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What Do Octane Numbers Mean?

You probably visit a gas station every few days, but have you ever paid attention to octane numbers?

sign at gas pump

Photo: nsub1 (flickr)

Gas with an octane number of 87, when tested, acted just like the comparison mixture with 87% knock resistant fuel.

At the gas pump, you have a choice of octane numbers. 87? 93? What’s the difference?

To explain octane numbers, let’s take an imaginative leap into the internal combustion engine of a car.

Gasoline And Air

Inside the cylinders are combustion chambers where a mixture of gasoline and air burns. Normally, the spark plug ignites the mixture and the pressure from the resulting explosion pushes the piston down. However, if the fuel is not matched to the engine, the heat in the cylinder can ignite the fuel even before the spark plug fires.

This early ignition causes pressure waves that rattle the piston, making a knocking sound. It can also overheat the spark plugs, erode the combustion chamber, and make the car run inefficiently and roughly.

Antiknock Ratings

Octane numbers, or antiknock ratings, as they are called, tell you how resistant gasoline is to knocking. Gas at the pump has been tested in a lab, where it’s compared to two reference fuels. The first is very knock resistant, rating 100 on the knock resistance scale.

The second knocks easily and rates a lowly zero. The two are combined to make comparison mixtures. Gas with an octane number of 87, when tested, acted just like the comparison mixture with 87% knock resistant fuel. The higher the octane number, the more resistant to knocking.

If your engine doesn’t knock with lower octane gas, there’s no reason to spend more on higher octane. Since knocking depends partly on the engine’s compression ratio, some engines need higher octanes to prevent it. Owner’s manuals often specify the right octane for your engine, and your ears are a good measure, too!

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