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Timing Curves

From: Hank Dozier
Remote Name:
Date: January 23, 2003
Time: 06:56:48


The advance mechanism in the distributors consists of two sections. The vacuum advance (timing versus engine vacuum, also called "load" advance) is controlled by the vacuum "motor" diaphragm unit on the side of the distributor. It has a spring inside it of a certain "K" factor which controls position of the arm versus vacuum. There is a limit stop on it to set maximum advance (look on the arm, it will have a number like 9.5, 11.0, etc). This is the total avance in DISTIBUTOR degrees, so multiply by 2 for crank timing. The vacuum unit also has a preload on it to prevent advance below a certain vacuum (called the "takeoff or breakaway" setpoin in Engineering-ese). The othe part of the distributor advance is the centrifugal advance (also called mechanical or sometimes governor advance). This consists of two flybob weights with slots in them which connect the upper breaker point cam to the lower distributor drive through a pivot point and a pin in the slots. As the weights fly out with increasing speed (centrifugal force), the cam is rotated to advance it by the weight slots traversing the pin. There are two springs in the system, one light and one heavier, and these provide the resistance to the weights flying out and therefore create the centrifugal advance curve, which generally has a knee in in (where the lighter and heavier springs start to work together). The advance curve is a function of the springs, the weights of the flybobs themselves, and is limited by the lentgh of the slots or a positive mechanical weight stop in some distributors. As I mentioned in my earlier post, if you only stick in the reluctor and the base advance plate, and keep the old vacuum unit and old weights/springs, then the old distributor looks original, acts original from an advance curve standpoint, but now uses the electronics for the spark plug coil control. The only thing that is otherwise different is that DWELL is now controlled electronically through the breakerless box, and is more precise than the old point unit. Hope this explanation helps.


Last changed: July 19, 2018