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underdog305 01-03-2005 07:17 PM

Vacuum adv..
I'll keep this short and sweet. Vacuum advance... Does anyone have any personal preferences to a set-up? Vacuum source, point of initial travel and total vac advance. This is for a mild SBC 305, w/ cam, air gap manifold, and headers. I've read conflicting stuff so give me the straight skinny. Thanks!

1BAD80 01-03-2005 07:37 PM

Here is a site search on this Forum that has about 600 of the same questions, one should answer it the way you want plus a few good sites. There are sites/info in the Knowledge Base.

underdog305 01-03-2005 07:45 PM

I know the same ole question gets old. Thanks for the effort. :thumbup:

I know the same ole question gets old. Thanks for the effort. :thumbup:

1BAD80 01-03-2005 07:45 PM

Taken from one of the postings, everything you should know about HEI ignition's, the timing is still the same set-up on advance.
Part 1

hosep 01-03-2005 11:40 PM

manifold vac hook up, below carb

underdog305 01-03-2005 11:45 PM

Why the manifold? This will potentially introduce advance too soon, wont it? or is their a trick to keep that from happening?

Triaged 01-04-2005 12:18 AM

You want advance at idle.

This is quoted from one of the threads that comes up with a search.


This is a great article!!!!I had my vacuum advance hooked up to the ported vacuum port after reading this article I hooked it up to full manifold vacuum adjusted the idle rpm.The car has so much more power now.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.


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.

malc 01-04-2005 03:08 AM

So you disconnect your vacuum advance and set the initial timing, then with springs / weights set the total timing ?
When you reconnect the vacuum advance to a full manifold vacuum you immediately induce advance at idle, so there goes your initial advance up 10º or whatever the vacuum cannister is set at.
As you state "if your static timing is at 10º, at idle it´s actually around 25º with vacuum advance connected" as I see it with the vacuum advance connected it is not static and 25º at idle is way too much.This is why the vacuum advance is disconnected at idle to get a correct static timing reading.
I disagree with the statement of seeing 50º advance if you were going down the highway, (10º initial, 20º - 25º mechanical and 15º from the vacuum) you stated elsewhere when the mechanical advance kicks in the vacuum is not adding anything.
It is also important to get all the advance in before your cruising rpms.
Also if the total timing was set at 34º - 36º how can it get to 50º?
Remember total timing has been restricted to 34º - 36º and even more rpms won´t advance it anymore.
The vacuum advance is an aid to get from idle to mechanical advance and also operates during low engine load when the vacuum is high.
Setting vacuum advance is not measurable it has to be done on the road until pinging is eliminated.
Get yourself a book dedicated to ignitions, I´m going to throw out my computer controlled distributor and go with PORTED vacuum advance, try Ignition Systems by Todd Ryden.
BTW there are several sites in the knowledge base on this.

ccnova 01-04-2005 06:51 AM

I have a small block in a 63 Nova with aftermarket intake ,carb,cam, headers etc. I use manifold Vac. and a stop plate to limit the amount of vacuum advance to 8*. I set my initial timing with advance unhooked at 12*. With the cam we have it idles a little rough and when you add the extra 8* degrees advance at idle it smooths out and is much more drivable. I have the mechanical advance set up to work right at wide open throttle such as when at the track. You just need to make sure to use an adjustable vacuum advance and have it set to provide the advance at idle and low rpm's but also not so low that it adds at your cruise rpm. This setup seems to work great for us. ccnova

malc 01-04-2005 07:12 AM

So you are in fact running 20º initial timing.
Initial timing is set at idle, no advance, to then add vacuum advance ? seems odd.
There is no transition from no advance into the mechanical advance.
How does it react when you pull away from a standstill slowly?
How does it react when you stomp on it ?

454C10 01-04-2005 08:02 AM

GM uses ported vacuum for the advance system. So you need to plug the vacuum line from the distributor to the port above the throttle blades (no vacuum at idle). When the throttle is openned (off idle) the distributor will advance. Pretty simple.

Connecting the vacuum advance to the manifold makes no sense. If that is done then the timing will retard when the throttle is openned which is the opposite of what is needed. Plus the mechanical advance will be advancing at the same time the vacuum is retarding. What is the point of that?

Fords use the manifold pressure for their vacuum advance system. It this case, the high manifold pressure, holds the vacuum advance in retard. When the throttle blades open and manifold pressure drops then the distributor will advance. Different method but the same effect.

red65mustang 01-04-2005 08:39 AM

from my install instructions with adjustable advance
set the base timing to 8-10
as delivered mech is 24 @ 2800
as delivered vacuum is 14* @ 12 Hg (4 turns)(8 turns possible)

"If surging or pinging is noticed at CRUISE rpm's turn the vacuum adjustment conterclockwise"
If more advance is needed turn clockwise

from the chart table for 1,2,3, etc turns with manifold vacuum

1 turn=3 deg all in by 11Hg, starting at 8Hg
4* turn = 14 deg all in by 12Hg starting at 6Hg, (45deg slope)
8 turns = 28 deg all in at 17Hg but a long slope starting at 4Hg

The vacuum advance is off when you floor it, the Hg is below 6, timing advances only from the mech to the 34 total.

When you back off the gas, as the Hg builds, the vacuum advance works against the mechanical for a smooth transition back to base timing

Your balancing motor load (Hg) to mech advance at any rpms for best burn possible.

Reason I bought it (and tuned both mechanical and vacuum): I can burn 87 octane with 10deg base/36 total (in at 2400) with no ping because of the vacuum advance. It replaced a Mallory dual point mech only that wanted 93 octane at 12 and 32 total

So jealous, you Chevy guys can buy just the adjustable vacuum, I had to buy a distributor.....

underdog305 01-04-2005 09:19 AM

I'm impressed with the responses, but the question remains.... Ported or Manifold.. I'm leaning with the manifold source because it leans toward a more dynamic timing curve that does reflect load and throttle position. I'm so confused:confused:

red65mustang 01-04-2005 09:37 AM

won't hurt anything to try both,

you will see a change, ported vacuum is "0" until ? 1500 rpm+, you have no vacuum advance at idle
so when drive away normal at a green light, it takes a little more peddle or excellerator pump (richer) to compensate for no advance

Why is it there? I don't know this to be THE reason but there are "dual" vacuum advances on some Ford dist, it's a push/pull deal, never used one but makes sense.

My car sucks on ported, runs great on manifold, 90% sure yours will also.

Gona buy that Demon carb, ask them also, direct connection, vacuum advance and carb rich/lean

454C10 01-04-2005 10:05 AM

You be the judge.

Try it both ways and see which works best.

I'm betting you will find ported is much better.

I still think it is simple, when you give it the gas of idle, you need advance. And the only way to get that with a chevy is to use a ported source.

A ported source will also advance some when going down the highway because the throttle is openned. That combined with some part of the mechanical advanvce makes a nice amount of timing for good MPG.

Let us know what happens.


I guess you have a ford by your web name.
Yes Fords need to use manifold vacuum source. On a ford, the vacuum advance retards during high vacuum (idle). The diaphagm pulls on a spring and retards the timing, but once the vacuum drops, the spring in the vacuum advance system is what actually advances the timing. So when you give it throttle and the manifold vacuum drops, the timing advances.

Chevys work opposite, the spring retards and the vacuum advances. That is why a ported source is used on a chevy. But in both ford and chevy, the timing advances when the throttle is depressed.

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