As mentioned previously, the timing curve on pre-1990 automobiles was generated using a combination of vacuum advance and centrifugal advance. The centrifugal advance system in these engines generates the timing curve using a series of weights and springs attached to the shaft of the distributor which rotate outwards as the speed of rotation of the distributor increases (with increasing engine rpm). This force is used to rotate the distributor baseplate (and thus the relative position of the rotor) in such a way as to change (advance) the timing of the spark.
The vacuum advance system senses the vacuum in the air intake system, and uses this information to control the relative position of the rotor in the same manner as described above (via the distributor baseplate). However, unlike the centrifugal advance, the vacuum advance is designed to respond to the change in engine loads, rather than rpm.
Both systems are used in conjunction in order to control the timing parameters over the entire range of engine operating conditions. Essentially, the centrifugal system is responsible for generating the correct timing curve, while the vacuum advance system makes allowance for differing engine loads by shifting the curve accordingly.
Thus, to accurately control the timing of an engine, it is necessary not only to control the timing curve, but also to sense the vacuum, and be able to adjust the curve accordingly. This is taken into account on all modern ignition systems. Although we have not made direct provisions for this in our current design, we have allowed for the future incorporation of this feature. Until then, the user will have one of two options: