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  #16 (permalink)  
Old 01-29-2004, 09:17 PM
malducci's Avatar
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Actually, I set my timing to 10* without the vacuum advance. With the vacuum advance hooked up its around 18-20* @ idle and cruise RPMs. Also, you have to have the correct combo of springs, weights, and cuts in the weights for your curve to work with your vacuum manifold setup. My gas mileage and power response from cruise have both increased for my stock 283.

Check out this clipping:

<<It was written by a GM engineer.

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.

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.

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  #17 (permalink)  
Old 01-29-2004, 09:59 PM
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Thanx for the article malducci, that makes total sense. In my experience, manifold vacuum on a properly set up car gives absolutely the best performance- reduces knock, improves cooling, improves acceleration, etc.
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Old 01-29-2004, 10:02 PM
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I don't know who this guy is,and because hes a gm engineer don't mean squat to me,I know engineers that I wouldnt let chang my oil. In my opinion he is confused,but thats just my opinion. I agree with a lot of his statements because they are fact but some are off, the math don't work out on my calculator. I realize you can tweak your distributer to make it work properly,Ive done that to start very high compression motors,but Im speaking of stock mainstream gm distributors in relatively stock compression motors using pump gasoline.
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Old 01-29-2004, 11:41 PM
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What's makes sense to me is that in idle and cruise - the carb is still using its idle circuit which almost always lean. If you step on the gas, the accel pump squarts gas into the air/fuel mixture - richening it out. Also as the throttle blades open, the carb switches out of idle circuit mode - richening the mixture further. And to top it off if you have a holly carb, the power valve opens up as the thottle blades open up - richening the mixture as well. Richer air/fuel mixture requires less advance spark, so it makes sense that the timing back off as the fuel mixture richens. Higher RPMs need more advance spark and that is were the mechnical advance mechanizm comes in (weights and springs).
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  #20 (permalink)  
Old 01-30-2004, 12:50 AM
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Excellent article. Most important thing to remember is that it's the mechanical advance that is going to will races, and the addition of vacuum advance to increase fuel economy. Both should be set to a point where any additional will cause a loss in performance. Every engine will be different, and there is no formula for magically setting it. Setting up the distributor on a distributor machine will get you in the ball park, but the final tuning will have to be on the road. Leave the vacuum off while figuring out how much mechanical advance the engine will handle, and then after you get this set, then dial in the vacuum advance to improve your fuel economy. I also cannot see why anything other than a manifold vacuum source would be used unless you are subject to smog inspections. Manifold vacuum does the best job in filling in the additional advance during idle and light throttle conditions.
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  #21 (permalink)  
Old 01-30-2004, 07:54 AM
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As I recall, Doc Jacobs makes the same points as malducci's article does about the benefits of manifold vacuum in his very informative and in-depth book on ignition systems. Port vacuum is for smog motors, manifold vacuum is for performance and economy.
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  #22 (permalink)  
Old 01-30-2004, 10:37 AM
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Vacuum advance was created to aid in the low end acceloration,those whiz bang mechanical distributors work awsome on marine aplications and cars with bigger stall converters where the load is not applied until the rpms have increased. If you choose to go mechanical,curve it to match,but if you choose a vacuum distributer,your timing being lessened under acceleration is not a good choice,never,if your optimal advance is 34,and you tromp the throttle and get 24,your killing your performance. Efficiency is power,if your power valve needs to pick up the slack,all your doing is making is a big soot pump. Your mechanical advance weights work just like a mechanical only distributor,your vacume only helps get you going,because of the speed and nature of low rpm burning.

A lot of this has to do with the port velocity also.

If Im wrong,ask yourself,why would the major performance carburetor manufacturers provide such a ridiculous option,surly not to pass a smog test.

Last edited by rifraf; 01-30-2004 at 10:37 AM.
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Old 01-30-2004, 01:40 PM
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Asked myself, and the answer back was Yep, only because of smog and the desire to subdue emissions somewhat at idle. Another fallacy is that you would have vacuum anywhere in the carburetor with the venturis slammed full open. Hook a vacuum gauge up and check out ported vacuum. No vacuum at closed throttle, good vacuum at partial throttle, and no more than manifold vacuum with everything dumped fully open - and this won't be much.

I set my distributors up on the distributor machine, and I set them up for maximum usable advance curve regardless of whether they are straight mechanical, or will be used for the street with a vacuum advance. After the mechanical curve is set for optimum performance, I then adjust the vacuum curve to match the individual engine I'm working on. This might mean either limiting the total advance the vacuum motor can input, or even altering how much vacuum it takes to move the plate. My engines will produce maximum power with a correctly set up distributor regardless of whether there is a vacuum module, the only difference is about 2 MPG on a trip with the vacuum unit functioning correctly.
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Old 01-30-2004, 01:52 PM
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"those whiz bang mechanical distributors work awsome on marine aplications and cars with bigger stall converters where the load is not applied until the rpms have increased."

