Compression Ratio

Introduction

"Compression Ratio" is the term used to describe how much a piston can compress the air within its cylinder.

Static Compression

Static compression, otherwise known as calculated compression, is the rating which most companies use to advertise their product. It is a purely mathematical rating and should only ever be used as a general guide. A properly designed piston will have a true compression ratio within 15% of the static (calculated) compression evenly throughout the cylinder.

Caculating

Calculating the static compression is quite simple. You take the volume of the cylinder + combustion chamber at full piston stroke BDC (Bottom Dead Center), then divide by the volume of the cylinder + combustion chamber at full piston stroke TDC (Top Dead Center).

Exempli gratia: a cylinder volume at BDC is 400 cc (cubic-centimeter of volume). At TDC it is 40 cc. Therefore the compression ratio is 10:1. This is because it compresses 10 cc into 1 cc.

True Compression

Static compression is rarely the same as true compression -- otherwise known as dynamic compression. This is due to a few factors, such as engine efficiency and how the combustion chamber and piston interact.