Why octane rate must true for your engine

The term OCTANE refers to a fuel's ability to resist spark knock or detonation. The chemical properties of the fuel determine its combustion characteristics - the temperature and pressure at which it ignites and how quickly it burns. Higher octane fuels can withstand higher temperatures and pressures, and typically burn somewhat slower than lower octane fuels.

The least amount that's necessary to prevent detonation (spark knock). On most vehicles, that would be regular unleaded 87 octane gasoline. But on higher compression engines, or turbocharged or supercharged engines, the engine may require premium grade 91 to 93 octane fuel.

Detonation (also called spark knock) occurs when the octane rating of the fuel isn't high enough to handle the heat and pressure. Detonation is most noticeable when lugging the engine under load or accelerating. It may should like a pinging, clattering or rattling noise. Instead of a single flame front forming when the fuel is ignited, multiple flame front form spontaneously throughout the combustion chamber. These collide and produce shock waves that cause the noise. The hammer-like blows produced by detonation are very hard on the pistons, head gasket and bearings and may damage the engine if the problem is not corrected.

Other common causes of detonation (besides low octane gasoline) include: a faulty Exhaust Gas Recirculation (EGR) system, too much compression due to a build up of carbon deposits in the combustion chambers (use a top cleaner to clean the combustion chambers), a lean fuel condition (dirty fuel injectors), or engine overheating.

Most late model engines have a "knock" sensor to detect engine vibrations caused by detonation. When the sensor detects detonation, it signals the PCM to temporarily retard spark timing. This helps protect the engine from possible detonation damage, but it also reduces engine performance and fuel economy.