Which vacuum source is correct? or should i say which is preferred? i've always tapped into manifold vacuum for vacuum advance, some use ported, just wondered what the consensus was here, it's funny how something so apparently black or white could have so many differing opinions, but the best arguments seem to be made on behalf of manifold/full vacuum, just outta curiosty i hooked mine up to ported/timed vacuum and my truck ran like crap but i guess any given engine can be set up differently, but full vacuum seems to work best for me.

 

Ported is the proper way to run vacuum advance. If manifold was the correct way to go, why did not factory engines ever come that way? If the engine runs better with manifold vacuum, then you are just covering up a setup and tuning problem by running extra advance at idle and low speed. A smart way would be to recurve the dist and readjust the carb properly.

 

I ran across this article while researching this topic, i thought it may be of interest so i posted it here, he makes a pretty valid argument for manifold vacuum.....

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 engineering 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. 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 engine'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 hi-performance 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 any parts store) 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 the hi-performance catalogs about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

 

Very interesting, and I will say one thing....easy enough to to test the statements and find out IMO.. sounds like reasonable advice really. Thanks for the read..

 

Ditto!! I was just pondering this yesterday while setting the timing! If I had a vacuum tester or at least a vacuum line that would've reached to manifold port, we probably would be on the port for a trial. Later I found my timing chain must have a lot of slop, started running with the dogs again :-(!

 

It maybe be interesting but ignores the basic fact that at idle the engine is always unloaded and has lots of time to burn the mixture because the piston takes a long time to move from 10° BTDC to BDC. I jut love these theories that counter physics or at least ignore physics.

"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."

My dist. doesn't work this way and neither does anyone elses.

 

Montana, that tech piece you found looks like it was written by my high school auto shop teacher. If you look up history in this forum, you'll see that Bear and I have argued this issue for years. I have always been and still remain a proponent of manifold vacuum. Thats the way I setup my engines. If it makes you happy, you can try it both ways and see which way works best for your setup. I believe Bear's memory is faulty when he says factory setups never had manifold vaccuum. My memory and that of many others says they did. and now you found a very nicely articulated explaination of what happened to vaccumm advance and why the ported stuff exists. Ported VA functions exactly as your author describes. Bear, what part of this don't you understand or find correct ? DinosaurFan, on work's old cast off 'puter

 

Given that no references were provided, it is impossible to check the credentials of the author of this piece. He could be Dino's old shop teacher (in which case I would cast a hard and critical eye at what he wrote) or he could be Ford's chief engineer in charge of ignition design (in which case I would bow to his feet and accept without question everything he says). Until then, I can just say, nice article.

And just for the record, with STOCK engines which were DESIGNED for ported or manifold vacuum, there should be no question which one should be used. For highly modified engines, I guess it depends on what you are trying to achieve.

 

My 66 352FE VA operates with manifold vacumm.

 

I find the part where he totally ignores the fact that at idle you have much more time (real time like micro-seconds and not degrees) to burn the mixture than you do a WOT ot even cruise because the piston is moving much slower. How much time the mixture has to burn is important and not the number of degrees involved. That's the reason you run less timing at lower speeds, you don't need more time to burn the mixture. Has no one ever had an engine dynamics class? This stuff is all explained. engine timing uses degrees, because counting the micro or milli-seconds you have to burn the mixture is hard to measure, but degrees on the damper are easy. Timing is actually about piston speed.

 

Name one that came from the factory with manifold vacuum.

 

Going for popcorn....

 

Hello all,
In my 65 truck w/352 I have a mild cam, edelbrock 600cfm carb and all the rest of the bolt on goodies. I have had the timing set with the dizy vacuum line conected to the pasenger side of the carb, which is timmed vacuum. After reading the article I went out and flipped it over to the drivers side of the carb, which is manifold vacuum. I re-adjusted the timing, 10 degrees at idle with vacuum at carb pluged. With the vacuum line re-connected the timing at idle reads 30 degrees. Took the truck for a spin and was a huge difference. The motor runs really smooth and throttle response is much more snappy. Quess the motor is more happy with manifold vacuum.

Aaron

 

Aaron, thats the way your setup is suppose to work. Same as mine and it came that way from the factory.

 

OK, with a 390, Holley 600 cfm, I connected the vacuum to the passenger side, base or bottom port of the carb. Is this location manifold or other vacuum? Before I timed it, it was connected to a port higher on the carb. If I want true manifold vacuum, should I go to the vacuum ports behind the carb??
I am curious as to the original location for the vacuum on a 390?