Gain extra benefits by becoming a Supporting Member Click here find out how!
Classic Performance Products Classic Parts
American Auto Wire Classic Industries
Chevs of the 40sDanchuk Catalog
Hellwig Products IncPerformance Rod & CustomEcklers AutoMotive
Nu-Relics Power WindowsRain Gear Wiper Systems
Impala Bob's Bob's Chevy Trucks Bob's Chevelle Parts Bob's Classic Chevy

 Page 2 of 2 <12
Username Post: Vacuum Advance- how is it supposed to work?        (Topic#239321)
Posts: 730

Loc: Central New York
Reg: 07-15-09
04-07-10 01:33 PM - Post#1894487    
    In response to NYH1

IgnitionMan, e-mail sent.

'78 Camaro, mild Vortec head 385 stroker, 9.1 comp. Lunati Voodoo 262/268 hyd. ft. cam, RPM Intake, 650 AVS carb, TH350 Coan 11" 2600 stall, 8.5" 10 bolt. 3:42 posi, 1 5/8" Full length Headers, 2 1/2" Flowmaster Exhaust System.


Senior Member
Posts: 250
Loc: La Quinta Calif
Reg: 11-06-01
04-07-10 02:39 PM - Post#1894533    
    In response to IgnitionMan

Thank you Dave, Yes I still read the site and pickup tidbits of information...
I made a test long ago on the EGR valve right before my 87 motor home (454) was sold so here is what was discovered.....
On long pulls my vacuum was 6 inches on the westbound climb out of the Coachella valley (Palm Springs).
With the vacuum hose disconected & plugged at the EGR the next pull was 8 inches of vacuum so I left it disconected from that time on.....
Better vacumm on the flat lands was also noted cruising down the super slab from (12-14 inches to 14 to 17 inches) which of course means mo'power along with better fuel mileage.
Of course your mechanical distributor settings (375 & 41 weights) were in play prior which are perfect....
When I sold the 87 RV I think the EGR vacuum was never reinstalled


Posts: 291

Reg: 11-28-09
04-07-10 07:53 PM - Post#1894730    
    In response to NYH1

Missing: an engine with low vacuum, high overlap and high compression using full vacuum will have a tendency to hunt and stagger at certain combinations of throttle position and engine speed right above idle, making setting idle speed for auto trans very irritating.

Set idle speed to 700 with vacuum off.
Connect vacuum, extra advance raises speed to 1,000.
Set idle mix.
Scenario #1: if you touch the gas, the idle vacuum jumps, which spikes the vacuum canister and the RPM goes up way more than you intended.
You back off, but the vacuum goes higher momentarily, and the engine doesn't slow down. After 2-3 seconds, it does - and may stall.
Scenario #2: more throttle. Vacuum drops.
Can retards spark timing, which reduces vacuum, which retards spark timing, blah.
Result: soggy response in a narrow range of throttle motion above idle until RPM picks up.

Ported prevents the engine idle from being too "nervous" as to changes in idle vacuum. Yes, it generally requires more initial advance.

Forum Newbie
Posts: 17
Reg: 04-07-10
04-08-10 08:53 AM - Post#1894942    
    In response to kitabel

Wow, some interesting stuff in here. Here's my thoughts... One comment was made about basically trying to get max advance at idle to try and fire the mix. How much advance do you want at idle? Also a comment was made on exhaust gas dilution. Now if that's an issue, you have so much overlap in the camshaft, that vacuum isn't really much of a worry for what you're doing. The whole idea of the vacuum advance was for mileage and will make the engine run cooler, am I not right? Anytime I set a motor up with whatever distributor I used (HEI, MSD, Stinger, etc.) I dropped it in, unplugged the vacuum and brought the motor up to around 3000 rpm. I use a dial-back timing light so I can set the timing at what I want. I want total mech. timing at 36 degrees to start, and tighten the distributor down (put in lighter springs if it's not in by then). Check the timing at idle and on HEI's it's usually around 18-20 degrees. If you're running a Holley carb, they are pretty much known to run really rich at idle unless you live below sea level! Unless you buy an aftermarket HP or someones other carb body with adjustable air bleeds (need to put smaller ones in, to to make it crisper at idle) Now maybe some people want full advance at idle, but with a small load it could cause detonation (looking at the plugs 'cuz you can't always hear it) but 18-20 initial is more than fine with me. I prefer ported vacuum because it's throttle controlled and I don't want the extra 12 or so degrees advance unless the motor is under NO load and cruising down the freeway. The next thing to do and really make it right is to phase in the distributor so it's not trying to fire the next cylinder. I'll explain how to if anybody wants to know.

"It's all about stance!"

