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351C i was just experimenting and i found that if i hook up the vacuum advance to the manifold vacuum source it raises the idle speed because the manifold vacuum is advanciing the timing correct?.....i was just wondering if this is a bad idea(ie will it cause overheating, pingin etc et) or will it not harm anything.......??.....any ideas will be appreciated as usuallTHANKS
 

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no, the vacuum signal should be similar throughout the manifold. Generally, however, they're hooked to a ported source. You'd know if you had a problem-

K
 

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Vacuum

Doc here: :pimp:

If your car Is a SMOGGER ...leave it where you found it. Which is a ported source from the Carb or manifold...You WILL not pass your next CALIFORNIA SMOG CHECK II test on either Visual or HC.

If your Idle is higher than spec, which SHOULD be around 650...plus or minus 50 RPM with no excursions...the Computer will automatically fail you...

If your timing is plus or minus 1 degree advance or retard, the computer will automatically fail you...

And don't forget, over/under advance can be detrimental to your Cat (s)

At the very least mark the source and DON'T forget to replace it , reset your timing, and idle before your next test..

It Takes very little now to end up on the Gross polluter list In California anymore , and if you haven't recently (within 2 years or after July 15, 2003) tested you'll find out, if anything is other then stock, the Visual or DYNO pull will fail it.

Once you end up on the GP list...your locked out of testing from all but State Referee Test only, or Gold seal testing Facilities. And these folks are much more picky than a regular test station.

I had one Corvette I tried getting through Smog less than 6 months ago...It required 13 trips to the REFEREE and so much repair, it cut the Value of the car by half ($6,200 worth of tests and repairs..) and on the previous test (prior to SMOG CHECK II,) passed with high marks.

The Year tag was withheld by DMV, and during one trip to the referee, I was pulled over and told by the locals (C.C.C.S.O.)..Even though the DMV fees were paid, Next time they stopped it, they would impound it, 30 days at $33 a day plus fee's.

Running the Advance on a full Manifold source is a good performance addition, versus a ported source, but as I said..be advised of the future pitfalls it can bring...

A lot of guy's are modifying their daily driver's for a little more performance , and ignoring the fact that sooner or later, their gonna end up on that DYNO...and their either failing or end up as G.P.'s and don't know why...then Bi*ch about it after the fact...

Any more, the only way's you can get away with it, is have all stock stuff ready to go back in before the test, (and hope it still all plays together) Or own a car out of the 30 year window..

Food for thought!

Doc
 

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This is a great article!!!!I had my vacuum advance hooked up to the ported vacuum port after reading this article I hooked it up to full manifold vacuum adjusted the idle rpm.The car has so much more power now.



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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Two articles by "authorities" with differing statements.
 

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Correct me if I am wrong but those to articles say two different things, just like MI2600 stated, correct?

The article copy and pasted in this thread, states connecting the vac advance to direct manifold vacuum, increases vac advance at idle and as the vacuum drops so does the vac advance.

The linked article, says the direct manifold vacuum connected to the vac advance, holds the vac advance at zero and when the vacuum drops the advance increases.

Conflicting or what?


What I do know, when I connected my vac hose to the direct manifold vacuum, the engine rpms increased, so the timing must have advanced, and understadably will drop as the rpms increase. Weird. No wonder this crap is so misunderstood.
 

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Quote-From a hotrodding perspective here's the difference; From a dead idle if you were to stomp on the accellerator pedal with a vacuum advance you would get instantanious power all the way up to where the engine vacuum rises and pulls the advance back.

This sounds a little *** backwards to me. engine vacuum retards the timing? I don't think so.
 

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ratlover

good point and the vac doesn't increase when you stomp on the pedal...something is definitely bass ackwards with that link. later
 

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yeah,
this seems to be a pretty hot topic here, and there is no real way of knowing who's right..... does it matter?. If it works do it. I never took mine for a road test when I hooked up my can to manifold, because it made the car hard starting, and I could not get the idle down to an acceptable level. I may try it again just fir the heck of it.
 

