didn't want to bring it up on Cliff's forum, but yes, Lars has great information on tuning the Quad and distributor.
At this point, I've pretty much read everything on the net: Lars, Cliff, Wallace Racing, Tuner @ Racing Fuel Systems, etc. All are pretty sharp guys, but do have varying viewpoints. Lars doesn't discuss calibration of the idle circuit, Tuner likes the idle circuit lean, Cliff likes rich since rich runs, Wallace must be an engineer 'cause he has a calculator for everything.... :D
I'm stubborn as hell, and have to learn things by doing. So I take everyone's advice and then do what I want. Win some, lose some, but always gain experience. I'll tell you, I'm almost to the point where I will follow Cliff's advice to drop in the ZZ4 cam... almost.....
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I have found chasing a "magic" timing curve to ignite a too lean A/F mixture is a waste of time and leaves you endlessly chasing your tail, and an engine that will never run right or at it´s full potential.
Been there done that.
Nowadays i mostly use the proper factory timing curve, or very close to it, per engine application and add proper amounts of fuel into the mixture were needed, idle, off-idle, main and wot circuits, the engine will tell you were the need is.
Hesitation is an indicator of a too lean A/F mixture, and so is pinging.
Hats off for Cliff sharing his hands on experience with these things!
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Well said Kenth, we actually posted at exactly the same time so below I just left my comments on what you were trying to convey.
Cliff doesn't like "rich", I tune every carb and distributor exactly for the application.
As for predicting fuel delivery with metering area, well, sadly it just doesn't work that way.
Engine load, vacuum, airflow thru the carb/across the venturi areas, pressure differential above and below the throttle plates, fuel added by the transfer slot, idle mixture holes and LIAB plus the size of the idle and MAB's all have an impact on fuel delivery across the load/speed range.
I've NEVER once found success trying to use stronger distributor springs or VA cans with stronger springs in them (delay timing or have it fall out sooner) tuning one of these engines. Superficially yes, but in the long haul other changes were needed to get the combo up to par and the most out of it.
The problem here is fundamental, too much cylinder pressure for the octane of fuel being used. So you have to retard the total timing (mechanical) and limit the amount of vacuum advance added until it stops pinging. A/F is such a small player there not really worth mentioning as we NEVER flood an engine with fuel to cure a detonation issue, that just leads to heavy fuel consumption and in most cases it pings anyhow.
Playing around with distributor springs and VA cans is more of a "crutch" fix than to cure the issues and get the best end results.
A cam change will be the best move, and you will also be nicely rewarded with a LOT more power with the right camshaft. A small cam gets you in several ways, it does too good a job of cylinder filling early and with a tight LSA version you get a high spike in cylinder pressure early in the RPM range. A small cam in a large engine KILLS power clear across the RPM range especially when you have to start pulling out timing to make things happy.
Change the cam, bigger and wider LSA and your tuning efforts will be nicely rewarded with a BUTTLOAD more power over a broader RPM range, and no detonation anyplace.......FWIW.....Cliff
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Is there a specific cam you would recommend for this application? As I mentioned, I do have a ZZ4 cam. And I can get creative with the springs and retainers to permit greater than .470 lift. Does Melling or someplace like that make an "RV" roller cam? That is basically what I need. Would be great if I don't have to mess with the valve springs and retainers.
How about this one: https://www.summitracing.com/parts/mel-22129/make/chevrolet (https://www.summitracing.com/parts/mel-22129/make/chevrolet)
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I'm not even sure what's out there for hydraulic roller cams these days. I got out of the engine building business about 4 years ago. I've had very good success with factory roller cams dating clear back to when they started making roller blocks. I've never built an older Chevy with a roller in it, only the 1987 and newer blocks.
Going from memory some of the late 80's/early 90 TBI engines had some really nice cams in them on wider LSA's. There were pretty "low" lift too. I used one once in a 350 truck engine with early Vortec heads and it was an absolute HOME RUN in that build. I also used flat top pistons and thin head gasket, so the compression was up there some. The 350 went into my dads 1991 3/4 ton truck backed by a NV4500 and 3.73 gears. It pulled like a big block and got excellent fuel economy when he wasn't towing with it. I'm sure they make something these days along those lines. They didn't have a lot of lift and were easy on the valve train as well.
I'd recommend a cam with about 10 degrees more duration than you have now out on a 112. I hate the word "RV" but a 204/214 @ .050" cam would work very well in the 383 with 9 to 1 compression.......CLiff
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I took Kenth's suggestion and put it to GM factory specs. That means I've been driving around with the vacuum advance unplugged, only I did not install a 750 Holley. ;)
I swapped out to colder plugs R43LTS -> Still have detonation at WOT
I dropped static timing to 8 degrees -> Still have detonation at WOT
So, per Cliff's advice, have sought out a cam that does not suck. Called Howard's Cams and they recommended a 180225-12: .450/.465 207/213@.050 112 separation, idle - 4200. It should be about as good as it gets when used with factory Vortec springs and retainers.
