FINALLY - Vacuum Advance Dilemma Solved

[SuperHart]

After reading many articles on the subject of distributor advance, ported or manifold, and coming away still confused I came across this article on the National T Bucket Web Site and just HAD to share it with you all. In my opinion, this article spells the entire issue out in plain English and makes perfect sense. I hope anyone who wonders about where to plug in their advance will read this article. I just wanted it to become part of our information database. I have no idea who actually wrote this article but, in my opinion, they did a superb job and deserve our thanks. Enjoy!

--------------------------------------------------------------------------

"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."
__________________

[DAVID GAGNARD]

Thanks for the post,very informative in a way an idiot like me can understand.....

David

[DanEC]

That looks right on. Do you know who wrote it? It looks like information I've seen on the Corvette Forum.

Having said that and as a true believer in a vacuum advance system on street cars - I'm getting ready to try an original 427 dual point distributor w/o vacuum advance on my 460 FE motor. We'll see how it goes when I get it running.

[vettestr]

The decision to run or not run vac. advance on my/your engine should be a tempered choice. A complete performance tune needs to look at the pros and cons of the overall system with intended goals in mind.


I do think there are some benefits to VA (vac. advance) but the higher the modifications from stock the lower the VA benefits. In every case VA must compliment not complicate the performance of the total system. VA has multiple components that need to work well before we even discuss when and how advancing ignition timing is beneficial. I like electronic timing Maps or controls but mechanical .... not so much.



The items that allow vacuum advance include A> a rotating plate, B> an arm to mechanical move this plate, C> a vac. diaphragm housed in a canister, D> then a source for the controlling vac. In the old days the ignition points (1 or 2 sets) lived on this rotating plate and was a source for timing error and so on. In short each component and its movement/function can be eliminated without having VA on your engine. If present you must verify they each work without inducing other problems. To be sure you can argue mechanical Vacuum Advance aids in emissions reduction and can increase fuel mileage under certain conditions. Greenies may hate me but MY Engine was not built with emissions or fuel mileage in mind. I control ignition timing through my fuel injection controller and have a solid mounted pickup plate in the distributor for these reasons.

My opinion only..... mechanical vacuum advance is more trouble than value the way most folks bring me the car. I find less trouble when MSD with general spring changes were installed but they still need a lot of tweaking.

[jwd]

Originally Posted by DanEC

That looks right on. Do you know who wrote it? It looks like information I've seen on the Corvette Forum.

That article was written by John Hinckley and you can take it as the gospel. I've built and tuned many engines from a complete stocker to 800+HP monsters and every single one of them had vacuum advance hooked up to manifold vacuum. I would run from any engine builder that said "vacuum advance is a waste of time" or "this motor doesn't need it, it's a race motor" because they obviously have no f---ing idea what they're talking about and read too many Hot Rod magazines.
This topic has been beaten to death and the result is always the same - those who know, run vacuum advance. Those who don't, should.
I posted that same article a while back trying to convince some naysayers. I guess you just can't cure stupidity.

[DanEC]

Originally Posted by jwd

That article was written by John Hinckley and you can take it as the gospel. .

Thats what I suspected. John is the man and second to none technically.

[elmariachi]

Not the first time this topic was covered, with the results being the same:

Vacuum Advance - Manifold or Ported

[Silverback51]

Originally Posted by jwd

That article was written by John Hinckley and you can take it as the gospel. I've built and tuned many engines from a complete stocker to 800+HP monsters and every single one of them had vacuum advance hooked up to manifold vacuum. I would run from any engine builder that said "vacuum advance is a waste of time" or "this motor doesn't need it, it's a race motor" because they obviously have no f---ing idea what they're talking about and read too many Hot Rod magazines.
This topic has been beaten to death and the result is always the same - those who know, run vacuum advance. Those who don't, should.
I posted that same article a while back trying to convince some naysayers. I guess you just can't cure stupidity.

Gee Jim, are you saying I'm stupid?

Actually since I went back to the vacuum secondary carbs, I'm going to go back to the vacuum advance distributor to see how it works.

[jwd]

Originally Posted by Silverback51

Gee Jim, are you saying I'm stupid?

