[mreid]

Not quite. While mechanical and vacuum advance both advance the spark, they operate independently.

There are actually three advances at play: base, mechanical, and vacuum.

Base: is the advance in degrees before TDC (Z1) at idle.

Mechanical: advances timing bases on rpm. Adds to base on a predetermined rate with a maximum. The rate and max are determined by springs, weights, and bushings in the distributor.

Vacuum: advances timing based on engine vacuum. Since vacuum is highest under cruise, advance serves to optimize fuel combustion hence gas mileage. Most vacuum advance uses a "ported" signal. This means the vacuum signal is above the throttle plate(s) and only sees vacuum above a certain rpm. This keeps vacuum advance at zero at idle.

EFI uses the computer controlled spark to apply an exact mix of base, mechanical, and vacuum for every engine operating condition.