Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle; it is normally stated in cubic centimetres, litres or cubic inches. In a piston engine, this is the volume that is swept as the pistons are moved from top dead centre to bottom dead centre.
 Standard engines
In a standard piston engine (an Otto or Diesel engine), displacement is calculated by multiplying the number of cylinders in the engine with the area of a piston and the length of the stroke. With circular pistons, displacement can be calculated from the bore diameter and stroke using the following formula:
Displacement in other engine types (especially for a Wankel engine) is much more complicated. For instance,Mazda's 13B is a two-rotor engine with combustion chambers of roughly 0.65 liters in displacement (difference in max vs. min combustion chamber volume). At 100% volumetric efficiency, 0.65 liter per rotor face * 3 faces per rotor * 2 rotors gives a total displacement of 3.9 liters. It takes 3 rotations of the eccentric shaft to complete one engine cycle, however. In 2 rotations of the eccentric shaft, comparable to 2 crankshaft rotations on a 4-stroke piston engine, the 13B would displace 2.6 liters. Mazda advertises the 13B as a 1.3 liter engine, which is the volume displaced during a single rotation of the eccentric shaft.
Displacement is equal to the volume of combustible air/fuel mixture ingested during one cycle of all the cylinders at 100% volumetric efficiency. Thus, a four-stroke engine ingests its displacement in combustible mixture in two engine revolutions, while a two-stroke engine needs only one engine revolution to do so.
Engine power is thus dependent on the quantity of air/fuel mixture ingested and the efficiency of its combustion and conversion into power. To increase the quantity of mixture combusted, the engine displacement can be increased, the speed of operation of the engine can be increased, or the mixture quantity (volume) can be delivered at a higher pressure, which is the function of such devices as turbochargers and superchargers. See engine tuning.
All other factors being equal, a larger displacement engine is therefore more powerful than a smaller one. It is the easiest method of adding power since it neither requires higher rotational speeds nor complicated auxiliaries. The ease of adding power this way (along with the lack of performance effects such as turbocharger lag caused by the time needed to spin up the turbine of the turbocharger) led to the sayings There's no substitute for cubic inches or, alternatively, There's no replacement for displacement commonly quoted by devotees of large-engined cars.
The added mass and size reduce a vehicle's maneuverability however, and in applications where that is important, alternative methods for increasing power are commonly employed. Additionally, because the efficiency of the engine is not improved, fuel consumption rises dramatically.
In cars, engines over 8 litres displacement are extremely rare in the last half-century and most modern cars utilise engines much smaller than that: in the United States, 1 to 2 litres for smaller cars, 3 to 5 litres for larger and faster cars, and 5 to 8 litres in sports cars. In Europe, cars with a displacement larger than 2 litres are rare, due to taxation discouraging the use of fuel-inefficient cars.
Five to 10 litre engines are used in many single and twin engine propeller-driven aircraft. Much larger engines tend to be diesel engines fitted to trucks, ships, railroad locomotives and those used to drive stationary generators. The displacement of each cylinder in such an engine may be much larger than that of a whole car engine.
 Units of measure
The cubic inch was often formerly used (until the 1980s) to express the displacement of engines for new cars, trucks, etc. (e.g., the "426" in 426 HEMI refers to 426 cubic inches displaced). It is therefore still used for this purpose in the context of the classic-car hobby, auto racing, and so forth. The auto industry nowadays uses SI for this purpose (e.g. 6.1 L HEMI). However, the actual displacement measurements of an engine are still given by many manufacturers in cubic inch displacement (usually along with cc; e.g. the 6.1 L HEMI's published displacement is 370.0 CID/6,059 cc).
Some examples of common CID-to-litre conversions are given below. Note that nominal sizes are not always precisely equal to actual sizes. This principle is frequently seen in engineering, tool standardization, etc. (for ease of use) and in marketing (when a big round number sounds more impressive, is more memorable, etc.).
