1903 Won't Respond to Tip-In Procedure

[Cliff Ruggles]

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?......

[bry593]

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.

[bry593]

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.

[bry593]

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.

[bry593]

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.   

[Cliff Ruggles]

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....

[Cliff Ruggles]

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

[bry593]

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.

[Cliff Ruggles]

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......

[bry593]

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.

[bry593]

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.   

[Cliff Ruggles]

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......

[Kenth]

Did you disconnect the secondaries during this last test?

[bry593]

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. 

[bry593]

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....