Vacuum advance is not an either/or situation with centrifugal. At idle, there is NO vacuum advance at all, because there's no ported vacuum at the carburetor port. At steady throttle cruise, the vacuum advance ADDS about 8-10 degrees MORE advance than the centrifugal advance provides. If the throttle is opened for acceleration, passing, hill climbing, etc., the vacuum on the diaphragm goes away and the only advance is centrifugal. Retarding the spark a few degrees during acceleration prevents detonation from over-advancing under load.
Initial advance, centrifugal advance, and vacuum advance add up to give a total advance number at light-throttle cruise.
Jerry
