Abstract: An automotive engine control system is provided for an automobile having a variable rotation speed engine which has a plurality of spark plugs served by respective spark plugs. The system includes an optical rotary encoder having a rotatable pulse scale having a row of code pattern with specific light-permeable or light-deflection slits each for outputting a reference position signal and other light-permeable or light-reflection slits each adapted to reduce the amount of light for outputting an angular signal, both of the optical signals being photoelectronically converted and the waveform of the resultant electric signals being shaped based on threshold voltages into electric pulse signals indicative of a reference angle of 0.degree.