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ratlover

Well I set mine on direct vacuum this morning, left the vac canister adjustment, mechanical advance and initial as they were, and fired it up, let it warm and the idle jumped about 400 rpm, I brought it down to 650, then shut if off, restarted it after 15-20 minutes and the idle was a hair below 650 ~ 630-640 so I reset it and shut if off, it definitely reved quicker thats for sure. I will run it up the road later and see how she feels.

Lemme know how it goes for you , if you decided to give it another shot.
 

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yes timing does retard when you step on the pedal. Vacuum advance advances the HELL out of your timing at part and no throttle. Of course you're going to want less timing if you're doing a full out, WOT run, you'll get detonation otherwise. At part throttle on the highway, you can bet you're advanced well into the 40 degree range (people often don't this when I tell them, vacuum advance is NOT included in total timing). vacuum advance is not a performance item. All it does is influence a more complete burn when you're running a smaller charge to keep the plugs clean and get some decent gas mileage. Yes you can run a mechanical distributor, and set your initial, and total timing and be done with it. It will work, but it won't be load dependent.

Ever had a hotrod that would ping in third gear on the highway, but not in WOT runs? That's because your timing for the WOT was about 37, whereas your timing on the highway is probably in the 40's-

K
 

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Okay,
here comes the hard part.... The part where I have to admit I am a total idiot! For the heck of it I decided to try the vac swap one more time. Just straight pull the hose and switch it over, adjust idle..... It works fine. I think I over thunk it last time by retarding the timing back to 18 or 20 degrees as I was not comfortable running like 32 at idle. That would have made the hard starting issues. (crank easy no start).... what can I say, I guess I owe a few people on here an apology, sorry man, I guess it does work.:embarrass
 

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killerformula said:
yes timing does retard when you step on the pedal. Vacuum advance advances the HELL out of your timing at part and no throttle. Of course you're going to want less timing if you're doing a full out, WOT run, you'll get detonation otherwise. At part throttle on the highway, you can bet you're advanced well into the 40 degree range (people often don't this when I tell them, vacuum advance is NOT included in total timing). vacuum advance is not a performance item. All it does is influence a more complete burn when you're running a smaller charge to keep the plugs clean and get some decent gas mileage. Yes you can run a mechanical distributor, and set your initial, and total timing and be done with it. It will work, but it won't be load dependent.

Ever had a hotrod that would ping in third gear on the highway, but not in WOT runs? That's because your timing for the WOT was about 37, whereas your timing on the highway is probably in the 40's-

K

Seems we all agree, wonder where that linked article received his/her information from then?

ONe question though from your post, why would you want less timing at WOT than part throttle? More timing is needed when the engine is spinning so quick that the ignition needs to begin the combustion process even more (advanced) degrees before tdc to assure the combustion process reaches its peak right after tdc to get the greatest amount of force on the power stroke, correct, so why would you want more advance at less rpm? Just trying to figure all this out.




Ratlover


So do you like it better w/t full manifold or ported?
 

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I think we all agree that:

1. Applying vacuum to the can creates an advance in the distributor.

2. Total mechanical timing is the initial setting, plus the centrifugal weights.

3. Vacuum advance is dependent on engine load.

I've always thought that at idle and steady speeds, the engine could/would accept more than normal total mechanical advance. And, at WOT, the engine only wants total mechanical advance, otherwise it would detonate.

Connecting the advance can to a manifold source accomplishes that.
 

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Seems like you guys have got it figured out but, when hooked to manifold vacuum........

At idle the engine will be happy and much more responsive with a high intial advance.....maybe even close to 30. As you step on the throttle the vacuum decreases and engine speed increases. The weights advance timing and the vacuum decrease retards the timing simultaniously so it effectively timing will stay about the same. At cruise the weights and the vacuum are advancing timing. The engine will working with 45 or more degrees at cruise and will get better economy this way. Anther plus is it keeps EGT down.

Chris
 

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Okay...as far as I am concerned I think I get it...I was hung up on the idea of why would you need more timing at cruise and not wot, and then I realize it has everything to do with the fuel delivery of the carb.
 

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back in the day if you were out cruising looking for challengers you would swap your vaccum source while cruising and and swap it back on the way home.with manifold you had full adv. all the time.nice for racing but not gas milage.
 
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