Vacuum advance is mostly a fuel economy thing, and it's only needed at cruise RPM or light-throttle applications where the timing can be advanced 5 to 10 degrees over the centrifugal advance. Centrifugal is directly related to RPM, up to the point it hits its mechanical limits. As the throttle is opened above idle, intake manifold vacuum is applied to the ported vacuum hole, and the advance diaphragm is pulled in the "advance" direction against a spring. If the throttle is opened quickly, or it approaches full throttle, the vacuum on the port drops, and the vac. advance spring retards the spark a few degrees to prevent detonation during acceleration. Total timing at light-throttle cruise is a combination of initial advance, set with a timing light, centrifugal advance, related to RPM, and vacuum advance, related to engine load and manifold vacuum. It's not uncommon to see 45 degrees or more of total advance at cruise, dropping to 36 or so as a passing maneuver is made at highway speed. Most engines max out on centrifugal advance by about 2500 RPM or so.
Jerry
