Bob,
Vacuum advance helps fuel efficiency because cylinders do not fill with air/fuel very well when intake manifold vacuum is high. Under these conditions, the cylinders can use additional ignition advance in order extract the most work out of the combustion process. The idea is to time the occurence of peak combustion pressure during the proper position of the crankshaft as it rotates.
That's a fancy way of saying what you already knew, right?
You are crusing in your Pantera in 5th gear on the interstate. You're doing 70 mph or thereabouts. The motor is turning 3000 rpm. Your centrifugal advance is fully advanced. If your static advance is 16 degrees, and your centrifugal advance is 20 degrees, then at 3000 rpm you are at 36 degrees. But cruising at 70 mph, your foot is barely into the throttle, intake manifold vacuum will be high which will also operate your vacuum advance, which means you may have an additional 10 degrees of advance, or in other words, 46 degrees total under those conditions. But this is OK, because with that high intake manifold vacuum and lightly filled cylinders, your motor needs that additional advance to properly time the ignition of the air fuel mixture.
your freind on the DTBB