My initial tune will be similar to Cliff's original recommendation. Hopefully reduced dynamic compression will permit the engine to use 87 pure gas, without WOT ping and engine run-on:
IT .036
UIAB (horn) .050 (1.25mm)
LIAB .064
50C needles
74 jets
I'm feeling optimistic about this......
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cliff gave me some guide lines to help select a cam for my "vortech" heads. these are the TBI heads that GM used from 87-95 before the "vortec" heads came out. Most consider them door stops but I did some pocket porting on them and put them on my 355. Flat tops with a tight 0.034" quench, i think compression worked out to 10.3:1. Cam was also a Howards cam 213/217 @ 0.050" 112LSA 0.485"/0.495" lift. Built motor for towing, seems to run fine on 91,haven't towed anything yet though.
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I actually like the early swirl port "Vortec" heads for a truck/towing engine. They bring the power in early and didn't fair two bad anyplace in your 355 by looking at the dyno sheet.....
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Yes, that is impressive for TBI swirl heads.
Here is a useful reference from the Crankshaft Coalition discussing Cam Duration vs Compression:
https://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility (https://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility)
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Good info. I especially like this part:
"The chart will show why it is foolish to install a 230 degree cam in an 8.00:1 motor in an attempt to get a lope out of the motor. It might lope, but it will lack the power to pull the hat off your head. There seems to be an obsession with lope these days. Lope is simply the sound of the motor being inefficient at low rpm's because the ascending piston is pushing fuel/air mixture back up the intake tract through the still open intake valve and disrupting the metering abilities of the carburetor."
Although they didn't take CID into consideration the info is still valid and the chart reflects what you'd see from a 350cid. A 454 on the other hand will gobble up a 230 @ .050" cam and run very well with it, especially if it had moderate compression. Below is a dyno chart from a 455 Pontiac Super Duty build around 8.8 to 1 compression. The cam was a 230/236/112LSA HR with 281/287 @ .006" duration. It idled dead smooth making 16" vacuum at 650rpm's with only 10 degrees timing in it.
Put the same cam in a 350 engine with under 9 to 1 compression and I'll bet you'd be lucky to see 8-9" vacuum without running the initial timing clear off the scale at idle speed!......
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cliff, bry: i wish I would have known the dyno graph went lower than 3000. i suspect the peak torque is higher and a bit lower down in the rpm range. also, it made the most power at 26*. i had to persuade the owner/dyno operator to dial back the timing from 32*. he did so 2* at a time made about 10hp/15tq more peal power with 6* less timing!
EDIT: I currently have 3.42s with 30" tire, this thing needa more gear, I think i will goto 4.11s which will help a lot for towing IMO
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The better job that you do with the engine build the LESS timing and fuel is needed to make highest peak power. Those early swirl intake port heads must do a great job at cylinder filling early in the RPM range so it's now wanting a lot of timing and making a lot of power early.
Most of the engines I've done here in the last 25 years have made best power with around 28-32 degrees and another 10-14 from the vacuum advance.
My own engine runs equally as well at 28 vs 30 or 32 degrees at the track, so I run it with 28 degrees......Cliff
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Still waiting on the cam.
In the meantime, I learned about the GM 395 cam currently in the HT383. It was a Mercruiser marine cam originally. Boat cams are designed to prevent reversion. On shut down, reversion (due to overlap) will suck water back into the engine - hydrolock. Generally, a marine cam has very small overlap and early intake closing. That in turn makes the dynamic compression ratio extremely high. This is why the HT383 has engine run-on and WOT detonation (87 pure gas).
GM 395 Camshaft Specs:
246/256 196/206 109 3A
49 ABDC 8.10 DCR 33 OL
.431/.451
Howard's Camshaft 180225-12 Specs:
260/266 207/213 112 4A
58 ABDC 7.69 DCR 39 OL
.450/.465
The new cam will delay intake valve closure by 9 degrees, reducing the dynamic compression ratio by .4 points. Overlap is also improved by 6 degrees. Hopefully this will be enough.
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FWIW, you can get a GM roller cam p/n 94666492 for about $100 USD. While there are some various claims for duration, I recently put one in a friends boat as he was on a budget. My degree wheel said 202/210 @ 0.050” 112 LSA, 0.300”/0.310" lobe lift (.450/.465 w/ 1.5 rockers).
