Ignition Timing: HEI-Est

All models are equipped with a High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains a 7-terminal HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to the external electronic control module (ECM).

When engine speed reaches about 400 RPM, the ECM transmits a constant 5-volt signal to the distributor HEI-EST module. This activates the HEI-EST module by-pass circuit, switching spark timing control from the HEI module to the ECM.

The PROM unit in the ECM has a basic spark advance curve built into it. Engine sensor values are used by the ECM to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions.

Spark timing is calculated by the ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking).