The geometric ratio
The compression ratio is the cylinder volume with the piston at the bottom of the stroke divided by the volume with the piston at the top. The top volume is the combustion chamber, the gasket thickness, the deck clearance and any dish or dome in the piston. Change any one of those and the ratio changes. A skimmed head, a thinner gasket and a domed piston are not three separate upgrades. They are three ways of reducing the same clearance volume, and they stack.
NASA's ideal compression relation shows why the number matters. For a reversible adiabatic compression, the pressure ratio is the volume ratio raised to gamma, and the temperature rises with the pressure. Gamma for air at standard conditions is given as 1.4. A smaller clearance volume therefore produces a larger pressure and temperature at the end of the stroke, before any fuel has released heat. A real engine is not that ideal process, but the direction is the same: more geometric compression makes a hotter, higher-pressure end-gas.
Efficiency and knock
That higher pressure is the reason a higher ratio can make more work from the same fuel. It is also the reason knock arrives sooner. The end-gas temperature BMW discusses for a turbocharged engine is this temperature plus the heat the compressor has already added. Their water-injection account is explicit that full-load knock is what stops the compression ratio being raised, and that cooling the charge is one way to move that limit. See charge cooling and ignition timing.
A naturally aspirated engine that is not knock-limited can often use a higher ratio than a boosted engine built from the same parts. The boosted engine starts the compression stroke from a higher pressure and temperature. Copying a compression ratio from one to the other, or from a different fuel, is how the margin disappears. Octane and mixture, covered in fuel system upgrades, are part of the same decision.
Measuring it
The ratio is a volume measurement, not a skim depth. Chamber volume, piston dish or dome, gasket volume, bore and stroke all enter it. A deck that is not square, or a piston that sits at a different height from the one assumed, changes the result. So does a change of stroke, because the swept volume changes while the clearance volume may not have been altered to suit.
On an ECU that models the engine from the original geometry, a different compression ratio changes both the knock behaviour and the estimated cylinder filling. The calibration has to be checked against the engine that was actually built. Diesel engines are less knock-limited in the petrol sense and more limited by peak cylinder pressure, bearing load and the injection timing. Raising the ratio there is a mechanical limit as well as a combustion one.
Sources
- Isentropic compression or expansion. NASA Glenn Research Center. Pressure and temperature against volume ratio, and gamma of 1.4 for air. Accessed 2026-11-11.
- Water injection. BMW M. Compression ratio, full-load knock and charge cooling. Accessed 2026-11-11.