True. Mechnical advance "only" distributor is meant for race engines that live 3500-8500. You can run mechnical advance "only" distributor on the street, but your not benefiting from anything and actually gas mileage/cruise to WOT throttle response.

"but if you choose a vacuum distributer,your timing being lessened under acceleration is not a good choice,never,if your optimal advance is 34,and you tromp the throttle and get 24,your killing your performance"

I think your forgeting that a vacuum advance distributor has mechnical weighted advance mechanism that is RPM dependent. In other words, if your total timing is 34* @ 2500rpm (without manifold vacuum advance) , then that is the total timing - not 24*. If you were cruising at 2500rpms and had +6* vacuum advance, then your timing is 40* - when you punch the throttle to WOT, you loose all vacuum advance and only have mechnical advance which is 34*.



Actually, were are talking about stock to mild setups, not most strip or race engines.

Ported vacuum has its place, but for a different kind of timing curve - depending on the engine setup and desired RPM use, its mostly for more hi-performance or extreme engine setups.

Ported vacuum was used on older setups too ( pre 1971 I believe ), were you had higher compression and too much advance cruise rpms resulted in engine nock - in that setup mechnical advance + ported vacuum = total timing. Also if the base timing is 4* and the engine had higher compression, it would be easier to start at a lower timing, then when you had acceleration the ported vacuum would put the timing were it needed to be for performance (maybe 10* more than cruise/idle) and as RPM increased the mechnical weights would work out the rest of the timing curve.

You also have to remember that engine setups ran richer back then and emission weren't as strict if at all in effect. Newer engine setups with lower compression and leaner idle/cruise carbs circuits switched over to manifold vacuum. Newer computer controlled timing is even more efficient than manifold vacuum setup.

Like all good engine setups, even manifold vacuum setup needs to be tuned to the specific engines needs.
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Old 01-30-2004, 05:51 PM
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I give up,I haven't forgot,I do understand,and Ive spent more time on distributer machines and wheel dynos than I care to remember,you do it your way and Ill do it mine. Lust4speed,try your vacuum theory under load,see what you get between the two,I think you may find it eye opening.
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Old 01-30-2004, 07:36 PM
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rifraf, didn't mean to affend you. Just trying to explain that manifold vacuum port is there for a reason and most daily drivers would be better off using that kind of setup. Most people think manifold is utterly useless and its not. And usually those people that have done some mild upgrades to there engine have been told to run ported vaccum since it is a performance timing setup. Its like those guys that run over cam'd engines or single plane hirise on a engine whose valve train is limited to 5200rpm.

It's the fact that you said, when you see a car that has the distributor hooked up to manifold vacuum port - you change it to ported vacuum. To me that is crazy unless you know the engine setup from the get go( cam, carb, power band, car usage, etc) and ported would be more/better suited for it.

I'm not saying every engine setup should use manifold vacuum port. I'm in the process of building a strip/barely streetable 327 engine. 300* solid roller cam with .672 lift - victor jr. heads - tunnel ram - 11.5 static CR. Guess what, I'm sure as hell not going to use manifold vacuum port on it. Matter of fact I'm probably going to take out the vaccum advance and put in a mechnical pull switch so I can drive around town on pump gas to avoid engine knock - retarding the timing by 10-12*. And when I get to the track and fill up on some race gas, I can manually activate the pull switch setting the distributor up by 10-12* were is would be optimal for that engine + race gas.
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Old 01-30-2004, 08:45 PM
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I have a 63 Nova with small block that is street driven whenever possible. I have HEI distributer with Crane adjustable vacuum advance and vacuum advance limiter plate. I set my initial timing at 12 degrees and adjusted the limiter to allow 8 degrees of vacuum advance. When idleing it has 20 degrees advance. It is hooked up to full manifold vacuum now after trying many combinations of ported. I have also tried different spring and weight combos and have the total at 34degrees and it runs better now than it ever has! Better mileage, runs cooler, better low end response. I know I have almost no vacuum at full throttle and only mechanical advance is working then with 34 degrees total. I understand this may not work for everyone but for me it has worked great. Just my opinion. ccnova
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Old 01-31-2004, 04:51 AM
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so then, the reason i cant get my vacume advance distributer to work with with manifold vacume is because it was made to be run on the ported vacume connection and thus its not delivering a high enough mechanical advance. if this is the case how would i fix that? i would really like to have the added benefit of running on the manifold vacume.
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Old 01-31-2004, 10:12 AM
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Its been my experience that over advanced motors run hot,its also been my experience that rapid fluctuations in timing affect the performance,as it would hooked under the venturi causing bucking,hesitation,and an inability to dial in a carburetor,but I guess if it works for ya,knock yourself out,the next time I look at a 850 double pumper Ill keep in mind its smog ready.
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Old 01-31-2004, 11:12 AM
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Arrow

I think you have some of your facts confused, as well as your generalizing about vacuum advance. I always run manifold vacuum on street cars and a retarded ignition makes an engine run hotter not the reverse. Advance makes combustion cooler, up to the point of over advancing which can cause pre-ignition.
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