Because varying octane levels in gasoline can cause detonation (spark knock) in any engine, some of the fuel injected engines covered by this guide which are equipped with HEI (EST) are also equipped with an Electronic Spark Control (ESC) system. Spark knock is a condition caused by temperatures inside the cylinder rising so high as to ignite the air/fuel mixture prior to the spark plug firing. This early ignition causes a down force on the piston as it is rising in the cylinder toward TDC. In light cases, the only damage to the engine may be broken spark plug insulators. In extreme cases, pistons may become severely damaged (holes blown through the top of the piston) requiring the engine to be rebuilt.
The ESC is designed to detect detonation and retard spark timing up to 20° to reduce or eliminate detonation in the engine. This allows the engine to use the maximum possible spark advance the rest of the time (when there is no knock), therefore improving driveability and fuel economy. For more information regarding the ESC system, please refer to Engine Performance & Tune-up of this guide.