Author Topic: 7028276 air fuel ratios at cruise 15.9  (Read 722 times)

Offline Stripes

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7028276 air fuel ratios at cruise 15.9
« on: May 28, 2019, 02:48:28 PM »
My 7028276 Rochester carb I rebuilt with recipe#1, jets 74, rods 41, .036 idle tube, .046 down tube cups. The 455 bored 60 Pontiac has a mild cam, iron manifold, RA3 exhausts, 2.5 duels. Rear gear 3:55, 700R4 tranny.
8 degrees initial timing, 30 total, 7k3 iron heads, 9.5 ish to 1 compression. Vacuum advance disconnected due to detonation at light throttle.

Using an Innovate MTX-OL Plus Digital Air/Fuel Ratio Gauge.

The car idles well with 14 inches of vacuum at 650 RPM, tuned for highest idle rpm resulting in a 12.1 A/F ratio. However, at cruise, I am running about 15.5 to 16-1 a/f ratio. I get detonation accelerating using light throttle and suspect I'm too lean at cruise. When I floor it the a/f ratio is right at 11.9 to 12.1, which I think is dead on with pump gasoline (10 ethanol).

Since the primary rod diameters ends are the same, seems jets should remain the same. How far should I jump down in rod size to get to a 14/1 AF ratio? Running 41 primary rods, considering 39 or 36 rods? Or am I missing something else?
Thanks in advance.
(running Cliffs kit, book, & recipe #1)

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #1 on: May 28, 2019, 08:27:43 PM »
How much vacuum advance are you adding?

Any idea what camshaft the engine has in it?....Cliff

Offline Stripes

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #2 on: May 28, 2019, 10:12:17 PM »
The vacuum advance is disconnected, so zero vacuum advance. I can't run more vacuum without detonation. Started at 12 degrees advance and worked back to 8 degrees to stop the detonation on 93 octane, 30 degrees total. The engine has flat tops and 96cc heads. 2200 stall converter, 3:55 rear.

The engine likely has a 068 cam with 212intake/225 exhaust duration at 50, very mild for the 469.(14 inches vacuum, pretty smooth 650 idle)

I understand current pump fuel with 10% ethanol has a stoichiometric ideal mixture of 14.08 to 1, as opposed to straight gasoline 14.7 to 1. It would appear that a carb built before ethanol was added to the gasoline would not be calibrated correctly. To make the same power with the gas plus 10% ethanol mixture would require a richer carb mixture. Thus, I think 15.5 to 1 is pretty lean at cruise, and hope going to a thinner rod will help it run richer. 

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #3 on: May 29, 2019, 02:27:39 AM »
Forget for a moment about all the info on A/F ratios and what is being posted about it on the websites.

It doesn't really apply as we tune for results not any particular A/F ratio.  There are many things that effect the outcome, and carburetors are not a fixed metering device.

A/F ratios can and will vary considerably over the load/speed range of the engine depending on throttle angle, pressure differential above and below the throttle plates, airflow across the venturi areas, etc.

If you were way too lean the engine would be WAY down on power, "flat" on acceleration, hesitating, and bucking and kicking in protest.  A/F ratios at light engine load can run pretty lean and the engine show no ill effects.  Running w/o vacuum advance throws a monkey wrench into the equation because lacking another 15-20 degrees timing at light load means that it will not effectively burn a leaner mixture and require MORE fuel to make things happy.

So timing is also a big player in this deal as it allows for a much leaner mixture to be effectively burned, especially in the "normal" driving range and why engine vacuum is used to add more timing as it is a load sensing device.

There are also many other contributors to not being able to run effective ignition advance and observing some ping at light engine load. 

I've had dozens of Pontiac 455's in here for custom tuning, many were very sensitive to timing and fuel at light load and I've had a few that were NOT happy adding any advance to them at all with the vacuum unit.  We found that these engines had small camshafts in them and or camshafts with early intake valve closing points.  Some had a LOT of quench as well.  Others ran hot due to poor combustion efficiency, some had combinations of problems all making for a poor end result as far as getting them tuned was concerned.

In any and all cases we give the engine the timing and fuel that it wants.  If you had to pull the timing back to 8 initial and 30 total and it woln't tolerate any more at light load a flag goes up telling me it's making too much cylinder pressure to the octane even though the static compression ratio is 9.3 or so to 1.

