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post #1 of 16 (permalink) Old Aug 18th, 08, 10:35 PM Thread Starter
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Patric
 
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vacuum advance hook up

hi, i am use to connect vaccum advance on full vacuum side and i tought that centrifugal advance would compensate at higher rpm, but i have some friends that reccommands to connect on a ported vaccum. What shoud i do

Thank you
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post #2 of 16 (permalink) Old Aug 18th, 08, 10:46 PM
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Re: vacuum advance hook up

[quote=patvac;1049067]hi, i am use to connect vaccum advance on full vacuum side and i tought that centrifugal advance would compensate at higher rpm, but i have some friends that reccommands to connect on a ported vaccum. What shoud i do

Thank you[/quote

Take your friends out for lunch and let them pick up the tab for erroneous advice.

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post #3 of 16 (permalink) Old Aug 19th, 08, 12:30 AM
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Re: vacuum advance hook up

Manifold Vacuum. Make sure with the car in gear the canister pulls full vacuum. So if the car is say 10" in gear you need a canister that pulls full vacuum at 8". If not it may run rough. Do a search there are MANY threads on vacuum advance. 90% say MANIFOLD VACUUM.

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post #4 of 16 (permalink) Old Aug 19th, 08, 05:22 AM
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Re: vacuum advance hook up

post 11

https://www.camaros.net/forums/showth...&highlight=101

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post #5 of 16 (permalink) Old Aug 19th, 08, 06:07 AM
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Re: vacuum advance hook up

Its your call, The manifold vac gives better performance , the ported will give better emmisions ( thats the good side) and a bit worse fuel economy and power.
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post #6 of 16 (permalink) Old Aug 19th, 08, 08:17 PM Thread Starter
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Re: vacuum advance hook up

than you for fast response.

This is what a call an answer, not just a yes or a no.
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post #7 of 16 (permalink) Old Aug 19th, 08, 08:41 PM
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Re: vacuum advance hook up

The following was written by John Z. from CRG and is the most accurate description of vacuum advance:

Yes, ported spark was the design condition, as a crutch for emissions. Yes, the full manifold vacuum connection provides full advance at idle, which is EXACTLY what you want. My "timing 101" paper below explains how timing and advance work together, and explains the "ported" vacuum aberration.

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.

Last edited by Sauron67MM; Aug 19th, 08 at 08:42 PM. Reason: forgot puncuation
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post #8 of 16 (permalink) Old Aug 19th, 08, 09:10 PM
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Re: vacuum advance hook up

I think we refer to JohnZ's well written articles at least once a week. Shouldn't they be stickies?

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post #9 of 16 (permalink) Old Aug 19th, 08, 11:10 PM
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Re: vacuum advance hook up

For sure! Until then, if John doesn't mind, I'm gonna copy and paste to my 'documents'. (just for insurance, of course.)

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Last edited by Fred Ficarra; Aug 19th, 08 at 11:23 PM.
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post #10 of 16 (permalink) Old Aug 20th, 08, 01:29 AM
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Re: vacuum advance hook up

Where can you buy a vac can that pulls that? I have looked several places.
Are we talking vac cannister on the dizzy or a can to help with low vacuum?

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post #11 of 16 (permalink) Old Aug 20th, 08, 09:01 AM
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Re: vacuum advance hook up

This article explains it pretty well and has a list at the end. You can get these cans at Kragen but Kragen uses a different numbering system. Does anyone have the link that converts the B xx numbers or V xx numbers to Kragen's DR xx series?

http://65corvette.nonethewiser.net/t...um_advance.pdf

HTH

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post #12 of 16 (permalink) Old Aug 20th, 08, 04:43 PM
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Re: vacuum advance hook up

With all due respect to the author pdf link above
the general infor is correct
He mentions its taken a lot of work to compllie the part specs
I have beside me the AC Delco Electrical service manual and most bulletins from 1959 to 1982.
The list of vac advance units would only cover less than 10%, and I do not believe the general application in the list at the bottom is accurate...
I suspect the application list may have been taken from actual vechiles in many cases assuming the orginal dizzy /vac advance is the orginal factory unit.
eg if one compares the listed 1965 vac advance units (48 variations) to 1966 (29) none are the same part # thu many do have very simlar specs..may only vary 0.5" or a couple degs.

Looking down the list, the application would be very usefull in most cases to to be in the ball park of factory specs

My Spelling is not incorrect...it is creative

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post #13 of 16 (permalink) Old Aug 21st, 08, 11:44 AM
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Re: vacuum advance hook up

Quote:
Originally Posted by ace's68 View Post
Where can you buy a vac can that pulls that? I have looked several places.
Are we talking vac cannister on the dizzy or a can to help with low vacuum?


http://www.partsamerica.com/ProductL...rtNumber=DR305 $8.99

Pulls full 16* advance at 8" of vac. It starts to pull at 5". I have one on my MSD dist and works well. I tested it with a pump and gauge.

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post #14 of 16 (permalink) Old Aug 21st, 08, 12:48 PM
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Re: vacuum advance hook up

I think that is the VC 1810 can. We were talking about that can a couple of weeks ago.

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post #15 of 16 (permalink) Old Aug 21st, 08, 01:24 PM
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Re: vacuum advance hook up

Quote:
Originally Posted by Steptoe View Post
With all due respect to the author pdf link above
the general infor is correct
He mentions its taken a lot of work to compllie the part specs
I have beside me the AC Delco Electrical service manual and most bulletins from 1959 to 1982.
The list of vac advance units would only cover less than 10%, and I do not believe the general application in the list at the bottom is accurate...
I suspect the application list may have been taken from actual vechiles in many cases assuming the orginal dizzy /vac advance is the orginal factory unit.
eg if one compares the listed 1965 vac advance units (48 variations) to 1966 (29) none are the same part # thu many do have very simlar specs..may only vary 0.5" or a couple degs.

Looking down the list, the application would be very usefull in most cases to to be in the ball park of factory specs
I think that that list is pretty much a jumping off point in can selection. Remember, these cans were spec'ed for engines using specific cams, heads, carbs etc. Throw in smog tuning for the era and these cans were maximized for those parameters. Obviously, many people modify their power plants in many ways which would alter the interaction of these parts. The good news is that these cans are $8-15 so you can get a few different ones and play with them to see which one gives you the best performance.

I know in my case, I tried 3 different cans, different carb jets, acc. pump squirter nozzles and acc. pump cams before I got her dialed in.

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