Flatfour

Flatfour, also known as a boxer-four, is a four-cylinder internal combustion engine with two banks of cylinders arranged horizontally in opposite directions around a common crankshaft. The opposing pistons move in opposite directions, which reduces primary vibrations and typically results in smooth operation. The layout also yields a low, wide engine package, contributing to a low center of gravity that can improve handling in certain vehicle layouts. Flatfours can be air-cooled or water-cooled, with cooling method historically shaping their design.

In production history, flatfours gained prominence through Volkswagen’s air-cooled boxer engines used in the Beetle and related models, and through Porsche’s early air-cooled four-cylinder units in the 356 and 912. In the late 20th century, Subaru popularized the modern flat-four boxer engine, applying it across most of its lineup and often pairing it with turbocharging for higher performance. The configuration has also seen use in some marine and aviation applications and in other light engines.

Compared with inline-four engines, flatfours offer smoother operation and a lower center of gravity but require more lateral space, which can complicate packaging and exhaust routing. Modern flat-fours span naturally aspirated and turbocharged variants and are designed to meet contemporary efficiency and emission standards. The term “boxer” is sometimes used synonymously with flatfour, reflecting the opposing-piston motion that resembles a boxer's movements.