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Vac adv experiment on zz4 sbc

8K views 36 replies 11 participants last post by  Beepster 
#1 · (Edited)
Recently I upgraded to a MSD distributor w/ vac advance sdtup. The instructions that came in the box clearly stated to connect the vac adv line to a timed or "ported" source rather than a below the throttle plate constant vacuum source. Been driving GM cars mostly for 55 years. I remember always routing to constant vacuum port.

Yesterday I retimed my spark from 8 degrees to 12 degrees on the crank with 22 more in the centrifugal side for a total of 34 degrees, all in @ 2500. With the vac adv now on the ported side the idle is much more deep and agressive soundkng. After a full warmup lap around the neighborhood I reset all the idle controls to a slightly" fat " 900 rpm.
It seems to like this as well this way. It may reduce my highway mileage a bit because of reduced vacuum signal. With these double pumpers mileage is a sore subject.
Anyway, I am going to drive it like this for avtrial period then read plugs.
I have had no pinging at all but I am running a m20 muncie and I never lug my motors.
The motor seems to like the rapid advance curve. At a 3000 roll in first gear it will boil both big 10" tires just by mashing pedal GO pedal.

I know...people my age aren't supposed go do stuff like smokin rubber. To thaf I say : Bullfeathers
 
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#2 ·
Cool James, smoke em if ya got em.
 
#3 ·
I am trying to see for myself if one way is better. I have read discussions about this. Never did understand the conclusions.

Sean, I went iut thursday to a cruise in @ hardees in Weeki Wachee. Not much action. I hsd the only cruck. Lot's of lste 40's. Fords. I was hoping to run into another necoa membr.
 
#4 ·
Subscribed for your final evaluation.
In the 70's, I would always move my vac advance to a manifold vacuum port. It really only affects the amount of advance at idle, since the ported advance comes in just off idle. Some of the 80's non-computer truck carbs wouldn't idle down low enough, so I had to move from manifold to ported vacuum. I always felt that manifold vacuum gave a bit crisper response off idle, but it certainly wasn't a huge difference. I'll be curious to hear your thoughts after trying it both ways.
BTW, 34-36 degrees @ 2500 rpm sounds like a real good place to start for your total timing. IIRC, 2500 rpm was what you got with the middle springs in the old Mr. Gasket 3 spring kits. As long as you used decent gas, it worked well on a bunch of near stock street SBCs I tuned over the years.

Bill
 
#5 ·
Subscribed for the hell of it. Would love to hear what happens with your experiment.
 
#10 ·
Sorry about the second reply but I forgot to mention in my first post that I've also heard that full vacuum helps the small block (I think more on the 400s) run a little cooler. Keep an eye on temp.
Ang.
 
#11 ·
vacuum advance

Mechanical advance changes ignition timing based on engine speed, period. The benefit from vacuum advance is that it makes adjustments to timing based on engine load. The vacuum source that changes based on engine load is manifold vacuum. There is a great article on the subject written by a GM engineer that developed these systems. It can be found in several forums on the web and I have referenced it here, when these discussions have come up in the past. Here it is from a Nova site.
http://www.stevesnovasite.com/forums/showthread.php?t=11689
It's a bit of reading but a lot good info.
 
#13 ·
Mechanical advance changes ignition timing based on engine speed, period. The benefit from vacuum advance is that it makes adjustments to timing based on engine load. The vacuum source that changes based on engine load is manifold vacuum. There is a great article on the subject written by a GM engineer that developed these systems. It can be found in several forums on the web and I have referenced it here, when these discussions have come up in the past. Here it is from a Nova site.
http://www.stevesnovasite.com/forums/showthread.php?t=11689
It's a bit of reading but a lot good info.

I read the info once tonight and will read it again to digest more. It was informative. It was always my understanding that the ported vacuum was more for emissions than for optimum engine performance. I have read similar info on the subject and tend to be in the full vacuum school. My motor likes it and as they (who ever they are) say if it ain't broke. Real world/time results are always interesting and good to see and hear.
Ang.
 
