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Vacuum Advance

Posted By: mopargem

Vacuum Advance - 06/12/08 02:36 AM

Hey guys, my 383 Challenger runs better with the vacuum advance disconected and hose plugged up. It has a crane cam well under .500 lift, not sure. 68 hp 10:1, timing is around 12-15 advanced seems to run best. MP electronic ignition. I checked the advance diaphram with an allen wrench and its now only 1/2 turn out. Car surges at minor rpm with it hooked up, runs great without it. Am I missing something, do I need it? Thanks always, Tim
Posted By: Anonymous

Re: Vacuum Advance - 06/12/08 02:57 AM

SETTING YOUR TIMING THE PROPER WAY



First you must set the mechanical curve. Worry about the vacuum advance later.

1. Disconnect the vacuum advance.
2. Hold REVS at 3000 RPM or greater.
3. Set timing to 34° BTDC.
4. Return to curb idle. Recheck timing. If its between 12° and 20° BTDC, you are
probably done, reconnect vacuum advance. If timing is less then 12°, mechanical
advance curve (weights, springs) in the distributor needs to be reworked
(for less total advance so you can run more initial spark).

5. Road test. If you hear detonation, disconnect vacuum advance again, retest.
Knocking gone? Readjust vacuum canister spring to slow vacuum advance rate.
Knocking still there? Rework mechanical advance curve for a slower rate of
advance (stiffer springs).

The above assumes a typical "street" state of tune (cam with advertised duration
between 280° and 300°).

Of course, you can always fine tune to your engines particular needs.
Posted By: dave571

Re: Vacuum Advance - 06/12/08 04:02 AM

Quote:

Car surges at minor rpm with it hooked up, runs great without it.



What is "minor rpm"

It sounds like you have the vaxc advance on a manifold vacuum source. It shouldn't be.
The place you should hook it on too, should have virtually no vac at idle and lower speeds.

Properly set up/installed, the vac advance shouldn't effect performance, or general running. The vac advance is primarily for economy under cruising and light load conditions.
Posted By: VanishPt

Re: Vacuum Advance - 06/12/08 10:17 AM

back the adjusting screw out 2 more turns and retest for surge. still there? try 2 more and retest .
Posted By: dgc333

Re: Vacuum Advance - 06/12/08 11:57 AM

Quote:

SETTING YOUR TIMING THE PROPER WAY

First you must set the mechanical curve. Worry about the vacuum advance later.

1. Disconnect the vacuum advance.
2. Hold REVS at 3000 RPM or greater.
3. Set timing to 34° BTDC.
4. Return to curb idle. Recheck timing. If its between 12° and 20° BTDC, you are
probably done, reconnect vacuum advance. If timing is less then 12°, mechanical
advance curve (weights, springs) in the distributor needs to be reworked
(for less total advance so you can run more initial spark).

5. Road test. If you hear detonation, disconnect vacuum advance again, retest.
Knocking gone? Readjust vacuum canister spring to slow vacuum advance rate.
Knocking still there? Rework mechanical advance curve for a slower rate of
advance (stiffer springs).

The above assumes a typical "street" state of tune (cam with advertised duration
between 280° and 300°).

Of course, you can always fine tune to your engines particular needs.



If you want the best throttle response and idle quality you start with the initial first. Do these steps with the vacuum advance disconnected.

1. Use a vacuum guage and increase initial in small increments while keeping the rpm constant until you find the timing that produces the max manifold vacuum (it will likely be between 12-and 20 but if the engine wants 20 and you have 12 the idle quality and throttle reposne won't be good) at idle. If this timing causes the engine to kick back when starting you will need to back off until it doesn't.

2. Now check the total by reving the engine until it does not adavance anymore. If it's between 34-36 for sb or 36-38 for a big block you are done. More likely it will be quite a bit more. If you have a stock distributor you will have to reduce the slots in the adance mechanism, if you have a mopar performance or after market distributor they are adjustable.

3. Now you want to set the rate at which it advance comes to be in the range of 2000-2500 rpm for a street motor. As you reduce the amount of advance in the mechanism inside the distributor you will need stiffer springs to maintain the same rpm.

4. Now test drive if you get detonation as the rpm increases but goes away higher up the rate is to quick. If you have detonation at max advance higher in the rpm range you will need to back off the total some (or use higher octane gas). Once you have this worked out to do better will require dyno time or many trips down the strip making small changes to ial it in.

