Electronic Spark Timing (EST)

Operation
The Electronic Spark Timing (EST) distributor contains no vacuum or centrifugal advance and uses a seven-terminal distributor module. In addition to the connectors normally found on HEI distributors, the 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.
The EST system provides better control of exhaust emissions and improves fuel economy by optimizing spark timing. The ECM monitors information from various engine sensors and computes the spark timing changes accordingly. In case of EST failure, a backup spark advance system is incorporated in the module.