The purpose of vacumn advance is to give easier starting. In other words, the timing is retarded when there is no engine vacumn. After the engine starts to run there is manifold vac. and the distributor advances.It also retards the timing under heavy load conditions when the mainfold vacumn drops low.In the performance world more accurate,constant timing that relates to engine RPM is nessecary, so I prefer to use mechanical only. Its largely preferance and a person's use application of the vehicle when making a decision pro or con. Maybe this will help--Paul
