Octane number is a measure of a gasoline's ability to resist knock or pinging noise from an engine. In older vehicles, knock may be accompanied by engine run-on, or dieseling. Knock is the sharp, metallic-sounding engine noise that results from uncontrolled combustion. Severe knocking over an extended time may damage pistons and other engine parts. If you can hear knocking, you should have your engine checked or use a higher octane gasoline. In most vehicles no benefit is gained from using gasoline that has a higher octane number than is needed to prevent knock. However, in some vehicles equipped with a knock sensor where the car's computer controls the engine to reduce or eliminate knock on lower octane grade fuels, a higher octane number gasoline may improve performance slightly if the cars's computer is controlling the engine to reduce or eliminate knock on lower octane grade fuels.