Vacuum advance is a fuel economy thing. When an engine is running at steady RPM, light throttle, such as level-road highway cruising, the ignition timing can be advanced about 10 degrees or so beyond the "best-power" setting that the centrifugal advance provides for the same RPM range. When even a slight load is applied, such as hill climbing or passing, the vacuum drops off enough to kill the advance, and the engine goes back to making power, not gas mileage. Deliberately running without vacuum advance will cost the average engine 2 or 3 MPG at light-throttle cruise by being unable to advance the timing into the good gas mileage range. OK for racing, lousy for street cruising. For instance, small block V-8's make their best full-throttle power at around 36-37 degrees of advance, while best fuel economy happens at 45-48 degrees with a light, steady throttle setting and minimum load. The vacuum advance provides that extra timing when it's appropriate to have it.
Jerry