#12 · (Edited)
Jim, thanks for the link. I can't wait to srudy it later.
So far the car seems content @ 12 static / 22 crntrifugal total of 34degrees in @ 2500. I made a beer run an hour ago and siezed the opportunity to turn up the wick. I ran a marked 1/4 on a backwoods road with no traffic. Motor sang to 6000 every shift. I might add two more degrees static to it yherr was no complaint from my mini starter. The strangest result is the really nice change to the exhaust note and rumble. I run long tube Hookers with two chamber flowmasters with 31/2" exit just behind rear wheels. I will run a mileage test soon asap.
Performance wise I don't feel any change as I shouldn't because the vac adv is moot when @ WOT. I still have no pinging at all with a costantly changing blend of 89/93 alleged octane. And my pipes are now snarling!
 
#14 ·
Awesome read, Jim. Thanks!
 
#16 ·
CONCLUSION: I have gone back to my traditional setup. After noticing loss if fuel mileage and reading this great technical article Jim posted I am convinced that manifold vacuum is what I should be using, not potted ir timed vacuum. With fuel prics so high and nothing to gain I am back to where I started. I set the static spark @10 with 22 in the centrifugal side. Runslike a top!
Thanks, Jim for the tech support.
 
#18 ·
I saw no difference in performance at all. I liked the exhaust note, but that was the only real differrnce. Highway mileage down by almost 2 mpg.
 
#20 ·
Ang, I gotta admirable the motor really seems to like the vacadv. Smoother throttle transition when shifting and it seems more eager to take input changes with the manifold vac source.
Final final conclusion: If you have vac adv...leave it!:beer:
 
#21 ·
#23 ·
Changes needed

So, I'm opposite of most of you guys and gals. I started with ported vacuum, and transferred over to full manifold vacuum. Exhaust note definitely quieter and smoother, etc. Since I switched this yesterday, I've driven about 150 miles. The car on manifold vacuum doesn't really want to idle, although it idled fine on ported vacuum. When I say it won't idle, I mean that what I think is a reasonable 750rpm that I ran on ported vacuum isn't working now. Intelligence would tell me that I need to back the timing off a couple or few degrees on manifold vacuum because the distributor is now getting a vacuum reading at idle. However, that REALLY didn't make the engine want to run. I have it set at 14 degrees base timing with 36 total. Does my switch to manifold vacuum require changes to the idle mixture screws? Increased timing? I am using Proform distributor guts (same adjustable-type vac canister as others). Other possibly helpful info: .460 lift cam, 220 duration at .050, 185 intake 64cc chamber Edelbrock heads, 600 cfm Holley/Summit carb. Thanks for your help, the possible increase in fuel mileage is definitely worth losing the "bad to the bone" exhaust tone.
 
#24 ·
You will need to reset your idle soeed and mixture. Most likely your idle speed is too high after switching to manifold vac.
 
#27 ·
You should have no problem using the same idle speed. The base timing is set at idle speed with vacuum disconnected at the distributor. At idle, using a ported vacuum source there will be no vacuum to the distributor so there will be no change to timing or idle speed when you connect the line. With a manifold source, when you connect the line, the timing will advance which will increase the idle speed. You then back off on the idle screw to get the idle speed back down. Once all that is done, you can adjust mixture. If your mixture adjustment changes idle speed, readjust the idle screw.
 
#28 ·
Blame the carb

I just wish it was all that easy! I understand what should happen but I can't understand why it's not happening. I think I can rule out the timing and ignition system and blame the carburetor. Even at 750-800 rpm the engine surges and attempts to shut off. Once I'm off idle and above 1000 rpm the car behaves normally. :dontknow: I'll take it to the GM engine guy tomorrow to have him tune on the carburetor. Might need to increase my idle feed restrictions or base nozzles.
 
