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Old 08-17-2013, 03:46 PM
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Mechanical advance in HEI Distributer

Working on a 355cid small block chevy for a friend of mine, still waiting on hedders before I can fire it up. But I was hoping someone here could tell me if it's possible to determine how much mechanical advance is inside of an HEI distributer. Is there any numbers I can check out on the advance mechanism inside the distributer that can tell you how much it has?

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Old 08-17-2013, 04:13 PM
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On your hei you always get 20 degrees of mechanical advance regardless where you have it set. What determines your timing is like this. Say you have 10 degrees initial timing you will also get 20 degrees of mechanical advance timing added to that as the rpm increase. The rate of how quick you get your mechanical advance is by the spring rate. Lighter springs bring in the timing quicker and stiffer springs make it come in slower and way in the upper rpms. You also have vacuum advance timing that is added as well under part throttle and high vacuum conditions and comes from vacuum signal. I would get into all and how it all works but the only way to tell how much timing you have at idle is with a timing light. Mechanical advance will always be 20 regardless of how the distributor is setup unless under really high performance situations where you have to limit it in order to get more initial timing with out giving to much over advance timing.
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Old 08-17-2013, 04:53 PM
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How to limit or lock out the nech advance on a GM HEI distributor.
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Old 08-17-2013, 09:08 PM
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Quote:
Originally Posted by pjc360 View Post
Working on a 355cid small block chevy for a friend of mine, still waiting on hedders before I can fire it up. But I was hoping someone here could tell me if it's possible to determine how much mechanical advance is inside of an HEI distributer. Is there any numbers I can check out on the advance mechanism inside the distributer that can tell you how much it has?
The amount of mechanical advance in a GM HEI distributor varies by application. The amount of mechanical advance is controlled mainly by the shape of the cam in the center of the weights, and to a lesser degree by the shape of the weights themselves. The weight of the weights and the spring tension controls the rate of mechanical advance.

There is a chart (link below) that show the amount of advance that different cams and weights supplies, but the problem here is the testing was done in a CCW rotation Pontiac distributor. The best way is to make a timing tape or use a dial back timing light and see what your distributor has.

Weights and cams: http://www.chevelles.com/forums/showthread.php?t=327577

More on the GM HEI distributor including initial, mechanical and vacuum advance is here.

Last edited by cobalt327; 08-17-2013 at 09:20 PM.
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Old 08-18-2013, 10:45 PM
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Well I took the cap off of the hei distributer I am working with and both the weights have the number 60 stamped into them and the centerplate has 369 on it. And this is a clock wise small block hei distributer.
I have a dial back light that I can check the amount of mechanical advance with, but I am still waiting for hedders in the mail before I can fire it up.
So I am just tying to get an idea of what the mechanical advance is now so that I know how much initial timing I will be able to run.
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Old 08-19-2013, 05:58 AM
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Quote:
Originally Posted by pjc360 View Post
Well I took the cap off of the hei distributer I am working with and both the weights have the number 60 stamped into them and the centerplate has 369 on it. And this is a clock wise small block hei distributer.
I have a dial back light that I can check the amount of mechanical advance with, but I am still waiting for hedders in the mail before I can fire it up.
So I am just tying to get an idea of what the mechanical advance is now so that I know how much initial timing I will be able to run.
The way to best go about this is to set the initial to as much timing as the engine/cam/combo wants, then limit the amount of mechanical advance so the total of initial and mechanical advance equals 34-36 degrees (or whatever the engine needs). So don't limit the initial advance to just what the stock mechanical advance will allow- in many cases that will not give you enough initial advance unless you're running a very mild camshaft.

A page on this can be seen here.
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