Valued Contributor
Posts: 3507

Reg: 04-15-05
04-08-10 07:52 PM - Post#1895281    
    In response to wayloud72

Ported vacuum sourcing does not work in early engines, only those with functional EGR systems, and that is that, period, proven time and time again, by not only me, but many others as well.

Dial back timing lights on any ignition system BUT digital distributor-less systems, do not accurately tell the timing.

LOTS of holes in the above post as far as correctly doing ignition curve setup. Sorry, that is just the FACTS of it.

C10 Sleeper 
Valued Contributor
Posts: 3426
C10 Sleeper
Loc: Redding Ca
Reg: 03-17-09
04-09-10 08:12 AM - Post#1895499    
    In response to IgnitionMan

I had a dial back timing light a few years ago I could never get it to work. I thought it would be a good short cut from using a timing tape. That thing is probably buried deep in the land fill where it belongs. I agree with ignitionman he knows his stuff and against my backwards way of thinking I tried setting up my ignition system his way and haven't had a problem more like big improvement. If I wouldn't have followed his way I would probably still be guessing at how to make my truck run better.

Valued Contributor
Posts: 3507

Reg: 04-15-05
04-09-10 05:50 PM - Post#1895832    
    In response to C10 Sleeper

I keep everything above board, and public, and I don't really do much with stuff sent to me in private messages, like below.

"Wow! I got shot down! lol Hey, what I posted on there has always worked for me and my stuff has always run pretty good. I'm never closed off for great ideas or actually any ideas, I just never understood why you'd want full vacuum advance at idle. Maybe I'm missing something. Would be nice to sit down with coffee or a beer and toss some stuff around, 'cuz I do listen! What don't you like about those timing lights? If you have your front tab and damper set up at exact TDC, then where would it not work? If I'm missing something and can eek an extra 5hp out of my stuff, tell me! You ever do much with distributor phasing? The old fashioned way of drilling a hole in the cap and watching the rotor swing and making it even from side to side by adding or removing from the vacuum rod? Hope you have a great weekend and hope to talk to you soon. Brad p.s. You don't live in So Cal do you?"

No, I no longer live in So. Cal., I live in No. Nevada. I don't drink beer or coffee. And, YES, I do, do quite a lot of "phasing", as I am a person that does small body HEI conversions to points distributors (I perfected them), for a living. I also worked for Duntov at GM (engine development, includes ignition systems and curves), and, at Holley, and have been a professional mechanic for just at 38 years now, and inventor, etc.

Being a person that does things the right way, I don't have much patience for "runs pretty good". I do more fix it work on "runs pretty good" stuff, and it is getting very, very old.

Posts: 142
Loc: Folsom Ca
Reg: 02-16-07
04-13-10 03:37 PM - Post#1897970    
    In response to NYH1

I don't pretend to be an expert on this at all. but here are a few thoughts

1. you want vacuum advance at idle to keep the engine temps low. Pulling in advance does this unless you want to set you static advance high and ignore Vacuum advance.

2. i have found manifold vacuum works best, even on my smog engines. after switching to manifold i have been able to set my static timeing a little higher.
The end result was my car idled better ran cooler and also got a little better gas mileage. If i were do due things 100% right I would also reset the distributor total advance lower by the amount I set the static higher. But i haven't pushed the static too high, on my 72 buick i went from 4 to 8 TDC

3. to figure out what can you would want to use the first thing you should do is figure out what amount of vacuum you pull at idle when motor is hot. on my buddies 396 we found we needed a can that pulled the vacumm in a about 8 Hg. additionally since we set intial Static at 10 and measure 35 full in, we picked a can that only added 8 more degrees which based on notes above would give and end result of 16 for a total of 51.
Once we did this his car was much more streetable.

4. you need to figure out what the total advance your distributor has set and determine if you need to change this based in the can selciton. For example if have intial Static timing at 10 with the distributor total advance at 38, you would not want a can with a rating of 12 degress of advance. Remember the can rating will all 2x the vacuum. this would give you about 38+24 for total of 62 degrees when both Vacuum and Mechanical advance are present. either buy a can that only pulls in about 10 degrees or limit your total static from distrbutor to about 33.

5. one other decision you need to consider is how fast the centrifugal advance is pulled in. Example if static is 10,final is 35. do u want the 35 pulled in at 3000 RPM or 4000 RPM. On a car used for peformance runs you may want the advance to pull in early, in this case since you will on the throttle the hole time. the vaccum advance at the top end will not really kick in since you won't have vacuum from the manifold.

so many options.

I would love some day to take my distributor to a and old guy with a curve machine and dial in both my impala and Buick. but for now neither car has a ping a high load, and both run ok. But i know i still don't the optimum setups.

