The arithmetic is straightforward. Take the volume one cylinder's piston sweeps as it travels from the top of its stroke to the bottom, then multiply by the number of cylinders. That total is the displacement, written as litres when the figure is round enough to be readable and as cubic centimetres when precision matters. A badge reading two litres is that sum and nothing else.
What it does not tell you is output. Displacement sets how much air and fuel an engine can draw in per cycle unassisted, which used to make it a decent proxy for power, but forced induction broke that link. A small engine with a turbocharger pushing extra air in can comfortably out-produce a larger naturally aspirated one, which is why modern figures and modern badges no longer track each other.
It is still a useful number, just not the one people often take it for. It describes the physical machine rather than the performance, in the way that torque describes force rather than speed.
Written by Craig Fearn, Petrol & Ink. Last updated .