vaccuum advance

[53burb]

Before I replaced my engine, I had the vacuum advance from the distributor hooked up to the ported vacuum spot on the carb. When I dropped off my old engine for the core, the guys there said it should be on full time vacuum. They moved it and readjusted the idle. I put it back on ported vacuum last week and adjusted the idle again. Which way is correct, ported or full time??
This link also shows full time.http://www.73-87.com/7387garage/drivetrain/hei.htm

[woodridge]

Ported is correct, full vacume advance at idle is not necesary and can cause increased emisions. On A/C vehicles I have seen it cause the engine to try do die when the compressor engages, more load, less vacume, less advance, less rpm, less vacume,etc. The only applications I have seen where full vacume was applied at idle was on a dual diaphram and it would actually retard timming at idle.

[JimKshortstep4x4]

We run all of our trucks and cars at full manifold vacuum, (17) and have for years. Our experience has been that this is the best way for performance and gas mileage. When we curve a distributor the amount of vacuum advance is set also as some advance too much or not enough. There may be situations where partial vacuum works best but in general full vacuum is what we find works best.

Jim

[53burb]

Thanks for the replies. I guess whatever you are most comfortable with. Anyone else???

[tmcgrawlookalike]

ported works best on mine!

[Solver]

Some info from a post a couple of years ago that I copied and saved
Didn't save name of poster sorry
Here it is good info on vacuum:

Ok, here's the question. When I disconnect the
vacuum advance line from the distributor
I see NO change in the timing mark for number one
cylinder. The line is connected per Holleys
instructions to the Vacuum Advance Port. The port
below is the full manifold vacuum port that the
modulator for the transmission (TH350) is
connected to.

Hi..your hooked up on ported vacuum right? This port has no vacuum at idle, it builds vacuum as the throttle is opened..I use the ported vacuum for my th350 and the full vacuum (the lower port) on the vacuum advance

I am not a motor head but I don't understand why you would want full vacuum at idle. Is'nt the whole idea of vacuum advance to increase timing at higher speeds (ie higher vacuum)?
If you hook it up to full vacuum port isn't that the same as just advancing the timing to that point initially and skipping vacuum advance unit?Like I said I am no motor head but was just wondering about this

not quite..if you have your initial at 10 deg. then add full manifold vacuum limited to 10-12 degrees this gives you 20-22 degrees at idle...where your motor likes it...then when you accelerate the vacuum drops off and the mechanical picks up..giving you 20-22 degreees of mechanical advance plus the initial of 10..giving you a total of 32 at 3000 rpm..now if you use ported vacuum..you will have 10 deg initial plus 20 degrees of vacuum plus 20 degrees of mechanical thats 50 degrees at 3000 rpm..can you say detonation??? GM used ported vacuum with real stiff springs on the mechanical for emissions...they would have 2-4 deg. of initial with the vacuum doing all the work until 5000 rpm...you will find more performance with non proted vacuum..

Now, for the rest of us: Vacuum advance.
Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.
Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.
The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.
In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.
So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.
As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.
If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.
OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.
Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.
All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.
99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.
Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.
If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake

[4x4Poet]

Heavy reading, Solver.

Very informative. It sounds like the author of the last comments thought that factory distributor mounted vacuum advance canisters have too much advance out of the box, so to speak. Never knew that, if true.

[neonlarry]

^ That's why almost all the aftermarket vacuum cans are adjustable.

[69 longhorn]

^ what they said! My 383 runs an old points dist , with a pertronix conversion.....10 initial, 36 total @ 2800 + 16 on the can. the old booger is on manifold vac, & runs 26degrees @ idle/ 52 degrees part throttle @ 2800-3000, with a light load. That 10 to1 aluminum headed sb is diggin the toad snot out of all that ign lead L

[dwaite72lnghrn]

I always look at vacuum distributors as giving you timing retard. At idle you pull full vacuum 20" (If in full vacuum port and stock cam). As you step on the gas vacuum drops so does your advance, aka retard, once you get to speed the vacuum climbs back up giving you your advance again. Load the engine vacuum drops timing retards giving you your mechanical advance.
Ported vacuum gives no advance at idle (good emissions numbers) as you come off idle vacuum jumps to 18"-20"and as long as the throttle plates are open you will get vacuum just like full vacuum port, only it seems to drop faster than the full port and under heavy load vacuum will drop of to near 0" faster.
So in the past the suggestion has been to run ported on HEI and full on Points. It doesn't seem to matter much as with cruising speed both ports pull 15"-20" depending on condition of engine. If you were to coast down a hill though full will be around 20"-22" and ported will be at or near 0". just my 2 cents.