Compression Ratio

The compression ratio compares the total volume in the cylinder when the piston is at the bottom of its stroke with the volume left when it is at the top (the combustion chamber volume). A ratio of 12:1 means the mixture is squeezed into one twelfth of its original space.

A higher compression ratio extracts more energy from the fuel, improving power and efficiency, which is why modern sports engines often run 12:1 to 14:1. The limit is knock (detonation), where the mixture ignites uncontrollably; high-compression engines therefore need higher-octane fuel and precise ignition timing.

Always use at least the fuel octane rating given in the owner's manual, usually RON 95 or 98 in Europe. Raising compression by skimming the head or fitting a high-compression piston is a common tuning step, but it increases engine stress and fuel requirements.