Now to the vacuum advance. Drive the car and determine what the manifold vacuum is at cruising speed. Using a hand vacuum pump adjust the advance pot until you have all the advance at this vacuum. Hook up the vacuum advance to a timed port (no vacuum at idle), drive the car. If you have no part throttle detonation or detonation at part throttle tip in you are done. If you do you will need to make of your adjustments in small increments until it stops. This can be further fine tuned with pots that have different amounts of advance and ones that have differnent advance curves. This is time consumming and tedious because it will require first finding the pots and then trying them.

some other things to keep in mind; higher cylinder pressures generally require less total timing for max power. Closed chamber heads gerneraly require less timing for max power. Total, rate and vacuum advance all have some interaction with each other so nothing is absoultly hard and fast but the process I mentioned is the best way to get the whole timing curve very close.
One you have the timing all sorted out then you can fine tune the carb. The initial timing will have a significant impact on the idle and transition circuits in the carb and most all carb tuning guides will tell you to get the timing correct before worrying about the carb.
Posted By: 65cuda

Re: Vacuum Advance - 06/12/08 02:21 PM

there was a post on here couple weeks ago said almost the same thing except he said to use manifold vac for the vac. adv.. i tried it and it was too much timing at idle. but the engine run cooler for the first time sense i put it together. i have been fighting a running hot problem for 5 years. i recently put in a MP dist.
my motor now runs around 180. still creeps up in traffic. i have it set about 12 basic 33-34 all in. but when i hook up the Vac. adv. to manifold vac it gives me 32 at idle. so i put it on ported. that works good at cruse (runs 180) but still creeps up at idle in traffic. didn't when i had it hooked to non ported. i adjusted the vac can all the way both ways didn't seem to make a diff. so how do i run a little more adv. at idle to keep it cool there to? the vac. can says 9.5 on the arm. doesn't that mean it will pulll in twice that in degrees? i am happy as he## about geting this thing cooled down but i am trying to fine tune it now.
Posted By: Anonymous

Re: Vacuum Advance - 06/12/08 05:08 PM

Yes the 9.5 on the arm is saying that you can get a max of 19 degrees . The allen hex in the end of the nipple regulates how much vacume it takes to get that. The more counter clock wise you go the higher the vacume will need to be to reach the amount of advance you want. Clock wise will make it take less vacume to get the unit to move. Your right the more initial timing the motor will run cooler. But I would do that with the advance slot in the distributor. Then play with the vac can. www.4secondsflat.com has a new no welding required advance limiter kit that you can use to dial in the advance .Comes with springs, limiter and instructions. Works on a standard Mopar Electric dizzy. Lots of argueing about manifold vs ported. I've done them both ways and had good results . From the factory they were set up ported.I guess its a matter of what your trying to accomplish with it. If the car runs better with it on manifold it wont hurt it as long as your not getting detonation.
Posted By: 70SportSatellite

Re: Vacuum Advance - 06/12/08 06:13 PM

Quote:

here was a post on here couple weeks ago said almost the same thing except he said to use manifold vac for the vac. adv..



Here is a link to that post you may be referring to. Although there is some disagreement all I know is it worked well for me. web page
Posted By: 65cuda

Re: Vacuum Advance - 06/12/08 08:35 PM

yes that was it. i saved the text from that one and this one. lots of good ideals even though they disagree some. i am still using both as a guide. changing things little at a time. i need a little more at idle a little less at crusing say 35-40 mph good at 65-75. i have cranked the vac. can both ways and it didn't seem to help. but like i said i am happy to get the darn thing to run cooler
Posted By: Magnum

Re: Vacuum Advance - 06/13/08 02:30 AM

Setting up the initial first or the total first makes no difference. For a fully tuned distributor they both need to be done. Plus timing at idle and light throttle constant speed cruise are also important.

As for running bad with advance connected. It's probably pulling the plate and bringing the reluctor out of phase. This can be checked by drilling a big hole in the dist cap. Shrine timing light in the hole and check the rotor to tip alignment with and without the vac advance.

Ported and manifold advance is exactly the same except the ported offers no advance at idle.
Most engines if not all engine will idle better at
20-40 deg than at 8.

My engine idles smoother and I can close my throttle blades more when using manifold vac advance. Most high stall auto guys would never appriciate this but mine is a stick and I have more power at light throttle, in the very low rpm range (accelerating in 5th in the city).
Posted By: dgc333

Re: Vacuum Advance - 06/15/08 12:50 PM

Quote:

Setting up the initial first or the total first makes no difference.




In the end no it doesn't but: Depending on the heads being used you know the range the total needs to be in before you start and you don't with the initial. So by starting with determining the initial first you only have to play with setting the mechanical once.
Posted By: Anonymous

Re: Vacuum Advance - 06/15/08 02:59 PM

I'm thinking the other way.. do it as I posted ealier, and the initial will automatically be set.
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