All 1981 models use EST. The EST distributor, as described in an Engine Performance And Tune-up , contains no vacuum or centrifugal advance mechanism and uses a seven terminal HEI module. It has four wires going to a four terminal connector in addition to the connectors normally found on HEI distributors. A reference pulse, indicating engine rpm is sent to the ECM. The ECM determines the proper spark advance for the engine operating conditions and sends an "EST" pulse back to the distributor.
Under most normal operating conditions, the ECM will control the spark advance. However, under certain operating conditions such as cranking or when setting base timing, the distributor is capable of operating without ECM control. This condition is called BYPASS and is determined by the BYPASS lead which runs from the ECM to the distributor. When the BYPASS lead is at the proper voltage (5), the ECM will control the spark. If the lead is grounded or open circuited, the HEI module itself will control the spark. Disconnecting the 4-terminal EST connector will also cause the engine to operate in the BYPASS mode.