The octane number of a gasoline is a measure of its ability to resist detonation during the combustion process in the cylinder of an engine. The octane required by an individual engine is subject to many external variables such as atmospheric pressure, relative humidity, and air temperature. Important engine design and operating factors which influence octane requirements are the spark timing, intake manifold pressure, water jacket temperature, air-fuel mixture ratio, combustion chamber design, including valving, head geometry, and spark plug location, and presence of combustion chamber deposits.

Octane numbers of gasoline are measured in the laboratory using a single-cylinder engine. Depending upon the set of operation conditions of this engine, the Motor Octane Number (MON) or the Research Octane Number (RON) is measured by comparing the gasoline sample against a series of reference fuels. There is no alternative means of measuring the octane numbers other than by the single-cylinder engine. Both MON and RON are measured for Sunoco Racing Gasolines.

When applying these laboratory octane numbers to automotive engines, it is found that in some cases RON better predicts the performance of the fuel while the other cases the MON is more important. Many factors influence this, but engine design and operating conditions are the most important.

For racing engine applications the MON is generally considered to be the most important. Over the years, the Octane Index, defined as the average of the RON and the MON, or (R+M)/2, has evolved as the accepted predictor of the fuel’s performance in passenger cars.