Timing Mech vs Vac advance Good Read

A

Anonymous

Guest
This is a reprint from another board author unknown but wanted to share this with the group as there has been some recent discussions on this.

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.
 

Lilracr

MalibuRacing Junkie
Jul 7, 2003
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That supports my theories! :D

Good article.
 

TURNA

Moderator
Jul 6, 2003
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after reading that my eyes now look like this :shock:
I have been starring at the screen to long
 

Al

Pro Stocker
May 25, 2003
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Harker Heights, TX
Can this be made into a sticky so that it stays at the top of the posts? Helpful info for anyone who is browsing the board.
 

malibu71

Pro Stocker
May 25, 2003
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Bloomington, IN
that explains alot of things i knew that happened but didn't quite understand why or how. hmmmm, i guess this guy doesn't work for MSD? lol! now this brings up another tuning question: do you set the air/fuel mixture with the vacuum advance hooked up or not? i would guess that if you're going to run it off of manifold vacuum then you should have it hooked up when you set the mixture screws? what do you guys think?
 
A

Anonymous

Guest
Al I can do that.

BJ thought you guys might like to see this when I came across it I was suprised by a few things myself. Could not remember the people that were discussing this in the group the other day. These are not my ideas only a repost. :lol:
 

Pushrod

Frequent Racer
May 22, 2003
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Joliet IL.
malibu71 said:
do you set the air/fuel mixture with the vacuum advance hooked up or not? i would guess that if you're going to run it off of manifold vacuum then you should have it hooked up when you set the mixture screws? what do you guys think?
I think your right on the money with that guesstimation :p
 
A

Anonymous

Guest
did malibu71 put you up to this?lol

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.


In responce to this i would have to say to the one who wrote this is that an experienced mechanic would know that when as the article says when you get on the gas manifold vaccum goes away unless you hook to the ported side that is.This means on many 80 s model fords and i believe mopars also only have vaccum advance and no centrifugel weights at all so if you hooked to manifold vaccum you would be at total timing at idle and base timing any other time the gas is applied and i promise you will tell a difference as i own two of them that work this way.That is my only argument to that.But if you had an engine like malibu71,s the vacuum goes up as the rpms go up so it would not matter either way and that would be fine.
 

malibu71

Pro Stocker
May 25, 2003
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Bloomington, IN
under part throttle EVERY engine will slightly increase it's vacuum, doesn't matter which port you use. under full throttle EVERY engine will lose it's vacuum, no matter which port. everyone can check it and see for themselves and run it however they want, doesn't matter to me. and no i did not put Fred up to this. i fight my own battles. :D
 

78 Elky

Frequent Racer
May 27, 2003
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Silver Lake, WI 53170
I have a question on this. When I first put my engine together, it ran, but it ran like crap. So I took it to a guy to have it tuned up. I ran a mallory unilite with vacuum advance, a 750 speed demon carb, and a roller cam with 232/240 @.050. When I picked it up, he had taken off the advance line, and said he set the by reving the engine to 3000 or around there and set it at 34 degrees. The initial was around 18 degrees. What I know is that it ran ten fold better. Throttel response every where improved and the car no longer bogged when I stabbed the gas. Maybe vacuum advance doesn't apply here, but I know that my engine runs a lot better without it.
 

Supe

MalibuRacing Junkie
May 21, 2003
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Charlotte, NC
Sounds like your timing was way off to begin with, or your vaccum advance weights were wrong for your combination.
 
A

Anonymous

Guest
I must agree with BJ on that not enough vaccum for the dist to work on, I won't even start on that Mallory Unilite :roll:
 

78 Elky

Frequent Racer
May 27, 2003
372
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Silver Lake, WI 53170
Actually I was able to pull more vacuum than you would think. I got about 12 inches at idle. And just out of curiosity, is there a problem with unilites, or is it a preference issue?
 

Supe

MalibuRacing Junkie
May 21, 2003
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Charlotte, NC
Lots of horror stories about the pickup and the module burning out due to being very sensitive to voltage.
 
A

Anonymous

Guest
All I will say on this is go to the race track and look at the dist in all the cars and see what they are running. There is a reason why they run them and not the others. :)
 
Set your initail timing with the vacuum advance unhooked. I answer this question about timing and vacuum advance on a different topic on like page 3 or something. He just did it in more detail.

Well said
 

Got Torque

Top Fueler
Oct 29, 2003
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Stockholm, Sweden
Thank's to Fred, I now know enough to ditch my "Harry High-school" carburator setups that I used until now.

Thank's again Fred for explaining the vacuum advance bit the understandble way!

I think this knowledge will come in handy when it's time to beef up the caddy engine i hope to be the proud owner of this weekend.