Vacuum Advance
Library.VacuumAdvance · r1.1 · · blundar
Vaccuum Advance is a purely mechanical method for varying the amount of Spark Advance based on engine load. A vac-advance distributor is connected to a source of engine vacuum. As vacuum increases, forces of suction rotate the distributor, creating more ignition advance.
The ECU cannot control timing at all with a vacuum advance distributor.