Octane Tests

Octane testing measures the compression ratio at which a gasoline will knock. The octane number is an average of the Research Octane Number (RON) and the Motor Octane Number (MON). Both tests use a single-cylinder variable compression-ratio test engine to compare a fuel sample to a known mix of two liquid hydrocarbon fuels (isooctane and normal heptane). Isooctane is arbitrarily rated at an octane number of 100 and normal heptane is rated at zero. A test sample of 87 percent isooctane and 13 percent heptane receives an octane number of 87. Any gasoline sample which matches the laboratory sample's performance in the engine will be rated 87 octane.