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Topic Review (Newest First)
02-07-2007 07:17 PM
Originally Posted by red65mustang
(Top Fuel, that's my soap box, people spend $5k+ on a motor , don't spend $35 to actually know what it is doing on the road or on the track)
Truer words were never spoken.

It's amazing how many people think tuning an engine means changing plugs and wires.
I had a customer who had a good running brakcet car. He decided to purchase an advance re-curving kit and install it. He called me and told me his car had mysteriously slowed 6 tenths in the quarter. He ended up doing leakdown tests, pulling the carb apart, changing plugs, lashing valves, etc., doing everything he could possibly think of because he didn't/couldn't believe that the distributor modifications he made had caused the car to slow. After he found nothing, I advised him to put his distributor back to the way it was. The next week, it ran within hundreths of the time he ran before recurving the distributor. The only thing he said was "It pulled harder after re-curving it. I can't understand it". The clock didn't lie, but the "butt dyno" did.

02-07-2007 01:58 PM
oregon chevelle Thanks all I think I get it now!!!!!! No vacuum advance if I am getting on the pedal hard. Guess I will leave it as is for now and then see how it runs when I get a chance to get to the track.
02-07-2007 11:59 AM
xntrik When messing with old Fords,

If the vacuum can has a HEX 3/4" nut that unscrews where the hose connects, these cans are adjustable for rate and degrees.

Don't swap these cans for the later type or aftermarket type that are adjustable for degrees only. It usually is a waste of money.
02-07-2007 11:28 AM
xntrik Thank you JimFulco for finding that information. (Mine was lost in the computer crash)

Such eleoquent words = From post # 24

QUOTE = 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"

- there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, QUOTE

02-07-2007 06:55 AM
red65mustang oregon,
geeeeze...if you had posted back earlier "no it doesn't have a hex".....
that would have been the end of the discussion....
my post would have been "no it will not help, because you can't tune it with the hex to work with your performance curve"

repeat after me: vacuum advance ONLY works when using a "little" gas pedal to give the car a little more "zip" accelerating around town....vacuum advance "disappears" when I do mash the pedal

drive safe
02-06-2007 09:53 PM
oregon chevelle Thanks for all the info all! I still need to learn so some of this is going over my head but I am trying to read and reread and reread all your posts to get a better understanding. RED no I do not have a vacuum adjustable cannister. I put the hex in and it just goes all the way in. I went out a couple days ago and ran the car hard a time or two to see if there was any change. I do not "feel" any change but to be honest I expect the change will be in times if there is going to be any. No pinging and no valve rattle. I seems to be pretty much the same car as with the vacuum advnce hooked up. Except It may get hotter at idle quicker now? Not sure as I had been driving it & it was pretty cold and the other day after the vacuum removal it was a good 20 degrees warmer out? Any how I need to get a different fan set up. With the 7 blade clutch fan I had no heating issues at idle. Then when I tore that up I put the flex fan back on and it heats up at idle again. (that was from this summer)
02-06-2007 07:07 AM
red65mustang jim,
thanks for posting the corvette link xyntrik mentioned......great reference list for GM cans spec's

I hadn't seen it, have saved it and forwarded it to a bunch of "orange blood" friends and racers......

oregon, I wish the camero author had offered the "counterpoint" info for ported vacuum: If you have a great idle carb pulse Hg (like your 18Hg) for atomization and a strong idle spark (your HEI) and the practical max above stock base timing (your 17*) to help it idle cool......ported does work

what he wrote is true.... but... "everything" in those early 70's "stock" motors was a "aberation", cam was retarded, crank was retarded, really low CR's, crummy ignition....and ported vacuum....switch to a manifold vac dist from a earlier car onto a early 70's motor.....motor is still a dog turd
02-05-2007 10:30 PM
jimfulco This may be the article xntrik mentioned from the Corvette site:

This is from the forum, posted by John Z:

"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.


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."