BTW, this cam has NO fuel pump lobe. An electric pump will be required
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Thanks, I did see that. It's a Crane Compu-Cam 2032. Back in the day, this was a lumpy street cam for an L98.
GM 492 Camshaft
270/276 214/220 112 4A
63 ABDC 7.44 DCR 49 OL
.452/.465" LIFT
Primary reason I did not use is lack of fuel lobe (plus a need for new valve springs). However, the specs are decent for a street 383 with 9.1 CR. The 383 would tend to mellow it out.
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Like I said, my degree wheel said differently. Don't know if they have changed the grind,but I was nearly 10* less than the 2032 grind on intake&exhaust. Hydraulic camshafts are $400 to $600 Canadian so its a no brainer for EFI SBC. It's a shame about the fuel pump lobe or I probably would have ran with with some 1.7 roller rockers for a bit more effective duration on my recent truck engine build. I ended up using a Howard's 180235-12 on my 10:1 sbc. Looks like 1 step up from the one you are waiting for. Anyway,goodluck!
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Interesting. How do you like that cam for a daily truck?
I considered the 180235-10. It looked like a fair match with 49 overlap and 57 ABDC. I saw Howards also offers a -12 (112 lobe separation).
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I unfortunately only put 1000 miles on it last summer, had some issue with lifter bores and am only now getting it back together (waiting on exhaust). I will keep you posted during the next month.Hoping to get it on the road by the end of the month
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Still waiting on the cam. Continued disruption in supply chain due to C19 response? Gates, Xiden and the CDC certainly want to extend the pain until everyone receives experimental gene therapy. Incremental destruction of your constitutional rights. Not to mention, I still don't have a cam. >:(
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My vote is still for the GM cam i posted above even though a few less degrees duration. Than the howards cam. I have been waiting dor a missing piece from my flowmaster exhaust kit for over 3 months now. also helping a friend with his 1950 dodge and we bought a parts carb for it and have been waiting over 6 weeks. Yet my rockauto orders are delivered in 3 or 4 days.....
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Finally received the cam end of June. Got it installed and running a few days ago.
Before I removed the 395 cam, I measured cranking cylinder pressure. It was 210 psi which is too high for 87 octane. Why did one of those HotRod magazines claim it was 195 psi? I haven't measured the pressure of the Howard's cam, but shutdown run-on and WOT detonation are no longer an issue (Cliff, thanks for recommending the cam swap!).
My guess is the new camshaft will slightly reduce gas mileage since I no longer have "beast mode" torque right off the line. It used to almost always chirp the tires on initial acceleration. I'd guess the power band has moved up about 500 rpm.
New cam definitely idles smoother with 112 vs 109 lobe separation, even though intake and exhaust duration are increased about 10 degrees.
Exhaust note is quieter under normal driving conditions.
Still running the same carb tune and it seems okay so far.
Overall, I believe the 395 cam would work great in an EFI daily driver, but it is a poor match to an old school carb and HEI.
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Good news. You really didn't give up any power, it's making more power. Wider LSA simply spreads it out some and doesn't spike it nearly as high. This reduces octane requirements at the same time.
I'm NOT surprised idle quality is improved with more seat timing. The longer seat timing and wider LSA will also make for a nice improvement in upper mid-range and top end power.
The engine may not "feel" quite as strong IF you are judging improvements by the "seat of your pants". Do NOT be fooled by this. Smooth/strong power over a broad RPM range will trump throwing all the power at you early and in a narrow RPM range.
This simply happens because you are applying more power to the pavement on a full throttle run vs throwing ALL the power at you quick and early. When this happens the quick blast of power "feels" like a shot of nitrous and we evaluate it as an improvement over the "locomotive" power from a smooth/flat/strong power curve.
I've tested camshafts on the dyno and it also reflects the same thing. Tight LSA and shorter seat timing spike peak torque high and it happens early.
One test I did back to back a few years ago was testing a Pontiac RAIV cam against a custom ground Comp cam with their "best lobes" (at that time).
The factory cam was 304/316, 231/240 @ .050" and 113 LSA.
The Comp cam was 284/296, 240/248 @ .050" and 112LSA. We had it ground to replace the Crower RAIV clone cam thinking the bigger Comp cam would make more power and improve both ET and MPH at the track. The Comp cam also had a LOT more lift than the short lobed RAIV cam. Even with high ratio rockers the RAIV cam was only .516" lift. The larger lobe Comp cam was over .560" lift.
EVERYONE involved in the swap was betting the "modern" Comp cam would outrun the old RAIV grind everywhere.
