A compression ratio is a measure of how much the engine compresses the air/fuel mixture on the compressions stroke. The ratio is the volume of the clyinder when the engine is at TDC(top dead center) over the volume of the clyinder when the motor is at BDC. The engine has 4 strokes. They are in this order intake, compression, power and exhaust. The stroke is hte movement of rocker arm, crank and piston in the clyinder. Hope this helps
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