Definition: The term compression ratio is used in reference to internal-combustion piston engines. It indicates the ratio of volume if a combustion chamber when the piston is at the top and bottom of the stroke.
On a 1000cc engine if the piston moved from bottom to top and compressed the air-fuel mixture down to 100cc it would be a compression ratio of 10:1. Even on a small engine can benefit from a high compression ratio, like the 2011 Yamaha Raptor 125's 10:1 CR.
Having a bigger compression ratio is usually better because the engine makes more power. The bigger the compression ratio the more possibility there is for knocking, or pinging. This can be dangerous for an engine, but if the engine is from 1996 or later, it already has a knock-sensor that will retard the timing.