Actually since I went back to the vacuum secondary carbs, I'm going to go back to the vacuum advance distributor to see how it works.

I'd never call you stupid John. Maybe misinformed though. Smart move on the carb. change but don't get me started on mech. secondaries on the street. Let me know if you have any problems with the dist. changeover.

Jim

[elmariachi]

Originally Posted by Silverback51

Gee Jim, are you saying I'm stupid?

Actually since I went back to the vacuum secondary carbs, I'm going to go back to the vacuum advance distributor to see how it works.

John, which distributor are you going to use?

Jim

[CHANMADD]

Nothing beats my 800 double pump (mech) spreadbore!!!!!

[jwd]

Originally Posted by CHANMADD

Nothing beats my 800 double pump (mech) spreadbore!!!!!

Glad it works for you. Ever wonder why, even in the horsepower wars of the 60's and early 70's, that ALL manufacturers used vacuum secondary carbs.? That includes the GM L-88 (550HP), LS-6 (450HP) and your Ford, 428 SCJ, 429 SCJ and the dual quad 427 SO. Maybe you're smarter than those factory engineers that spent hundreds of thousands of hours designing those motors.
A street driven car with a DP isn't living up to it's potential.

[DanEC]

Originally Posted by jwd

A street driven car with a DP isn't living up to it's potential.

Jumping from vacuum advance to mechanical secondaries and now to double pumper carbs. I'm not tracking. I've never heard anyone critisize a double pumper carb. It may be overkill for some motors but having a good shot of fuel on acceleration is never a bad thing.

[Silverback51]

Originally Posted by elmariachi

John, which distributor are you going to use?

Jim

Both distributors are MSD units for the 460. One with and one without vacuum advance.

If the weather ever turns nice in this festering crap hole, maybe I will get a chance to try it out.

[jwd]

Originally Posted by DanEC

I've never heard anyone critisize a double pumper carb. It may be overkill for some motors but having a good shot of fuel on acceleration is never a bad thing.

I've converted many people to vacuum secondaries and every one was more than happy with the results. That "good shot of fuel" is used to cover up the short comings of forcing more air into the motor than it needs. Just another case of "if racers use it, it must be good".

[jwd]

Originally Posted by Silverback51

Both distributors are MSD units for the 460. One with and one without vacuum advance.

If the weather ever turns nice in this festering crap hole, maybe I will get a chance to try it out.

MSD uses AC/Delco vacuum advance units. You need to know what your engine vacuum is at an idle in order to choose the correct unit.

[Silverback51]

Originally Posted by jwd

MSD uses AC/Delco vacuum advance units. You need to know what your engine vacuum is at an idle in order to choose the correct unit.

I know it sucks.

Otherwise it would be a diesel.

[jwd]

Originally Posted by Silverback51

I know it sucks.

Just gotta know how hard it sux.

[Wbulk]

OK, that is funny.

The article is right on. Most don't know that many vacuum advance canisters are adjustable with a 3/32" allen wrench inserted in the canister vacuum port.

[TButtrick]

Thanks for posting that article! I've been running ported vac on my 650DP forever. Been hunting down a dieseling problem (run-on) which forced me to look at all the base line settings once again. Eliminated any possible vac leaks (new intake and carb gaskets, PCV and lines, etc.) Glad I suspected the intake gaskets first. They were FelPro 1250s whose coolant ports had disintegrated in just 2000 miles. (common problem with these) New dizzy and set base and total timing to 12 degrees base and 34 total all in at 2900rpm. I have a new Quick Fuel 650DP with annular boosters. Idle airs are 1 1/8 turn out (just below max manifold vac) Pri jets are 70 and 76 for secondaries on a 392W with AFR185 heads. Dieseling is all but gone as I've run some octane booster and CRC carbon cleaner through a tank and slurped through the manifold intake to help clean out any carbon build up. New AutoLite 3924s gapped at .045". Still feel like I'm on the edge of dieseling as it coughs from time to time but mostly after a long cruise and MUCH better than before. Maybe full manifold vac advance to the dizzy will help carbon build up and fuel mileage. As of right now, fuel mileage seems to REALLY suck when just cruising.