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The octane rating of fuel is a measure of its performance in engines. A higher octane number means that the fuel-and-air mixture can withstand more compression before detonating. Premature detonation can result in an engine making knocking, pinging or rattling sounds. High octane fuel, therefore, helps reduce or eliminate knocking, pinging or rattling in engines.
Which octane level is required for a particular engine is determined by its compression ratio; higher-compression engines require higher-octane fuel. For example, the engine in a basic sedan generally requires the standard octane fuel offered at gas stations, but a high-performance sports car or race car might require a high octane fuel. Most gas stations offer three octane levels of fuel: regular, mid-grade and premium. The owner’s manual lists information on the type of fuel that should be used in a particular car.
There are different number systems that are used to indicate the octane rating of fuel. In some countries, including Australia and many European countries, the octane rating used is the research octane number (RON), which reflects how the fuel performs under controlled conditions in a laboratory. Another system uses what is called the motor octane number (MON), which also is determined in a laboratory but uses engines running at higher loads and under greater stress. In many countries, including the United States and Canada, the octane rating that is used is the average of the RON and the MON. This is often represented by the equation R+M/2, or it might be called the pump octane number (PON) or the anti-knock index (AKI).
For most types of gasoline, the RON is higher than the MON. Some other types of fuel, however, have higher MON ratings or have ratings that are about equal. The standard octane fuel for most vehicles as of 2012 had a RON of about 92 and a MON of about 82, giving it an R+M/2, PON or AKI of 87. High octane fuel for cars typically is considered anything with a RON of 95 or higher, a MON of 85 or higher and a R+M/2 of 90 or higher.
Many people believe that high octane fuel will make a car go faster or get better fuel efficiency. This generally is not true, however, unless the engine knocks, pings or rattles when regular octane fuel is used. If the engine is functioning properly, then using a high octane fuel typically will not improve its performance, although some special types of high octane fuel can improve an engine's combustion efficiency. Using a low octane fuel, however, could causing knocking, pinging or rattling and reduce the engine's performance. If an engine makes those sounds when fuel of the recommended octane is being used, it probably needs a tune-up.