Vacuume Advance Question

[sgaylord]

I have my vac advance can plugged into manifold vacuum. When I put a timing light on it there is no difference in initial timing with the vac can attached or disconnected. My understanding is with the can attached to manifold vacuum at idle I should see an advance in timing. Is this correct?

Bone stock 69 396, no headers, Q-Jet, Accel HEI dizzy. Puling 16Hg vac.

Thanks,
Steve

[Leviticus]

Yes, you should see an advance when the vac is hooked back up IF you are hooked to Manifold vacuum. If you are hooked to ported vac then you won't see any advance, or very little.

May be time for a new Vac advance.

[sgaylord]

Leviticus,
Yep, that's what i'm thinking. I know when I attach a vac gauge to the same port the vac can was attached I get 16Hg vacuum at idle as well as I feel the suction when I put my finger up to it. I know there's vacuum there so my guess it's the vac can.

Thanks,
Steve

[68gmsee]

Take the distributor cap off and suck on the hose with your mouth.. Sounds gross but it doesn't take much vacuum to move the vac advance and that way you can see if it is leaking or stuck and not moving.

[sgaylord]

Broken vacuum advance can. My guess is the diaphragm was toast. I followed the advice and placed suction on the hose without the dizzy cap on and nada no movement. I had another non-adjustable vacuum advance on an old dizzy and it tested fine. Installed it and now I have 32 deg initial advance at idle with vac advance hooked up to manifold vacuum and 12 deg without.

Now I have another problem. With this new found advance, I have pinging under light load. I know the solution to this so I ordered an adjustable vac advance tonight. When it comes in I will tune out the ping. Thanks to all who replied for your help.

Steve

[Jwill41]

I have never understood why it would be connected to full time vacume instead of ported vacume. Doesnt this mean that it would be advanced at all times. Excuse my ignorance if I am out to lunch but this has always bothered me so I connect mine to ported vacume so it advances when I accelerate.

[Leviticus]

Originally Posted by Jwill41

I have never understood why it would be connected to full time vacume instead of ported vacume. Doesnt this mean that it would be advanced at all times. Excuse my ignorance if I am out to lunch but this has always bothered me so I connect mine to ported vacume so it advances when I accelerate.

I believe it was an emissions thing from the 70's. mine runs and idles much better IMO when hooked to manifold vac.

[sgaylord]

JWill41, my understanding is (and let me say I am not an expert by any stretch of the meaning) that full manifold vacuum will only come into play when the throttle plates are closed or mostly closed. Meaning that vacuum advance will only happen at cruise or light throttle conditions. When the throttle plates are open manifold vacuum drops and eliminates vac advance.

By using full manifold vacuum this allows the engine more advance which assists with fuel economy at cruise speed. The problem with ported vacuum is that it does not allow any vac advance at idle. This was part of a short lived emissions change that GM did back in the 70's that allowed the engine to run hotter at idle. In conjunction with the A.I.R. pump this would help to burn excess fuel that did not get fully consumed in the combustion chamber. The problem most people have with running ported vacuum is 1) they no longer run the A.I.R pump, and 2) running retarded timing causes the engine to run hotter at idle than they are comfortable with.

Full vacuum will not cause you any problems as long as you use an adjustable vac can so you can limit the amount of vac advance so you don't ping. Adjust down the amount of advance till the pinging goes away. This is what I need to do.

Think of it this way, when you open the throttle plates under load the vacuum drops and you go back to your initial + mechanical advance which should be around 36 deg. This is why you disconnect the vac advance can when setting timing.

There are many on this forum that know a lot more about this than I. If anyone see's any problems with what I just said, please correct me so we all will have the right information. The understanding I have came from reading the many posts on this subject here on this site. If I misunderstood anything I would really like to get that corrected.

Thanks,
Steve

[70jnktrk]

manifold vacuum is the answer....systems will operate either way though...I ran on ported for years then decided to try manifold...much better,,,read the piece below pulled from another forum and digest the contents...
TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.



__________________
John
'67 Convertible

[Jwill41]

very interesting. I just changed my opinion on vacume advance and will be trying mine connected to full manifold vacume. Thanks for the explanation. See there, you CAN teach an old dog new tricks.

[mdwilf]

Excellent! Thanks for taking the time to post that!!

[CE20934]

I was on the fence about this whole debate, so I did a bit of snooping to find the author of that article. His name is John Hinckley, and his as well as the articles from a few other folks can be found here. Click on "Timing & Vacuum Advance" for even more discussion about manifold vs. ported vacuum.

In this article, another tuner (Lars Grimsrud) says:

If, however, your engine idles best in the 12-16 degree range due to a mild cam, plug the vacuum advance control unit to a ported vacuum source to eliminate the vacuum signal at idle. You will still obtain the 36-degree WOT total, and you'’ll still have 52 at cruise. Also, if you need to pass an emissions test, use the ported source to reduce your hydrocarbons.