Electronic Spark Timing (EST)

The Electronic Spark Timing (EST) distributor contains no vacuum or centrifugal advance mechanisms. In addition to the connectors normally found on HEI distributors, the HEI-EST distributor also has four wires going to a four terminal connector. A reference pulse, indicating engine RPM and crankshaft position is sent to the ECM. The ECM determines the proper spark advance for the engine operating conditions and sends an "EST" pulse to the distributor.

In the case of EST failure, a back-up spark advance system is incorporated in the module.