Compression Ratio as Performance Modification


When considering an engine rebuild or just looking for some additional performance, the first thought is often to increase the compression ratio. It's generally accepted that sports and race engines have higher compression ratios because, provided the volumetric efficiency is optimized, i.e. you can get enough mixture in, it will produce more power with better throttle response.

There are two types of compression ratio, static and dynamic. The latter is also known as running compression ratio and is dependent on camshaft design and the actual point that the inlet valve closes on the compression stroke. It is therefore lower than the static compression ratio.

Increasing the compression ratio is limited by several mechanical factors, including cylinder head material, surface area of the fins and combustion chamber design, though these days the main problem is the octane of the fuel.
Once the compression ratio reaches a certain point, detonation occurs, which can literally wreck an engine in seconds. Greater compression ratios produce increased cylinder pressures, which produce higher inertia loadings on engine components, thereby reducing engine longevity.

It is evident that with any particular fuel there is a limiting ratio that cannot be exceeded without the risk of piston trouble, but the ratio that the engine will tolerate is frequently only slightly lower. An engine causing concern at 9:1 may run perfectly at 8.8:1. To some extent detonation due to excessively high compression ratios can be suppressed by retarding the ignition and richening the mixture, but this will also result in loss of power. There is nothing to be gained in running excessively high compression ratios balanced against the risk of serious engine damage.
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