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Compression Ratio Vs Boost

 - High compression ratio means more pressure in the cylinders and more power.
 - On NA engines the higher the compression ratio possible, the more power.
 - On turbo engines a high compression ratio means better off boost power.
 - On turbo engines a low compression ratio means higher max boost is possible.

Every engine expert has their own ideas about the correct compression ratio to use for set levels of maximum boost. There are lots of mathematical formulae which try to provide a way of calculating the best ratios but none of them make adequate account of variable air density and temperature.

Alloy heads, long duration cams, 4-valves per cylinder all make higher compression a possibility. Cast heads, 2-valves, short cam durations, carbs and mechanical injection and mechanical ignition advance mean that a lower compression ratio must be used. Vehicle weight, aerodynamics, gearing all affect compression ratio choice.

Finally consideration must be given to the vehicles intended use – city driving, racing etc… The next factor is fuel economy. Higher ratios mean better fuel economy. For turbo cars, the compression ratio can be set so low that off boost performance is abysmal.

On competition engines, the rules can dictate what compression ratio is best to use. On group A engines the inlet is restricted to 34mm, so charge density rapidly decreased past max torque so a high compression ratio of 9:1 must be used.