How Much Gasoline Octane Rates

One way to prevent detonation is to use a higher octane fuel. The octane rating of a motor fuel is a measure of its detonation resistance. The octane that's posted on the filling station pump is "pump octane," which is an average of something called "research" and "motor" octane ratings (which are two different laboratory methods of measuring octane). The higher the pump octane number, the better able the fuel is to resist detonation.

A gasoline's octane rating depends on the blend of hydrocarbons in the fuel and other ingredients that are added to it. Tetraethyl lead was long used as an anti-knock additive to improve gasoline octane. In fact, it was the most effective and least expensive octane-boosting additive that could be used for this purpose. But leaded fuel cannot be used in a vehicle with a catalytic converter because the lead fouls the catalyst. So unleaded fuels in the U.S. were phased out back in the early 1980s.

Most regular unleaded gasolines today are rated at 87 octane (or less in higher altitude locales), which is sufficient for engines with compression ratios of up to about 9 to 1, and up to 10 to 1 compression on some newer engines. Higher compression engines, engines with turbochargers or superchargers, or ones used frequently for towing usually require a higher octane premium grade of gasoline (91 octane).

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