One of the strokes in the 4-stroke gasoline engines is the compression stroke, where the engine compresses air and fuel mixture into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio. For example a typical engine might have a compression ratio of 9:1.

The octane rating of gasoline tells you the amount that the fuel can be compressed before if spontaneously ignites. When gas ignites spontaneously without the spark plug, it causes knocking in the engine. Knocking damages the engine, so it is important to always use high enough octane gasoline.

The compression ratio of your engine, the shape of the combustion chamber, and the position of the spark plug determines the octane rating of the gas you must use in the car. Basicly 95 octane gasoline is gasoline that contains 95% octane and 5% other combination of fuels, but in real life the amount of octane is not the same as the octane rating.

For example with lead the gasoline's octane rating can be increased even over 100, but since lead is very toxic, it is banned.

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