They're quick to modify their distributor's mechanical advance curve for maximum full-throttle performance, but often leave the original vacuum advance canister in place.

The throttle position system uses a vacuum can. At cruising speed, the manifold vacuum is high, which, when applied to the advance canister, advances the ignition timing.

Next month we will cover checking the centrifugal and vacuum advances, installing your distributor, all of the timing variables, EST distributor testing, and converting a points distributor to HEI.

As the distributor spins fast enough to activate the mechanical-advance weights, the engine receives initial timing, mechanical timing, and vacuum timing under part-throttle conditions.

There’s nothing wrong with connecting the vacuum advance to manifold vacuum if the engine needs this additional timing at idle but these applications are usually found with engines equipped with ...