What Is Engine Displacement?
Engine displacement is the total volume swept by all of an engine's pistons as they travel through their cylinders. It is determined by the cylinder bore, piston stroke, and number of cylinders.
Displacement is commonly expressed in cubic inches (CID) for many American engines, while cubic centimeters (cc) and liters are widely used for both automotive and motorcycle engines. A 5,000 cc engine, for example, has a displacement of approximately 5.0 liters.
Bore and Stroke
Bore is the inside diameter of the cylinder, while stroke is the distance the piston travels from top dead center to bottom dead center. Because cylinder volume depends on the square of the bore diameter, even a relatively small increase in bore can produce a noticeable change in displacement.
Changing either bore or stroke changes displacement. This is why boring cylinders oversize during an engine build or installing a crankshaft with a different stroke can result in a displacement different from the engine's original specification.
Oversquare, Square, and Undersquare Engines
An engine is described as oversquare when its bore is larger than its stroke. If bore and stroke are approximately equal, the engine is considered square. When the stroke is longer than the bore, it is described as undersquare.
These terms describe the engine's basic bore-and-stroke geometry. They do not by themselves determine horsepower, torque, RPM capability, efficiency, or how an engine will perform.
Actual Displacement vs. Engine Names
The displacement used in an engine's name isn't always its exact calculated displacement. Manufacturers and enthusiasts commonly round displacement when describing an engine in liters or cubic inches.
For engine-building calculations, use the actual bore and stroke measurements rather than relying on the engine's advertised or rounded displacement.
Measuring a Modified Engine
When calculating the displacement of a rebuilt or modified engine, use the finished cylinder bore rather than the original factory bore if the cylinders have been enlarged. Likewise, use the actual crankshaft stroke if a different crankshaft or stroker combination has been installed.
Accurate measurements become increasingly important when comparing small displacement changes, since rounding the bore or stroke before calculating can change the final result.