Compression Ratio
The ratio of the maximum volume in the cylinder when the piston is at the bottom to the minimum volume when it is at the top.
The compression ratio is a fundamental engine specification that measures the ratio of the volume of the combustion chamber when the piston is at the bottom of its stroke (Bottom Dead Center) to the volume remaining when the piston is at the top of its stroke (Top Dead Center). It indicates how much the air-fuel mixture is compressed inside the cylinder before ignition.
A higher compression ratio increases the thermal efficiency of the engine, allowing it to extract more energy from the fuel and produce more power and better fuel economy. However, higher compression ratios also increase cylinder temperatures and pressures, raising the risk of pre-ignition and engine knocking (detonation). For this reason, high-compression engines require higher octane fuel to prevent premature ignition.
Symptoms of compression ratio mismatches (often due to installing incorrect pistons or skimmed cylinder heads during a rebuild) include persistent engine knock, poor performance, or starting difficulties. If an engine has lost compression, it is diagnosed using a compression gauge, which measures the pressure built up in the cylinder during cranking to verify the sealing of the valves and rings.