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Topic Review (Newest First)
08-07-2006 08:56 PM
jimfulco Just don't tap off the PCV hose.
08-07-2006 07:29 PM
72Orange WOW, look what I started.

I think there are a few posts towards the beginning of this thread that just do not make sense. And that article by the GM tech is exactly why I am looking for manifold vacuum, and not ported. I have a vacuum gauge in the car and have double checked each vacuum port. I DO NOT have a manifold vacuum port on my carb ... which confirms what Holley said in the instructions.

I have since learned I can actually tap into one of the threaded holes on the intake and run a vacuum line off of it. I may try it that way.
08-07-2006 05:39 PM
Rick WI On almost all vehicles you will disconnect the vac line at the distributor and plug it, even my Pinto was that way.

If you were setup as true ported vacuum you'd have no vac signal to the vac advance at idle, assuming throttle blades are adjusted correctly. That ported source would not be exposed the the pressure differential.

No matter the port location, manifold or ported, once you are under wide open throttle either location will give the same signal to the vac canister. The reason, simple, they both are looking at the same thing. Absolute pressure in the intake.

If your carb is a restriction then you can have a vacuum reading at wide open throttle. It may only be 2 or 3 inches of pressure drop but may be enough to pull in some of the vac advance.

Hooking a vacuum gauge to the manifold port on the carb and taping the gauge to the windshield is a good way to figure out what is going on.

I'm not quite clear yet though on what your saying the GM Tech guy was saying versus the way you're thinking the 302 is setup.
08-07-2006 05:20 PM
cliff tate
vacume advance

on the fords i amworking on you set timing with vacume line off, inst line and you get some advance even with throttle plates closed as much as poss [ using holley 650]their is some signal,itis my understanding 1 gets advance via the vacuum as well as centrifical until near full throttle. the gm teck is saying toatle advance is not effected under full throtle on manifold vacume. on the ford ported signal it seems that it combines to give toatle advance as the vacume only drops off under heavy load as engine lugs. my question is am i missing something. iam useing a older vacume unit on my 302 with shims to delay vac advance as with latter sealed unit had to much advance part throttle. engine is in a 32ford infront of a c6. again the gm guy explained ther manifold system very well.any ways this is a good discusion as always i learn somethig cliff
08-07-2006 01:52 PM
Rick WI Cliff, I bet your wondering where you get the parts to "correct" the advance curves on a stock Ford distributor? That, I don't know. Almost every Ford engine we have done in the past 2 or 3 years has had an aftermarket unit on it.
08-07-2006 01:29 PM
cool rockin daddy Huh? What I think Rick is trying to say is that if you have vacuum advance on your Ford (and most Ford's do), the same principals would apply as in the chevy article. I don't know squat about Fords and maybe the mechanical advance comes in differently or is a lot less or more than a typical chevy distributor but vacuum advance is vacuum advance!
08-07-2006 01:20 PM
Rick WI What I am saying is the fundamentals are the same. Chevy used ported vacuum as well on a variety of applications.
08-07-2006 12:58 PM
cliff tate
Originally Posted by Rick WI
What is different between a Ford 4 stroke cycle and a Chevy? The fundamentals are the same.
maybe you should read the post again, ford uses a some what different advance system than gm. ford has a centrifical advance like gm but they use ported vacume rather than manifold vacum such as some gms,so your rply does not adress the question, i am sure you are not tying to be*****
08-07-2006 12:42 PM
Rick WI What is different between a Ford 4 stroke cycle and a Chevy? The fundamentals are the same.
08-07-2006 12:24 PM
cliff tate
vacume advance

a very good artical by the x gm guy. but what about us ford guys wher they have ported advance, any one out ther to go over that system to enlighten the blue oval crowd. cliff
08-07-2006 12:21 PM
Rick WI The all time (yes that's a pun) classic: Advance it till it pings and turn it back a tad.
08-07-2006 12:01 PM
cool rockin daddy yes, all the exciting anecdotes and yarns on what 'ol granpappy did to the Rambler to get it timed or that Fonzi down the street went from full manifold vacuum to ported vacuum and picked up a tenth in the quarter and could run it on last nights beer! LOL
08-07-2006 11:32 AM
Rick WI There will be lots of folks that will post it's BS, and that's too bad. For whatever reason folks have a very hard time wrapping their heads around how timing works and the purpose of it. You ALWAYS want to run vacuum advance on the street. Properly setup up base, mechanical and vac advance will always make a car run better and get better mileage.
08-07-2006 09:49 AM
cool rockin daddy OUTSTANDING!!!!!

This article clears up so many questions on this subject in a easy to understand way. The way this gentleman described it, I could actually picture the air/fuel mixture and the spark plug igniting it in my mind! Thank you for sharing this article. Full manifold vacuam for me from now on!

As an aside, I will be very curious as to how many people on the board are going to post views that refute what this ex-GM engineer has to say!
08-07-2006 08:40 AM
Originally Posted by badhabit
.DO NOT HOOK YOUR VAC ADVANCE UP TO NORMAL VACUUM--- this will cause the idle to race and really run flat, because your advance is all in at idle-- not as you accelarate.
This will only happen if you select the wrong VA can. Having full vacuum advance at idle will increase the VE, reduce the EGT's and make the idle smoother. A former GM engineer wrote a nice piece on vacuum advance:

Quotation thanks to JohnZ

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.
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