Distributor Advance Unit: Description and Operation

Centrifugal Advance:

Centrifugal advance of spark timing, with increasing engine speed, is achieved by rotation of the rotor and armature shaft in relation to the distributor shaft. The rotor shaft is driven by the distributor shaft. The two are connected through centrifugal flyweights, pivoted on a plate which is an integral part of the distributor shaft. Drive pins on the flyweights engage slots in a plate at the base of the rotor shaft. Springs pull the flyweights in toward shaft center when at rest and at low speed, holding the rotor shaft in the base timing or full-retard position. As engine speed increases, centrifugal force causes the weights to pull outward against the springs. At 1050 rpm they begin moving the rotor shaft in the advance direction.


The dual-diaphragm vacuum advance mechanism acts on the stator plate which is free to rotate in the distributor housing. The larger diaphragm chamber is connected to the spark port on the carburetor, just above the throttle at the idle setting. The smaller diaphragm chamber is connected to the intake manifold, below the throttle. The resultant advance response is based on throttle position, engine speed, and engine load.

High Altitude Spark Advance:

The high-altitude spark advance correction is all-electronic, and does not affect the relative positions of armature or stator. The ignition module goes into high-altitude mode when barometric pressure drops below 670 mm-Hg (26.4 in-Hg). On carbureted vehicles, barometric pressure is calculated using input from a manifold absolute pressure sensor mounted on the bulkhead to the right of the steering column. EFI vehicles are equipped with a barometric pressure sensor housed in the ECA. The normal delay circuitry of the ignition and engine control modules is then bypassed to advance the initial ignition timing to approximately 6° BTDC.