Engine Displacement

Displacement is the volume swept by every piston as it travels from top dead centre (TDC) to bottom dead centre (BDC). Each cylinder is treated as a plain cylinder of diameter $B$ (bore) and height $S$ (stroke).

• $B$ is the bore and $S$ the stroke. With both in millimetres, divide by 1000 to get cubic centimetres.
• $N$ is the number of cylinders.
• $1\ \text{L} = 1000\ \text{cc} = 61.02\ \text{cu in}$.

Bore-to-stroke ratio

An oversquare engine ($B/S > 1$) has room for big valves and keeps piston speed low, so it likes to rev. An undersquare engine ($B/S < 1$) has a longer crank throw, which tends to favour low-rpm torque and a compact combustion chamber. A square engine sits in between.

Because bore is squared in the formula, a small overbore adds more capacity than the same increase in stroke: 1 mm on an 86 mm bore is worth about 2.3 % more displacement.