Distributor Advance Unit: Description and Operation

Fig. 15 "Regular Fuel" Ignition Map:

Fig. 16 "Premium Fuel" Ignition Map:

The ignition timing is determined by the ignition control unit based on inputs from various sensors. The basic timing is calculated according to;
Engine speed
Engine load
Ignition timing is advanced as engine speed increases, and the amount of advance is reduced as engine load increases.
Two different timing maps are stored in the control unit memory, one for regular (low octane) fuel, Fig. 15, and one for premium (high octane) fuel, Fig. 16.
The engine will develop the same horsepower when operated with either fuel, but will develop slightly greater torque when high octane fuel is used.
The ignition control unit will initially run with the premium timing map whenever the engine coolant is greater than 65°C (149°F). If the knock sensor senses detonation ("pinging") repeatedly, the ignition control unit will automatically switch to the timing map for regular fuel. The ignition control unit will switch back to the premium fuel timing map after a certain amount of time, or when the engine is restarted.

Fig. 17 Ignition Advance Graph:

If the knock sensor detects detonation in a cylinder, the ignition control unit will go through a set of pre-programmed steps to keep the timing as advanced as possible, Fig. 17. The timing is retarded in 3.4° increments (up to a maximum of 12°)until the knocking stops, then advanced again in 0.56° steps until the timing value is back within the normal timing map, or detonation is detected. If the timing is retarded to the 12° limit and the knocking continues, the engine warning light in the instrument cluster will light, telling the driver to ease off the throttle and reduce the engine load.