THis contradicts a lot that I read on Moparts, but I think a lot of Moparts guys are racers and that is why they dump the vacuum advance...


TIMING AND VACUUM ADVANCE 101

John Hinckley


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.

Good article, thanks for posting.

I don't think the ported vacuum vs. manifold vacuum argument will ever be settled. I've read articles similar to this as well as articles that argue the exact opposite - all written by people who are smarter than me, all making good arguments for "their side".

Nevertheless, it's still quite interesting reading.

Who is John Hinkley?

yep....I'll take ported vacuum thank you.

This was from the Corvette Forum. I don't know if GM motors respond differently than Mopars.

My Edelbrock carb manual states to use ported on stock motors. With bigger cams it states to use manifold vacuum...

This would be a good one for Jeffrey Diamond aka Panic.

I have to admit, I'm interested enough to try some it on my own stuff.

overall, this article basically says everything that people on moparts (including me) have said on the subject.

this paragraph:

Quote:

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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.

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this is precisely why I recommend ported vacuum, as with even the mopar adjustable vac advance, it's difficult, if not impossible, to adjust it down so the vac. advance is all in by 10-12" of vaccuum or so. with any sort of performance cam, you probably will end up with the erratic idle, excessive idle drop, and potential stalling in gear problems.

im at about 10" at idle (jumping around at that).

I have mine unplugged. But I have the crane vacuum pod on there so maybe I can adjust that one down. I will try it but I doubt I'll like it

Quote:

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Who is John Hinkley?

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The guy who shot Reagan?

If your vacuum advance is affecting your idle, it's set up wrong.

There should be no vac advance at idle. Even on a rock stock motor, there is no vacuum on the ported line at idle.
That's one of the reasons why they aren't on manifold from the factory too(so it isn't on at idle), and why articles like the one posted, aren't as accurate as thier writers think they are.

And yes, I also thought John Hinckly was the guy who shot Reagan. Is he even out of prison yet?

JUST LIKE CUCU I HAVE 10 LBS VAC AND MY SIX PACK IDLED DECENT BUT WHEN I REV IT THE THING DOESN'T ALWAYS COME BACK TO THE 900 RPM THAT IT IS SET AT AND WHEN I PULL IT IN GEAR THE RPM DROPS TO THE POINT WHERE SOMETIMES IT WANTS TO STALL WITHOUT PEDALING THE GAS. SO DOES THE THE MOPAR PERFORMANCE DIST. HAVE ANYTHING TO DO WITH IT?

THANKS MOPARTS

no, the fact you have vac. advance hooked to manifold vaccuum, and at 10" you're partially on the advance curve is why. when you're at higher RPM it has more vac. advance because it's pulling more vaccuum. but as RPM drops in the idle range, vac drops, so timing is pulled out, making RPM drop more, and it continues to spiral down until you're back (probably) to base timing. switch your vacuum advance to ported vacuum, and your unstable idle return/excessive in-gear idle drop will go away.

PATRICK

I TRIED NOT RUNNING THE VAC ADVANCE AND IT DOES THE SAME THING. I WILL CERTAINTY TRY WHAT YOU SAY SOUNDS LIKE YOU KNOW MORE THEN I DO. SO WHERE IS THE BEST PLACE TO CONNECT TO A PORTED VACUMN PLACE ON A SIX PACK?

THANKS HAROLD

I have some questions about this.
But first he needs to define rich and lean.
A lean mixture, in my book, will light off quicker because the molecules are smaller.
A rich mixture will burn longer because the small molecules will burn first then light off the bigger ones.
We can't have a lot of timing at idle because it will cause kick-back a gainst the starter.
If vacuum advance is good at idle then if you run 12 degrees mechanical and 15 vacuum, set it all at 27 without vacuum and try it.
I say the vacuum needs to be there at light loads to start the fire early and create cylinder pressures that aren't good at full load.

At WOT there is little manifold vacuum so what happens to all the advance you are getting from the vacuum advance? You could tune it for manifold vacuum but to me you should set the mechanical for 36 degrees or so by 2500 rpm and then run the vacuum to ported and fine tune from there. I use to run no vacuum advance but recently started running it and will run it from now on.

I know Mopars came stock hooked up to ported advance, but Mid-year Vettes were on manifold vacuum... I think the guy who wrote this was an early Vette engineer. I will find out more.... I am tired of trying to tune my stumble out and will probably jsut throw money at it w/ a new carb and MSD distributor....

I was going to say it looks like a chevy guy wrote that and then someone said thats where it came from. So just ignore it and stick with ported vacum because thats how things were designed and thats what works.