I hope this makes sense. again i don't pretend to be an expert, but i have tried to spend a lot of time reading this material to understand it in a simple way that my brain could digest


Posts: 557

Reg: 07-06-05
05-04-10 10:10 AM - Post#1909773    
    In response to steeler_fan

Hi all

I have a 58 Impala with a 348 stock motor and stock distributor. When setting up the initial location of the vacuum advance, does it matter which position it is in?

The book I am using says to start with the can at approximately 45 degrees to the engine, go back about 1/2 inch and move the can forward until the contact is broken (?) (I'm using a light bulb connected to the coil etc.When the light goes out, I'm in the right spot)

Anyhow, if I follow their advice, the can winds up directly over the distributor hold down bolt, making it difficult to tighten down without moving the distributor out of alignment.

Can I move the can closer to the fire wall such that the it is approximately 90 degrees to the engine, again adjusting it so the contact is broken? This leaves room to tighten the bolt.

What I think this also does is move the #1 spark plug wire spot on the distributor from the first spot left of the little door on the cap to the second. The rotor remains roughly at 6:30 on the cap.

Does this make sense?

Thanks for your advise

Forum Newbie
Posts: 24

Loc: Bear Creek NC 27207
Reg: 11-26-10
01-11-15 06:28 PM - Post#2513947    
    In response to Mr. Sinister

I want to tank you for your response to the Vacuum advance . I made a post here about my 427 heating at idle and it never ran like I felt a 427 should . Even though its in a full sized 69 CST long bed at 4500 lbs it should get out of its own way . I was a GM mechanic for many years and I knew the vacuum advance was critical to gas mileage and idle quality . I made a mistake by hooking it up to ported vacuum ! I hooked it up to manifold vacuum and my idle quality returned even with a mild cam . My heating problem vanished and my horsepower on take off went through the roof . That old truck will lay down 300 feet of posi marks if you can hold it straight !! Im a very happy camper now :0) thank you so much !

Posts: 595
Loc: Los Angeles area
Reg: 11-12-09
01-11-15 07:05 PM - Post#2513963    
    In response to markamatic

There's a tremendous wealth of knowledge here, especially in regards to timing and ignition. I learned everything I know about it on this here forum from guys like IgnitionMan; almost exclusively from him actually.

Now that I have a computer controlled distributor, and EFI, I can better appreciate seeing results instantly with the tap of a key rather than messing with weights and plates and springs and needles and such. Anyone wanna buy a well-tuned HEI distributor? He he he.

FWIW, I run 25° timing at idle and I've never seen more vacuum in park with as steady a needle, plus my truck launches and runs like a scalded mule. I still drive with a vacuum gauge and find it the most honest and reliable, realtime feedback I can get from my engine. Gotta love technology, old and new 😊

'59 Chevy 3100 Stepside
0.030-over 350, mild (214/224 @ .050) 112 LC cam,
vortec heads, air-gap, 600cfm carter, Holley Avenger EFI, 2-1/2" rams horns,
Bowtie Overdrives 700R4 w 2200rpm stall, 3.42 posi rear on top of 31.5" BFGs

Very Senior Member
Posts: 4139

Reg: 12-29-02
01-11-15 07:42 PM - Post#2513974    
    In response to octanejunkie

  • octanejunkie Said:
FWIW, I run 25° timing at idle

I've got 28* at idle programmed into my LT1. That timing gives about the lowest injector pulse width. It could idle forever on the hottest day without overheating only using a 16" electric fan strapped to the radiator. And that fan doesn't even have to cycle on that much.

Posts: 595
Loc: Los Angeles area
Reg: 11-12-09
01-11-15 08:20 PM - Post#2513985    
    In response to 65_Impala

  • 65_Impala Said:
  • octanejunkie Said:
FWIW, I run 25° timing at idle

I've got 28* at idle programmed into my LT1. That timing gives about the lowest injector pulse width. It could idle forever on the hottest day without overheating only using a 16" electric fan strapped to the radiator. And that fan doesn't even have to cycle on that much.

Nice. I know I will have to re-tune it when the warmer weather comes to So Cal. 100°+ ambient temps make hot rods so much fun to manage.

'59 Chevy 3100 Stepside
0.030-over 350, mild (214/224 @ .050) 112 LC cam,
vortec heads, air-gap, 600cfm carter, Holley Avenger EFI, 2-1/2" rams horns,
Bowtie Overdrives 700R4 w 2200rpm stall, 3.42 posi rear on top of 31.5" BFGs


 Page 2 of 2 <12
Icon Legend Permissions Topic Options
Report Post

Quote Post

Quick Reply

Print Topic

Email Topic

FusionBB™ Version 2.1
©2003-2006 InteractivePHP, Inc.
Execution time: 0.257 seconds.   Total Queries: 16   Zlib Compression is on.
All times are (GMT -0800) Pacific. Current time is 02:39 PM