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Pressure Volume Cycle

EFI Tuning Fundamentals

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Id just like to get some clarity here on the comparison between higher and lower compression ratio.

In simple terms

a) does lower pressure in the cylinder on the intake stroke of a higher compression ratio mean it will pull more air in to the cylinder than a lower compression ratio?

b) What are the pros and cons of a high compression ratio over a low compression ratio, and are there any benefits of a low compression setup


Compression ratio has no direct effect on how much air the engine can take on intake stroke although on high compression engines the piston to valve clearance can dictate the camshaft lobs profile and that in its turn can effect on valve timing events and the amount of air entering engine at specific RPM.

High compression engines have better thermal and mechanical efficiency but prone to knock and overheating, requiring high octane fuels to produce better torque and power over low compression engines. They also have better fuel economy.

Low compression engines produce less power and torque but are very forgiving for low octane fuels. Also they allow very high boost level to be used on forced induction applications.

Thank you for your reply,

Do you have a specific engine in mind, or is it just a general inquiry?