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Original Message
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| More to the point, Joel... |
| By Gerry Proctor - 05/24/2002 7:45:54 AM; IP 207.133.188.254 |
There are two sources for vacuum advance. 1-Ported vacuum and; 2-Manifold vacuum.
Ported vacuum is a tap below the carburetor's venturi and above the throttle blades. Vacuum here is nil at idle and inreases with throttle opening and diminishes with load. In other words, no vacuum advance at idle, a lot of vacuum advance under light cruise load, and little to no vacuum advance under engine load.
Manifold vacuum advance is a tap below the throttle blades and responds directly to manifold pressure...It rises and falls as does manifold pressure.
The factory used ported vacuum and in your particular case, thermostatically controlled ported vacuum. In almost all cases, it's best to leave the vacuum source as ported vacuum.
But it gets more complex since vacuum advance is only one of the three timing components. You have your initial, your centrifugal, and your vacuum advance. All three of these timing advance elements work together with the intent of giving you the highest cylinder pressure possible consistent with the engine design and available fuel. Well, the fuel that your engine was designed to run on is not readily available anymore so you're going to have to make some adjustments to reduce cylinder pressure. You do that through mechanical modification like cam and/or pistons or by tuning your advance.
You want to run as much initial advance as possible and work on your centrifugal and vacuum advance tuning. In almost all cases, the only difference in a factory performance distributor and the one they put in a station wagon is in the centrifugal advance curve. In the day your engine was made, you had a lot of centrifugal advance that came in very early in the rpm curve. The wagon's distributor had less centrifugal and it came in at higher rpm. In many cases, the wagon had nothing until over 2,000 rpm and full centrifugal advance at well over 4,000 rpm. Most vacuum advance cans contribute from 15 to 20 degrees of advance but isn't referenced in the total advance you often see written in car magazines. The only way you're going to understand what your centrifugal advance is doing is to have a fully-degreed balancer or use a dial-back timing light. If you feel comfortable enough with removing and disassembling the distributor, you can tune your centrifugal advance yourself, otherwise you'll need the help of a professional tuner. Essentially you want to slow down the centrifual advance and in many cases, limit it as well. Remember, you want as much initial advance as you can run so if your optimal advance curve puts your total advance at 36-degrees and you're running 12-degrees initial, your centrifugal limit is 24-degrees. Vacuum advance is, again, not part of the picture.
You can tune your advance curve to accomodate your engine on today's fuels, but it will not perform up to factory power figures. The ultimate solution is to use lower compression pistons, update the cam, and tune your advance curve. You'll produce way more power with a complete package, but that's not always practical. |
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