I'd also add that 74 main jets and 41 rods are already pretty "fat" in that carb number, assuming it has the correct rebuild parts in it (fuel inlet seat diameter, float height, PP hanger arms not bent down too much, etc). 

I typically set those up with 73 main jets and 42 primary rods for set-ups like yours and they are flawless everywhere. 

Where to go from here.  As mentioned in any and all cases we tune for results, not any particular A/F ratio, especially at light engine load.  I've seen engines have perfect results with the A/F ranging from 13 or so clear to 17 to 1 at very light load, so 15.5 to 1 is not outrange the effective range, and that number is also effected by how much timing advance you are using.  LESS timing doesn't provide effective combustion so the results are altered vs adding more timing and seeing improved or better numbers on your gauge.

Audible detonation tells me it's not happy with something, and it may be more than just the A/F ratio the carb is trying to provide.......continued....

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #4 on: May 29, 2019, 02:45:56 AM »
I love to provide examples so here is one close to yours that we were unable to effectively tune that came thru the shop a few years back. 

The owner had built his own 455 Super Duty engine and raised the compression ratio with a slight dome on the pistons to about where yours is or just under 9.5 to 1. 

The camshaft chosen was from Comp, a HR with 224/230/110 specs.  He advanced the cam to 106ICL.  I built the carb and distributor for him like hundreds I've done before and after and almost immediately he calls the shop bellyaching about the carb being way too lean and his engine is pinging all over the place.  I fatten up the carb and pull timing out of the distributor and he calls right back up after a little testing and says it's just a tad better but still pinging.

So I have the entire vehicle brought here and go to work on it.  I very quickly discovered that even with pretty strong springs and pulling total timing back to 28 degrees and only 10 degrees from the VA it's not happy.  I throw larger jets in the carb and much smaller metering rods and it helps slightly but doesn't really fix it.

I start retarding the timing and when I got to about 26 total it quits pinging but also starts to act down on power some and I'm not able to add any timing from the VA.  I'm completely puzzled at this point and don't know why or where to go next.

So I throw in a set of high ratio rocker arms to give the engine some more effective duration and it immediately helps the situation but doesn't really cure the issues.  I'm able at this point to run 28 degrees total timing with a slow curve so it's not "all-in" till about 3400rpm's and put another 6 degrees or so to it from the VA. 

A "crutch" fix for sure but it told me the real culprit here was that the camshaft was too good at cylinder filling at low RPM (too small and LSA too tight) making the 9.3 to 1 or so compression ratio engine act much higher.

It was an early lesson for me in these things, but one I didn't forget.  Jump ahead to about a year ago and a customer calls up here with troubles on the dyno with his "fresh" 455 build.  The shop owner and builder is trying to dyno it and it's pinging on the dyno and it isn't making chit for power or close to what they expected.  Of course EVERYONE right down to the guy who takes out the trash is blaming the "Quadrajunk" carburetor.

I ask the builder for the details of the build and he says it's a 455 with 250cfm professionally ported #96 heads, Comp XR276HR cam (224/230/110LSA), RPM intake and Q-jet.  Before things get go very far they had attempted to advance the timing to "normal" total advance and it pinged so hard it spun the rod bearings in it right on the dyno!......continued

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #5 on: May 29, 2019, 02:46:32 AM »

So my advice to the builder is to remove the camshaft and replace it with a slightly larger cam on a wider LSA and I provided the lobe numbers and told him to put the ICL at 110.  They follow my advice and no other changes.

Back on the dyno it goes and no more detonation, plus it cranks out considerably more power everyplace and they noted it actually idled better with the larger cam out on a 114LSA than with he smaller cam on a 110LSA. 

The actual dyno sheets are at the bottom of the last page.

Not saying that this is exactly what your problems are, but I suspect that the engine isn't happy with the small cam and VE is too good at lower RPM's making it finicky for timing and fuel.  With that said I'll also bet it's close to being unhappy at full load as well and I'd be careful not to ping it and cause engine damage even though you have it pulled back to 8 initial and 30 total.  Those numbers are pretty low for a 9.3 to 1 455 on 93 octane fuel with those particular open chamber heads on it.........Cliff

Offline Stripes

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #6 on: May 29, 2019, 09:20:50 AM »
Thanks Cliff. I agree it needs a new cam. The rear seal is leaking also and thus i'll have to pull it out, and I guess I've been avoiding doing that with the good weather here...

