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
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