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The compression ratio is the key to understanding the suck, squish, bang, blow equation

A common relationship every internal combustion engine shares is that of the combustion chamber and static compression ratio. The static compression ratio refers to the swept volume of the cylinder (displacement) plus the volume of the combustion chamber when the piston is at Bottom Dead Centre (BDC), divided by the volume of the combustion chamber at Top Dead Centre (TDC).

It can be explained in simple terms this way, for example – if the piston in your engine’s cylinder is at BDC, you might be able to pour 1200ml of liquid into the cylinder (including the combustion chamber). With the piston at TDC, you would only be able to pour 120ml of liquid in. By calculation, that would make your compression ratio 12.0:1. but what does it actually matter?

Get the low-down on compression ratios in the latest issue…