Background
The fuel/air spark ignition systems of older, non-computer controlled gasoline internal combustion engines, consist of a mechanical device, known as a distributor, that triggers and distributes ignition spark to each cylinder relative to piston position—in crankshaft degrees relative to top dead center (TDC).
Spark timing, relative to piston position, is based on static (initial or base) timing prior to centrifugal advance. The distributor's centrifugal mechanical spark advancing mechanism allows the spark to occur sooner (relative to crankshaft degrees) as RPM increase. Many of these engines will use a vacuum advance that offers ignition advance during "deceleration" and/or low load "coast" time independent of the centrifugal advance. The above typically applies to automotive use. Marine gasoline engines generally use a similar system but with no vacuum advance.
In 1972, Chrysler introduced the electronic ignition system (as standard equipment on some production cars, standard across the board by 1973) that replaced the mechanical contact breaker ("points"). This provided for a stronger ignition spark, as well as virtually eliminating variations in ignition timing due to wear associated with breaker points.
Actual electronic control of ignition timing was introduced a few years later in 1975-'76 with the introduction of Chrysler's computer-controlled "Lean-Burn" electronic spark advance system. By 1979 with the Bosch Motronic engine management system, technology had advanced to include simultaneous control of both the ignition timing and fuel delivery. These systems form the basis for modern systems.
Read more about this topic: Ignition Timing
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