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #7 on: May 30, 2019, 04:33:15 AM »
In the mean time just be very careful with timing, and give the engine the fuel it wants to be happy.  If it's not wanting any additional timing from the VA then it's pretty close to detonation on the mechanical as well, especially at full load.

Related to your thread I have a customer with a 455 and mild cam.  He's got it in a later T/A with 2.5 something rear gears.  He has an A/F gauge and monitors it for tuning the APT system on his later Pontiac carb.

He told me a while back that at part throttle (primaries only) it will range from mid 13's to as high as 17's depending on engine load, throttle angle and vehicle speed.  He also spent quite a bit of timing tuning with the vacuum advance as well and found the "sweet spot" was to use a can that starts around 8" and all in by 12" vacuum and it adds around 16 degrees timing.  He also drove it several hundred miles to the T/A Nat's at got over 17mpg's and the car doesn't have overdrive.  That's pretty darned good for a 455 in my book........Cliff

Offline Stripes

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469 cam advise
« Reply #8 on: July 09, 2019, 09:53:32 AM »
Hi Cliff,
My 469 with the 7k3 heads and the very mild cam finally broke a valve spring and its time to replace the heads and move to a hydraulic roller.

I plan to use a set of d-port Edelbrock 87cc non ported heads on the 469, iron intake, long branch exhaust. Stock crank and cast pistons.  My 69 Firebird convertable weighs about 3300 lbs, has a 700r4, 3:55 rear gears, a 2200rpm convertor, and a quadrajet.

I know you like more duration in a big 469, and a wider centerlines. So, I'm torn between a 230/236 @50 hyd roller on a 112CL, or a 236/245@50 on a 114CL for a street fun type vehicle, not a race car. So really the stump puller or the old faithfull? I've got power brake, AC, and power steering.

I feel like the 230/236 would be enough duration, but with my past issues, I'm unsure.

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #9 on: July 10, 2019, 07:31:54 AM »
230/236 with at least a .360" lobe on a 112 LSA would be plenty for the big 469 with unported E-heads, especially with a 2200 stall converter....IMHO.

Offline PonchoVia

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #10 on: August 23, 2019, 02:45:07 PM »
This thread should be required reading. The explanation should really be tagged and posted on the home page.

The voodoo that is understanding a camshaft has been much more difficult for me to grasp than anything else. I   am just beginning to understand the basics.

What engine characteristics are affected by the centerline alignment of the camshaft?

thanks.
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #11 on: August 25, 2019, 04:44:40 AM »
Moving the cam around effects the intake closing point.  It also effects scavenging across TDC during the overlap period. 

There is a lot of things going on in one of these engines.  From what I've seen the big CID engines enjoy big cams on wide LSA's.  The Pontiac 455 by design is undersquare and fed by cylinder heads with no more cross section than an average small block Chevy head.

Good for cylinder filling down low, but chokes them up quickly at higher RPM's.  They were designed to be low RPM engines with tons of grunt for effectively moving heavy vehicles around. 

I have built a good many here, and consider any cam with less than 230 @ .050" duration too small.  I still see folks put tiny cams in them for "more low end power", but they will make more power than you could ever use at low RPM's with a larger cam, and the larger cam will make a butt-load more upper mid-range and top end power.

I attached a dyno sheet and cam card from a 455 we did here couple of years ago.  It was about 8.8 to 1 compression.  It idled with 14" vacuum and dead smooth clear down to 600rpm's.  I even had Harland Sharp 1.65 ratio rockers on that cam.......Cliff

Offline PonchoVia

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #12 on: August 25, 2019, 05:38:20 AM »
533 torque at 3500?!?! AND 454 horse at 8.8 compression.
These things are amazing when they
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline Cliff Ruggles

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Re: 7028276 air fuel ratios at cruise 15.9
« Reply #13 on: August 26, 2019, 07:49:56 AM »
Yep, actually undercammed it a bit, wishing afterwards I would have went to a .380" lobe with 289/308 seat timing, 236/245 @ .050" and 114LSA with the ICL at 110.  It would have at least 500hp and  still good idle quality and street manners.....Cliff