Advance and retard mechanisms in fuel systems with a distributor are used to vary the ignition timing to ensure optimum fuel combustion under different engine operating conditions.
The centrifugal advance mechanism rotates with the distributor shaft inside the distributor housing and advances ignition timing as the engine speed increases.
Ignition advance in relation to engine load is catered for by a vacuum advance unit which has a vacuum line connection to a port on the intake throttle body.
In a distributorless system, the ECU determines the optimum timing for ignition, based on input from various sensors.
