A 1975 California model automobile with an 8:1 C.R. 350 CID engine was modified by increasing the compression ratio to 9:1 which resulted in improved fuel economy. The higher NOx emissions were reduced to the base level by substituting a back pressure-controlled EGR unit for the original valve and increasing the EGR flow. Octane requirement was controlled by a knock sensor-actuated spark timing retard system. The knock sensor (accelerometer) is attached to one of the cylinder heads of the engine. When knock occurs, the vibration is picked up by the sensor, the signal is filtered to remove some of the engine background noise, and the knock pulse is detected. When the amplitude of the detected knock signal exceeds a threshold, the spark timing is retarded. When no knocking is detected over a waiting period, the timing is advanced back to its normal schedule. Using this system, the vehicle's octane requirement can be lowered several numbers with some performance debit, i.e., slower acceleration times.