Octane

Octane is a measure of a fuel's ability to resist spark knock. Spark knock occurs in the combustion chamber just after the spark plug wires, when the air/fuel mixture in the cylinder does not completely burn. The remaining mixture spontaneously combusts due to the temperature and the pressure. This secondary explosion causes a vibration that is heard as a knock (ping). A fuel with a high octane number has a higher resistance to spark knock. This vehicle requires 91 octane ([R+M]/2 method) in order to ensure the correct performance of the fuel control system. Using fuel with an octane rating of less than 91 can create spark knock, which would cause the PCM to retard the ignition timing to eliminate the knock. In a case such as this, poor engine performance and reduced fuel economy could result. Also, in severe knock cases, engine damage may occur.