Engine Compression Ratio
Work out an engine's compression ratio from its cylinder dimensions.
How much the engine squeezes
An engine's compression ratio compares the cylinder volume when the piston is at the bottom to the volume when it is at the top — how much the fuel-air mixture is squeezed before ignition. This works it out from the cylinder dimensions, a key figure in engine building and tuning.
Enter the cylinder measurements to see the compression ratio.
Why the ratio matters to an engine
Compression ratio has a big effect on how an engine performs and what fuel it needs. A higher ratio extracts more power and efficiency from the same fuel, but squeezing harder makes the mixture prone to igniting early and knocking, which higher-octane fuel resists — so high-compression engines demand premium fuel. Too high for the fuel, and the engine knocks damagingly. This is exactly why fuel grades exist.
Frequently asked questions
Why do high-compression engines need premium fuel?
Because squeezing the mixture harder makes it prone to igniting early and knocking, which higher-octane fuel resists.
What does a higher compression ratio give?
More power and efficiency from the same fuel, at the cost of needing higher-octane fuel to avoid damaging knock.
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