In this study, an investigation of the transient knock characteristics in a four cylinder DOHC spark-ignition engine has been carried out. We simulated the behavior of an engine during the rapid acceleration of a car using an eddy current type dynamometer. In a commercial ECU, spark timing was retarded from the steady state value during rapid acceleration to prevent uncomfortable feeling and knock. Knock usually occurred just after the start of acceleration when spark timing was advanced from the value set by the commercial ECU. We found that air/fuel ratio was about stoichiometric for a knocking cycle. Transient knock control logic was developed which used air/fuel ratio information of an O2 sensor as a control input. Torque was improved during rapid acceleration by using the transient knock control.