Well, NOTHING went well that day on the dyno. We made great power with the RAIV cam, right at 500hp/570tq. Once we dialed in the best numbers I installed the custom ground Comp cam and we fired up the engine. Immediately I noticed that the engine idled a little better and it was really "snappy" right off idle whacking the throttle quickly.
I checked the timing and we started making pulls on it. My heart sank as it printed out the first dyno sheet. The larger Comp cam was down nearly 15hp and 25ft lbs torque....WTF?
I made several adjustments, timing, fuel, and it just got worse. I even went in and move the cam ahead 2 degrees and then retarded it 2 degrees and things just got worse with everything I did.
I finally returned everything to the initial settings and fine tuned it to perfection and we were still off 10hp/22ft lbs torque that we just were NOT going to get back.
The engine also made peak power 400rpm's LESS in the RPM range with the larger cam, or done at 5200rpm's vs 5600rpm's with the RAIV clone. I just couldn't figure it out?
So I changed directions and installed a HR cam (we had one ground just in case this testing didn't go well).
The HR cam was a LOT bigger than in lobe lift than either of the flat cams. It used .361" Comp XFI lobes, 284/296 @ .006, 230/242 @ .050" and 112LSA.
........Continued
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I was expecting MAJOR improvements moving up to the roller. We did end up making a little more power with the HR cam, after spending the entire weekend on this testing it netted 3hp/4ft lbs better than the Crower RAIV cam we started out with.
Wow, I got a lousy 3hp/4ft lbs improvement spending well over $1000 on a cam swap...bummer.
Even though the HR cam made a few more HP it actually made it at 200rpm less than the flat cam. All of this baffled me for a while until I thought about what the engineers were doing with cams back then. They were using longer seat timing and wider LSA but not much lift. They must have known that the time the valves spend off the seat can very quickly trump shoving them around with greater authority and much further off the seats.
I suppose it all makes sense if you sit back and think about it. When we have 30-40 or even more cycles per second happening longer seat timing provides a considerable more amount of time for cylinder filling even if we aren't shoving the valve open as far or opening/closing them more aggressively.
Just some food for thought here as it's pretty rare to see anyone actually doing any back to back cam testing with no other changes. The folks selling cams ALWAYS change several items in the mix so their product wins the prize.......FWIW.......Cliff
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Wow that was a long read. Cliff was right when he said the carb should have been installed with stock calibration. Even if changes to calibration were wanted/needed you at least no where you started.
Unless you are as experienced as someone like Cliff, making more than one change at a time can be counter productive. Making single changes enables you to learn how each circuit works/reacts to changes.
.040" in the hole pistons and thick? head gaskets is a disaster for detonation/timing issues. Not sure I ever saw what the total ignition timing was. That led me to wondering why it wasn't backed off a bit.
Vortec heads do not require or like as much total timing.
Cliff's comments on ignition timing/curve, vacuum advance were on the money.
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Thanks. Correct about timing, and fuel requirements follow that as well.
The better you do with the engine build in terms of compression, quench distance and cam choice the LESS timing and fuel the engine will need to be happy.
If you were doing this thru the "smog" years you found out first hand how these things work.
Back in those days I owned and drove a 1970 Chevy Impala with a 350/300hp engine. It was powerful, idled smooth, great throttle response and nailed down 20-22mpg's on the highway w/o overdrive. It was stone stock with the only modification being dual exhaust, H pipe and a set of free flowing mufflers.
Those engines had flat top pistons, small cam, and excellent flowing small combustion chamber 041 heads.
I also owned and drove a 1970 GMC 3/4 ton truck with a 350 in it. Even with 4.10 gears it got pretty decent fuel economy and made great power. The engine started using a lot of oil and needed valve guides and seals. Back then I had no funds for this sort of thing. I was offered a set of finished heads from a local hot-rodder in trade for the heads from my 350 engine.
He just told me there were 1.94's and would work fine. Well I did the head swap one weekend using parts store "rebuilder" thick head gaskets. That move knocked what felt like about 100 horsepower out of my EXCELLENT running 350 engine. I couldn't believe it. What a "turd" it was with the new heads in place. Then I started looking at the heads I removed and noticed smaller chambers and much larger ports. I pulled the heads I was given back off the truck and compared them closer. The heads he had given me in trade were "882" castings.
I gave him back that junk and did a few side-jobs and got my 1970 heads rebuilt and put them back on the truck. I also used .020" thick steel head gaskets just like came off of it because I wanted everything right back the way it was. Power was restored and I logged that experience deep in my memory banks.
LOW compression, thick head gaskets and chitty flowing heads KILL power in these engines. The smog years were not good for the industry and they didn't get it figured out till almost 2 decades later........