An electronic device was constructed which, when used with the breakerless transistorized ignition system now common on U.S. cars, allows the octane requirements of the individual cylinders of a multicylinder engine to be determined. Basically the instrument functions by retarding the spark timing of all cylinders except the one being rated. A detailed schematic diagram of the Selective Ignition Retard Device (SIRD) is included in the paper to allow construction of the unit by interested parties. SIRD has been employed in studies aimed at elucidating the factors affecting octane requirements of engines. Some typical test results are presented.