|Make (±Division)||CID (actual) (nearest 1)||CID (nominal)||SI (actual) (nearest 0.01)||SI (nominal)|
|Honda, Kawasaki, others||something close to 61 CID||NA (not marketed in CID)||[something close to SI nominal]||1000 cc (= 1.0 L)|
|Honda, Kawasaki, others||something close to 98 CID||NA (not marketed in CID)||[something close to SI nominal]||1600 cc (= 1.6 L)|
|Honda, Kawasaki, others; Ford||something close to 122 CID||NA (not marketed in CID)||[something close to SI nominal]||2000 cc (= 2.0 L)|
|GM (Pontiac, Buick, Oldsmobile, GMC, others)||151 CID||NA (not marketed in CID)||[something close to SI nominal]||2.5 L|
|Toyota, Ford, Chrysler, others||something close to 183 CID||NA (not marketed in CID)||[something close to SI nominal]||3.0 L|
|AMC, Jeep, Chrysler (I6)||241.573 CID||242 CID||3,959 cc||4.0 L|
|Ford||something close to 244 CID||NA (not marketed in CID)||[something close to SI nominal]||4.0 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||250 CID||4.10 L||4.1 L|
|AMC, Jeep, International Harvester||[something close to CID nominal]||258 CID||4.22 L||4.2 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||289 CID||4.74 L||NA (not marketed in SI)|
|Ford (Ford trucks and vans)||[something close to CID nominal]||300 CID||4.92 L||4.9 L|
|Ford, GM (Chevrolet)||[something close to CID nominal]||302 CID (302 Windsor, 302 Cleveland, Chevrolet 302)||4.95 L||5.0 L|
|AMC, Jeep, International Harvester||[something close to CID nominal]||304 CID||4.98 L||5.0 L|
|GM (Chevrolet; Buick)||307 CID||307 CID||5.03 L||NA (not marketed in SI)|
|GM (Oldsmobile)||307 CID||NA (not marketed in CID)||5.03 L||5.0 L|
|Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||318 CID||5.21 L||5.2 L|
|AMC, GM (Chevrolet)||327 CID||327 CID||5.36 L||NA (not marketed in SI)|
|Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||340 CID||5.57 L||NA (not marketed in SI)|
|GM (GMC, Chevrolet, Buick, Oldsmobile, Pontiac, others)||[something close to CID nominal]||350 CID||5.74 L||5.7 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||351 CID (Cleveland or Windsor)||5.75 L||5.8 L|
|AMC, Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||360 CID||5.90 L||5.9 L|
|Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||383 CID||6.28 L||NA (not marketed in SI)|
|AMC, Ford, GM (Cadillac)||[something close to CID nominal]||390 CID||6.39 L||NA (not marketed in SI)|
|GM (Chevrolet)||[sometimes 396 CID, sometimes 402 CID]||396 CID||6.49 L||NA (not marketed in SI)|
|GM (Chevrolet; others?)||[something close to CID nominal]||400 CID||6.55 L||NA (not marketed in SI)|
|GM (Chevrolet)||[something close to CID nominal]||409 CID||6.70 L||NA (not marketed in SI)|
|GM (Pontiac)||[something close to CID nominal]||421 CID||6.90 L||NA (not marketed in SI)|
|Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||426 CID (Wedge or Hemi)||6.98 L||7.0 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||427 CID||7.00 L||7.0 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||428 CID||7.01 L||7.0 L|
|Ford (Ford, Mercury)||[something close to CID nominal]||429 CID||7.03 L||7.0 L|
|Chrysler (Chrysler, Dodge, Plymouth)||[something close to CID nominal]||440 CID||7.21 L||7.2 L|
|GM (GMC, Chevrolet)||[something close to CID nominal]||454 CID||7.44 L||7.4 L|
|GM (Buick, Oldsmobile, Pontiac)||[something close to CID nominal]||455 CID||7.46 L||NA (not marketed in SI)|
|Ford (Ford [trucks and vans]; Lincoln [cars])||[something close to CID nominal]||460 CID||7.54 L||7.5 L|
|GM (Cadillac)||[something close to CID nominal]||472 CID||7.73 L||7.7 L|
|GM (Cadillac)||[something close to CID nominal]||500 CID||8.19 L||8.2 L|
|Chrysler (Dodge)||506.5 CID||505 CID||8285 cc||8.3 L|
|Chrysler (Dodge)||509.8 CID||510 CID||8354 cc||8.4 L|
 Governmental regulations
In many nations levels of taxation on automobiles have been based on engine displacement, rather than on power output. Displacement is easy to identify and difficult to modify whereas power output must be tested. This has encouraged the development of other methods to increase engine power.
There are four major regulatory constraints for automobiles: the European, the British, the Japanese, and the American. The method used in some European countries, and which predates the EU, has a level of taxation for engines over one (1.0) litre and another at the level of about 100 cubic inches, which is approximated to 1.6 litres. The British system of taxation depends upon vehicle emissions for cars registered after 1 March 2001 but for cars registered before this date it depends on engine size. Cars under 1549cc qualify for a cheaper rate of tax .
