A boxer engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons but no cylinder head. Also, if a straight engine is canted 90 degrees into the horizontal plane, it may be thought of as a "flat engine", but this usage is not common.
True boxers have each crankpin controlling only one piston/cylinder while the 180° engines, which superficially appear very similar, share crankpins. The 180° engine, which may be thought of as a type of V engine, is quite uncommon as it has all of the disadvantages of a flat engine, and few of the advantages.
In 1896, Karl Benz invented the first internal combustion engine with horizontally opposed pistons. He called it the kontra engine, as the action of each side opposed the action of the other. This design has since been called the "boxer" engine because each pair of pistons moves in and out together, rather like the gloves of a boxer. The boxer engine has pairs of pistons reaching TDC simultaneously.
The boxer configuration is the only configuration in common use that does not have unbalanced forces with a four-stroke cycle regardless of the number of cylinders, as long as both banks have the same number of cylinders. They do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in most other engine configurations. However, in the case of boxer engines with fewer than six cylinders, unbalanced moments (a reciprocating torque also known as a "rocking couple") are unavoidable due to the "opposite" cylinders being slightly out of line with each other. Other engine configurations with natural dynamic balance include the inline 6, the inline 8, the V12, and the V16.
Boxer engines tend to be noisier than other common engines for both intrinsic and other reasons. In cars, valve clatter from the engine compartment is not damped by air filters or other components.
The low centre of gravity allowed by a boxer engine can reduce body roll in automobiles and enhance handling precision. Historically they could be found in cars manufactured by companies such as Porsche, Alfa Romeo, Benz, Ford, Tatra, Citroen, Jowett, Rover, Volkswagen, Chevrolet, and Ferrari. The most prominent manufacturers currently utilizing a boxer engine as their primary engine configuration are Porsche and Subaru.
When mounted longitudinally in a vehicle, flat engines with up to six cylinders are short, low, and wide. As a result, they have often been used in compact drivetrains where the engine is mounted outside the wheelbase and drives the nearer pair of wheels through the transmission without a drive shaft. The short length of a longitudinally mounted flat engine with six cylinders or less makes it ideally suitable for air cooling.
Examples with rear engine, rear wheel drive layouts include the 2 cylinder BMW 600 and 700, 4 cylinder Tatra 97, Volkswagen Beetle and Porsche 356, and the 6 cylinder Chevrolet Corvair, Porsche 911, and Tucker 48. All of these examples except the Tucker and later versions of the Porsche 911 are air cooled.
Examples with front engine, front wheel drive layouts include 2 cylinder Citroëns and Panhards and the 4 cylinder Citroën GS, Alfa Romeo Alfasud, Lancias from the Flavia to the Gamma, and Subarus DL and GL. The Citroëns and Panhards are air cooled while the Lancias, Alfa Romeos and Subarus are water cooled.
Boxer engines have also been used in cars with front engine, rear wheel drive layouts, including Bradford trucks and vans, the Glas Isar, Jowett cars and trucks, and early Tatras.
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