The centrifugal advance mechanism adjusts spark timing according to changes of engine speed, while the vacuum advance unit adjusts spark timing in relation to engine load. The two devices operate independently but the total spark advance is dependent on both.
An engine operating at 1500 rpm may be running under very light load with the throttle open only a small amount. This would be true if the car were being driven at a fixed speed on a level road. On the other hand, at the same speed, the engine may be under full load, as would be true if the car were being driven up a steep hill with the throttle wide open. In the first example, engine speed at 1500 rpm, engine under light load -the manifold vacuum would be high. This would operate the vacuum advance and the total spark advance would be the sum of the centrifugal advance plus the vacuum advance. In the second example, at the same engine speed but operating under full load, very little vacuum exists in the manifold. Because very little vacuum is available, the vacuum advance unit would return to its retarded position. Therefore, the total spark advance would depend on the operation of the centrifugal advance unit only.
Under any operating condition, the total spark advance is dependent upon the' speed of the engine and on the load under which the engine operates. Thee higher the speed, the greater the advance, and also, the lighter the load, the greater the advance. At any speed, the load on the engine may vary from zero to full load depending on the position of the throttle plate. When the engine is operating at high speed, the throttle will be open which results in low manifold vacuum. Spark advance is controlled entirely by the centrifugal advance mechanism at this time.
In any event, the total spark advance at any engine speed is the initial advance, plus the centrifugal advance, plus the vacuum advance.
Copyright 1967 Ignition Manufacturers Institute Printed in USA 197