The Japanese is similar to the European taxation by classes of displacement, plus a vehicle weight tax. In the American system, which includes Canada, Australia and New Zealand, there is not this sort of taxation per engine displacement. In The Netherlands and Sweden, road tax is based on vehicle weight.
Displacement is also used to distinguish categories of (heavier) motorbikes with respect to license requirements. In France and some other EU countries, mopeds, usually with a two-stroke engine and less than 50 cm3 displacement can be driven with minimum qualifications (previously, they could be driven by any person over 14). This led to all light motorbikes having a displacement of about 49.9 cm3. Some people tuned the engine by increasing the cylinder bore, increasing displacement; such mopeds cannot be driven legally on public roads since they do no longer conform to the original specifications and may go faster than 45 km/h.
Wankel engines, due to the amount of power and emissions they create for their displacement, are generally taxed as 1.5 times their actual physical displacment (1.3 litres becomes 2.0, 2.0 becomes 3.0), although actual power outputs are far greater (the 1.3 litre 13B can produce power comparable to a 3.0 V6, and the 2.0 litre 20B can produce power comparable to a 4.0L V8). As such, racing regulations actually use a much higher conversion factor.
 Example regulations
- Bulgaria: a special tax on non-European cars over 2.8 L, later amended to over 3.0 L
- Belgium and Portugal have a proportional tax including reference to displacement
- Ireland: There is one rate for cars under and including 1000 cc, then rates increase by increments of 100 cc up to 3,000 cc - 3,001 cc and higher are all charged the same. Goods vehicles are taxed by weight, and buses are taxed by the number of passenger seats (except school buses which are charged a small flat rate). 
- Korea: under 0.8 L; 0.8-2.0 L; over 2.0 L
- Netherlands: progressive proportional tax based on vehicle weight, fuel type and region.
- Philippines (proposal from 2000): under 1.6 L; 1.6-2.0 L; 2.0-2.8 L; over 2.8 L
- Spain: under 1.6 L; over 1.6 L
- Taiwan: under 500 cc, 500~600 cc, 601~1200 cc, 1201~1800cc, etc (increments of 600 cc up to 8400 cc, where generally the price difference is greater from one range to the next).
 Increase and decrease of typical engine displacement in the US
Once V8 engines became expected on large American cars in the late 1950s, and continuing to the oil crisis in the 1970s, there was an engine displacement race in the industry. Firms would put badges on the fenders of cars giving the displacement in cubic inches. This was also a sort of trademark as well. There's a famous Beach Boys song, "409", which refers to any full-size Chevrolet which had an engine displacement, in cubic inches, of that amount, regardless of trim level. This number was not the model number of the car.
In the mid-1960s, Chrysler offered a V8 engine of 426 cubic inches (6981 cm³) on its muscle cars and pony cars. Soon Ford came out with one of similar size, but it couldn't use the same label, so the engine was made and labelled as 427 cubic inches (6997 cm³). When Ford improved its engine by changing ancillary equipment, to indicate the change they put badges labelled "428" on such cars, and subsequently did the same to get "429". Engine sizes eventually grew to 440in³ in Chryslers, 500 in Cadillacs, 632 in Chevrolets, and 462 in Lincolns.
With the oil shocks of the 1970s, American firms started selling cars with smaller engines. The Chevrolet Vega was initially touted as having an engine of 1998 "cc" (cubic centimetres), given in metric because it equates to 122 cubic inches, which would have been considered laughable to declare in the American market. This also differs from the European convention of two significant figures, which was in the U.S. European car models usually have a number of three digits. In this instance, the numbers are considered trademarks. These two factors in the world marketplace contributed to American cars now getting labelled in the European manner. Engines like that of the Vega would now be called 2.0 (being litres).
- 1 L ~ 61 inch³
- 1 inch³ ~ 16 cm³
The big engines listed above are mostly 7.0 litres. The 3.5 litre engines listed on American cars today as being large are much smaller than the 350 cubic inch (5.7 L) engines that once were considered medium size.
The 3.5 litre engine is 213 cubic inches. The 1964 Mustang's smallest Ford V8 engine of 289 cubic inches is 4.7 litres.
However, modern engines are much more efficient, using such technologies as an ECU, electronic fuel injection, and variable valve timing. Also, the engines and the total weight of cars they are fitted in are lighter, so the difference in performance is not as great as might otherwise be supposed.