Engine displacement, also referred to by the less common, but more technically accurate term piston displacement, is the measure of the volume moved by the pistons of an internal combustion engine as they move through a single, complete revolution. Piston displacement, which is expressed in volumetric units such as liters, cubic centimeters, or cubic inches, is directly proportional to the overall power generated by an engine. It is calculated according to an equation taking into account various engine characteristics, including the number of cylinders, bore, and stroke.
Bore is the inside diameter of the hollow cylinders that house the pistons. Stroke is the distance a piston moves over the course of a revolution. With this in mind, the mathematical formula for displacement is: the number of cylinders multiplied by bore squared, multiplied by the stroke, multiplied by Pi divided by four.
Generally speaking, the larger the bore and stroke of an engine, the greater its displacement and horsepower. With technological advances, however, this correlation has become less absolute as variable displacement engines, which can turn entire cylinders on and off to provide greater performance or economy, have become more prevalent. Barring variable displacement, piston displacement can generally be used to estimate an engine's power and economy, and is particularly useful in comparative situations.
The units in which piston displacement is measured has undergone a historic shift, from cubic inches in the muscle car era of the 1960s and 70s, to the metric units — liters and cubic centimeters — of today. Car manufacturers have traditionally incorporated piston displacement into model nomenclature, marketing performance models in particular by highlighting the size of their engines. Famous examples of this technique include Ford's 427 cubic-inch engine, which was an eight cylinder V8 configuration motor featured in much of Ford's product line throughout the 1960s, and the 5.0 liter V8 that was a selling point in Ford Mustangs in the late 1980s and early 1990s.
Aside from its standalone utility as a measure of capacity, piston displacement is also closely related another common automotive measurement called compression ratio. Compression ratio is the comparison of a combustion chamber's largest capacity to its smallest capacity — from the top of a piston's stroke to the bottom. Bore and stroke are again integral components in the formula for compression ratio. A higher ratio means an engine is able to generate more mechanical energy from a given quantity of air-fuel mixture. Unlike piston displacement, which can be subjectively assessed based on the potential use of an engine, a high compression ratio is almost always considered a desirable trait.