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Old 02-09-2013, 07:39 AM
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vacume advance

I am running a msd pro billet with vacume advance I set the timing at 36 degrees total and when I hook up the vacume advance it jumps over 40, is this normal? or will it cause some detonation? its in a 383 chevy flat top two valve relief with aluminum 64 cc heads

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Old 02-09-2013, 10:43 AM
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Disconnect the vacuum advance and plug the vacuum source. Then see how it runs.

The initial timing advance should be set at 11-13 deg. BTDC with more than 9.8:1 compression ratio. I am assuming that your engine has about 9.6:1 static compression ratio so try 18 - 14 deg. BTDC if the static compression ratio is from 9.0:1 - 9.7:1.. The initial advance setting has more to do with detonation than the intermediate advance rate.

Last edited by MouseFink; 02-09-2013 at 10:58 AM.
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Old 02-09-2013, 11:31 AM
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I was guessing my static compression is around 10.5 with flat tops and .010 piston to deck with 64cc heads I set my initial timing at 12 degrees with the bushing I have in it makes the total around 36 it seems to run real good but was just curious when I hook up the advance why the timing jumps so much
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Old 02-09-2013, 11:41 AM
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It appears that your vacuum advance is giving you 28 degres or more advance. That is a lot of advance. if your initial is 12, mechanical is about 24, (36 total minus 12 initial) and your vacuum is 28, this gives you 64 degrees with vacuum advance. Too much! you probably need to limit your vacuum advance.
Initial plus mechanical = 34-36. Initial plus mechanical plus vacuum should equal 48-52.
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Old 02-09-2013, 11:45 AM
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how do I limit my vacume advance? with it unplugged my total is 36 until I hook up the vacume, my initial stays at 12
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Old 02-09-2013, 11:48 AM
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if I am understanding what you are saying is that with my vacume advance hooked up I should be around 48-52
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Old 02-09-2013, 12:38 PM
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Quote:
Originally Posted by Gus's 40 View Post
if I am understanding what you are saying is that with my vacume advance hooked up I should be around 48-52
Yes, with rpms high enough that all mechanical is in. Initial (@idle) with vacuum advance hooked up will depend on whether you are using ported or manifold vacuum for the vac adv. Using ported vacuum the initial should be the same whether vac hose is hooked up or not."
Using manifold vac, the advance reading at idle should equal your initial plus vac adv. Which are you using?
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Old 02-09-2013, 12:42 PM
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limit the maximum vac advance to around 10-12deg max possible.
vac adv travel stop.

What cam is in this motor? auto trans? manual trans?
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Old 02-09-2013, 12:52 PM
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The vacuum "advance" mechanism is an incorrect description of the device. It is a actually a "retard" mechanism it retards the rate of centrifugal advance by the amount of engine vacuum (throttle opening). The vacuum "advance" diaphragm has absolutely nothing to do with total timing or performance.

As the throttle is opened, the manifold vacuum drops to zero and the distributor vacuum diaphragm and the rod attached to the switch plate allows the centrifugal mechanism to continue advancing the spark timing an additional 12 degrees to at total of 34 degrees, including the degrees initial advance. Example: 10 degrees initial + 12 degrees centrifugal + 12 degrees with the manifold vacuum at zero. If you disconnect the vacuum device, the spark timing will advance to 34 degrees by centrifugal advance alone and it will be controlled by engine RPM rather than by RPM and engine vacuum. The centrifugal advance will advance the spark timing to 34 degree no matter what the initial timing is. If the initial is 14 degrees, the centrifugal mechanism will advance 20 degrees; if the initial is 8 degrees, the centrifugal will advance 26 degrees. Etc. I have determined on most high performance engines that they run better by putting more advance in the crank (initial) and less in the centrifugal advance, taking in consideration the compression ratio. .

If connected and properly functioning, the vacuum "advance-retard" mechanism will prevent the centrifugal advance mechanism from advancing the spark timing too far at part throttle. Therefore, you can disconnect the vacuum advance and plug the vacuum source and observe the total timing advance, and see how the engine performs.

Last edited by MouseFink; 02-09-2013 at 01:16 PM.
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Old 02-09-2013, 01:19 PM
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Read this. [url]www.crankshaftcoalition.com/wiki/Hot_Rodding the_Hei_distributor[url]

Last edited by jaw22w; 02-09-2013 at 01:25 PM.
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Old 02-09-2013, 01:26 PM
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Quote:
Originally Posted by MouseFink View Post
The vacuum "advance" mechanism is an incorrect description of the device. It is a actually a "retard" mechanism it retards the rate of centrifugal advance by the amount of engine vacuum (throttle opening). The vacuum "advance" diaphragm has absolutely nothing to do with total timing or performance.

