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Compression ratio

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals

Page 1

My question in reference to compression ratio, specifically in relation to turbocharged engines. In this lesson you elaborate on the principle of compression ratio and how it affects the cylinder pressures both at spark propagation and what’s left over after the combustion event. Theory being that the lower compression ratio will result in higher left over cylinder pressures due to slower piston decent in the cylinder bore. That being said does that extra left over pressure have any benefits to a turbocharged engine? Higher cylinder pressure during the exhaust stroke, thus creating a larger delta across the turbine? I understand the lower compression ratios in forced induction engines, as a means of preventing detonation, however I’m curious if that extra cylinder pressure after combustion may have a benefit as well

Hi Ian, interesting point you raise. First up a small correction - It's the larger expansion ratio involved with a higher compression ratio that results in a lower cylinder pressure when the exhaust valves open. the piston speed is a function of the stroke and rpm, not CR. A higher exhaust pressure early on during the period where the turbo is spooling up can potentially offer an improvement, this will end up being problematic at higher rpm where the exhaust pressure becomes excessive. You can actually achieve a similar effect by adjusting the cam timing for the exhaust cam.