#29 ·
I just wish it was all that easy! I understand what should happen but I can't understand why it's not happening.
I've heard you saying this repeatedly. I can't help but think that if your engine isn't playing "by the book" than there is something else going on. My guess would be that you should look for a vacuum leak, as that can really screw up idle and economy.

Just a random thought. I hope you find the reason. When you do, please post it for all.
 
#30 ·
Sorry for repeating myself. I'm the type that goes nuts if they can't understand a problem and solve it. My carb is fine, my vacuum lines are fine. Early in my engine's life (2-3 months ago), when first getting it going, I had some issues with my carb flooding. So, it's possible the integrity of the intake gaskets was compromised when the fuel laid between it and the head?? I also found a couple intake bolts last month that couldn't have been torqued to spec by the engine builder, and although I have tightened them properly, I'm not sure it didn't cause an issue. I'm anxious to get the manifold vacuum hooked up. Sorry again for whining and thank you for the advice! :smileyb:
 
#31 ·
Sorry for repeating myself. I'm the type that goes nuts if they can't understand a problem and solve it.
No apologies necessary. In fact it is refreshing to talk to someone that really wants to understand. I, too, want to understand more than I do, which is why this thread is of interest to me. I am by no means and expert. I learn by trying and talking to others more knowledgable and by reading.

If I were you, I would first do some diagnosing. Have you hooked up a vacuum gauge to your idling engine? I can tell you a lot. Read these:

gregsengine.com/using-a-vacuum-gauge

And From an ASE Study Guide

There was one really amazing page that I'm still looking for...

A simple vacuum test could reveal a wealth of info that you had no idea about. I'm assuming of course that you have a vacuum gauge. You may be able to rent or borrow one from a big box auto parts store for little to no $$$. Worth a try and it could definitively reveal issues for you.

After this initial test to see where you're at, the next thing I would do is to reset everything on your engine to factory specs (or if modified, recommended manufacturers' settings.) That means timing set back to factory, then idle, then idle mixture, then idle again. Recheck timing and test drive. If all feels good, then switch to your new manifold vac setup and do it all over again. At the very least, you're learning to live with/work on your engine. My Chevelle and El Camino are basically stock, so a baseline reset is easy for me to reproduce. I'm not sure what happens with "other" carbs/cams/distributors are introduced to the mix.

I hope this helps you at least get started. I'm sorry I'm not a more authoritative knowledge base for you. I would love to know how you do.
 
#32 ·
I do have a vacuum gauge and I have hooked it up a few times. My larger-than-stock cam only creates about 12" of vacuum at idle, so the Greg's article was a little hard to correlate with my engine. My vacuum readings are relatively steady--it will fluctuate only 1" at idle. The gauge returns to 12" after a quick snap of the throttle. The 12" reading is appropriate for what aftermarket manufacturers claim their cams with this amount of lift and duration will make.

Answer me this question if you can: does the vacuum advance rod that you adjust with the 3/32" hex wrench change the MAXIMUM vacuum advance or does it change HOW SOON the vacuum advance begins? I fully understand the centrifugal advance weights/springs are a separate system.

Thanks for your help!
 
#33 ·
Answer me this question if you can: does the vacuum advance rod that you adjust with the 3/32" hex wrench change the MAXIMUM vacuum advance or does it change HOW SOON the vacuum advance begins? I fully understand the centrifugal advance weights/springs are a separate system.
As I'm coming to understand, THAT is the manufacturer's difference. I purchased a Moroso unit that the "adjustment' was entirely "how much" related. Everything always happened at the same vac level. If other units change the "when" vacuum... I don't know.
 
#34 ·
As far as I know, adjustable vac advance cans can only vary the limit of advance, not the rate. We're going back a long ways so memory is foggy, but GM at one time offered different part numbers with different degrees of advance. Hi-performance cars, such as Z28s or Corvettes with higher initial advance settings (10-12 degrees vs the normal 4-6) got smaller vacuum advance ranges. Before adjustable cans, we'd fabricate stops in the slot to limit the advance at high vacuum conditions.

Bill
 
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