As the throttle is opened, the manifold vacuum drops to zero and the distributor vacuum diaphragm and the rod attached to the switch plate allows the centrifugal mechanism to continue advancing the spark timing an additional 12 degrees to at total of 34 degrees, including the degrees initial advance. Example: 10 degrees initial + 12 degrees centrifugal + 12 degrees with the manifold vacuum at zero. If you disconnect the vacuum device, the spark timing will advance to 34 degrees by centrifugal advance alone and it will be controlled by engine RPM rather than by RPM and engine vacuum. The centrifugal advance will advance the spark timing to 34 degree no matter what the initial timing is. If the initial is 14 degrees, the centrifugal mechanism will advance 20 degrees; if the initial is 8 degrees, the centrifugal will advance 26 degrees. Etc. I have determined on most high performance engines that they run better by putting more advance in the crank (initial) and less in the centrifugal advance, taking in consideration the compression ratio. .

If connected and properly functioning, the vacuum "advance-retard" mechanism will prevent the centrifugal advance mechanism from advancing the spark timing too far at part throttle. Therefore, you can disconnect the vacuum advance and plug the vacuum source and observe the total timing advance, and see how the engine performs.
This is all false and will only confuse..

Vac advance is a separate system from centrifical advance. They are two different things. Vac advance advances the timing when ever manifold vacuum is high. It does not change the centrifical advance or the amount of centr advance at all. ever
The timing will never be less than what the centrifical advance creates.
vacuum advance does not retard the timing.
it only increases the timing based on how much manifold vacuum it sees.

closed throttle =ls high vacuum open throttle ='s low manifold vacuum.

If this motor has a big cam in it the MSD distributor timing curve will need custom modification to get the right timing curve.

WHAT CAM?
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Old 02-09-2013, 01:52 PM
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Can't seem to make this appear as a link. Can someone else help?
www.crankshaftcoalition.com/wiki/Hot_Rodding_the _Hei_Distributor
For some reason it will not include the whole address.
It should confirm and help explain what fbird88 and I have said.

Last edited by jaw22w; 02-09-2013 at 02:00 PM.
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Old 02-09-2013, 03:28 PM
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Check a GM vacuum advance canister. It will have five numbers stamped on the mounting plate on each side of the push-pull rod. The set of 3 numbers indicates the last three digits of the part number. More important are the set of two numbers which indicates the maximum centrifugal advance allowed by the vacuum mechanism in distributor degrees.

Remember this: Distributor degrees x 2 = crank degrees. Maximum allowable advance is usual from 10 to 13 distributor degrees and is stamped on the vacuum canister mounting plate; or 20 to 26 crank degrees + initial advance = total advance. I would estimate that 80% of the people who have modified engines in their cars with double hump engines equipped with triple thumper camshafts don't even know what the initial advance is and don't know how to find it. That is because their engines cannot idle below 1000 RPM in order to check the initial advance with a timing light.
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Old 02-09-2013, 03:50 PM
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the cam I am running is a 480 lift 292 advertised duration 230 at .050 108 lsa I have the vacume advanced connected to the ported side per msd, when I timed it it idles about 900 rpm for the initial but with the vacume advanced plugged I just focused on the total and ended up with 12 initial it seems to start good and idle fine. I am just worried with the vacume advance hooked up that its too much
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Old 02-09-2013, 04:12 PM
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The cam needs a different timing curve . Requires modding the curve.
You want 22 to 26deg base timing at idle,,,, a short 10-12deg cent curve 34 to 36deg max WOT timing.
You must limit the mech adv travel to 10deg.

This increased idle timing will allow correct carb set up. readjust the pri and sec throttles positon at idle and use a 4.5" power valve.

Then...... vacuum advance will need to be custom dialed in. you want a max of 12-15deg at highest man vacuum. ( cruising and deceleration) you want around 8 to 10deg added in when cruisng.
The rate is found by drive testing.
Use ported vacuum ( auto trans)

right now your throttles are too far open at idle
The is cam will not allow correct idling in gear with a stock GM torque converter.

Needs a 10" 3000-3500 stall. 3500 stall is best, if you like a hard launch.
10" 3000 tall is the minimum requirement or it will not idle correct in gear (auto trans)

one of these is required for the MSD distributor to get it right.
Or you own custom bushing/method/modificatrion of limiting the advance travel.

10deg and 14deg advance bushings for the MSD distributor
www.4secondsflat.com

22 to 26deg base at idle 34 to 36 deg at max WOT advance

vac adv aprox 10deg at crusie 12-15deg max possible.
usually ported is better (auto trans).....the position of the carb throttles at idle matters. 4.5" power valve for that cam.

You will not get it right using the available MSD bushings and advance springs alone.
you will not get it right with a stock converter.

The timing must not decrease when you idle in gear. 22 to 26deg stable base timing , in gear.
if its hard to start when hot with 22 to 26deg base timing, get a starter motor heat shield and install the spark interupt switch

Last edited by F-BIRD'88; 02-09-2013 at 04:23 PM.
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