Compression ratio and displacement

phunk

CR has nothing to do with engine displacement.

Your displacement is the difference in area displaced by the piston from TDC to BDC multiplied by the number of cylinders, and thats it.

CR is is the ratio between that displacement plus the combustion chamber above it versus just the combustion chamber itself. The combustion chamber is often just measured in the cylinder head itself with the valves shut but also technically includes the bore x thickness of the headgasket and plus or minus how much the piston sits above or below the deck at TDC.

You can also consider dynamic compression ratio, such as how any fuel that makes it into the chamber in liquid form cannot be compressed therefore increases compression ratio while it is in liquid form. But 99.99% of the time anyone is talking about CR, they are talking about static compression ratio.

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