Note from jimfulco: I would have linked to the Camaro site for that one, but I think that old post must have gotten lost in a server change, so I copied it from my files. I thought both of these were in the Knowledge Base, but I couldn't find them.
02-05-2007 05:08 PM
Guy Hiltz
running no vacuum advance

I'm with 406 monte ss on this one,as I said earlier,vacuum advance doesent necessarily work better in every situation.
02-05-2007 04:13 PM
406 ss monte
Originally Posted by oregon chevelle
I have a 350sbc with an HEI that does have vacuum advance. However I had the distributor recurved to 36 total by 2600rpm. When this was done the vacuum advance was disconnected. Is anyone else running theirs this way?
son, asking a question about this on this board can get your balls cut off. a lot of people around here think it's a sin & will send you to hell for not running a vacuum advance. myself i don't run a vacuum advance & won't with the set up i have now. i've done it both ways in the past & really couldn't tell any thing eccept when i was running the vac advance it would tend to get valve chatter (detonation) under part throttle. without it, it didn't.
02-05-2007 09:46 AM
red65mustang a picture is worth a thousand words?

oregon, again, different vacuum advance cans will make different curves, 99% chance yours does not match this chart which happens to be a Accell for a ford 302

one dist degree equals two crank shaft degrees so if you hooked up this unit to ported vacuum and adjust this unit to 4 turns up from full off/counter clockwise (7* on the chart) (some are opposite, clockwise is off)......
your timing light will show 50* total timing at 2600rpms+ there is no load on the motor in your garage......vacuum will be approx 24Hg at 2600
it will stay at 50* at 2600+ until there is enough load on the motor to pull the vacuum below 12Hg (ex:pulling a big boat up a steep grade)
hope that helps....

mount a good vacuum guage on your dash hooked to manifold vacuum....shows you in "real time" Hg what the motor is actually will point out any (and all possible?) problems from a miss fire (bouncing needle) due to a loose plug wire to worn rings (cruise vac is down 4Hg?) to a minor vac leak (idle Hg is 2lbs low)....
it is very helpful for street and strip tuning.....what is the Hg at much is the Hg dropping when it shifts.....on your car, you probably want about 2"Hg at WOT for max power, is it 0" Hg

(Top Fuel, that's my soap box, people spend $5k+ on a motor , don't spend $35 to actually know what it is doing on the road or on the track)
02-05-2007 07:29 AM
red65mustang oregon,
no, adding vacuum advance will not change your present strip timing set up at all
yes, adding vacuum advance to the present strip timing will likely make the valves rattle
no, you can't set up vacuum advance to be a strip timing set up, vac advance only functions while driving around town and on the hwy at light throttle

yes, your present timing for the strip is as correct and is as "max" as it can be without doing actual dyno tests to tweek it for your mild car/combo....

do a few test runs with the present 17*base, then some runs at 15*base=34*total, results will be only a teeeeeeny bit quicker or slower with the 3.08 gears/3600lbs

many people who only occasionally go to the strip (me) use "a best on the street" timing combo: ex: on your car maybe(?) 14*base+19*cent+8d*vac hooked up to ported=50*total for light throttle "zip" around town at 1500-2600 rpms.....then just turn the base back up to 17* (my manifold is marked) when you get to the track for quickest ET.....

did you stick a hex wrench in the vac advance to see if it is adjustable?

good reason to buy the second you can stick the friggin' pain in the butt headers on it....put some stock vette manifolds on car #1
02-04-2007 10:11 PM
Rick WI Bahaha, no I agree, there are not too many folks running double Dominators on 433 big blocks around these parts of the BB world.

Agreed as well, the electronic ignitions allow much more flex. Lots of potential with the new boxes and especially with the new EFI's on the shelfs these days.
02-04-2007 09:53 PM
Originally Posted by Rick WI
Agreed TF, it's impossible though to get the vac advance dialed in correctly IF the base mechanical advance curve is foobared up. This particular mechanical advance curve in question I feel is foobared for vac advance.
Exactly Rick. The mechanical advance has to be correct before any vacuum advance tuning can be attempted. Just like running a point type distributor. Set the point gap/dwell then the timing. This is the main reason I feel that computer controlled ignition is a good thing. A certain advance curve can be selected or programmed in. The Crane HI-6M marine ignition system has advance profiles built in and all you have to do is select which one you need. Look at the engine in my photo album. that is run on the street every week in nice weather. It has a vacuum advance. I don't think too many other people here are running 2 Dominators on a 433 BBC on the street.
02-04-2007 06:30 PM
xntrik Vacuum advance cans come in various "types".

Total degrees of advance, the amount of vacuum needed to make the advance begin, as well as the amount of vacuum that makes it advance fully.

Adjustable types usually only alter the degrees.

There was a good Corvette site with a lot of specific vac can stuff listed. You might Google and find it.

Info is on this site too, IF it can be found. I know, I posted it.
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