Boxer Engine

A boxer engine, also called a horizontally opposed engine, arranges its pistons in two banks that lie flat and fire outward from a central crankshaft, with pistons on opposite sides moving inward and outward in mirrored pairs. This design cancels out much of the primary vibration that inline and V-configuration engines produce, since opposing pistons reach top and bottom dead center at the same time and their momentum offsets. The name comes from the visual resemblance to two boxers trading punches, cylinder banks striking outward in unison.

The flat, wide shape of a boxer engine sits lower in the chassis than an inline or V engine of comparable displacement, which lowers the vehicle's center of gravity and can improve handling balance, particularly in corners. Subaru has built its brand identity around boxer engines across nearly its entire lineup, including the WRX and STI, while Porsche uses flat-six boxer engines in the 911, Boxster, and Cayman. Both manufacturers cite the low center of gravity and the engine's distinctive exhaust burble, caused by the uneven firing pulses reaching the exhaust manifold, as signature traits.

Boxer engines also present engineering tradeoffs. Their wide shape complicates engine bay packaging and can make certain routine services, such as spark plug or valve cover access, more labor intensive than on an inline engine. Head gasket failures have historically been a known weak point on some Subaru boxer engines, particularly earlier EJ-series units, due to the long, thin head gasket surface required by the flat layout. Despite these drawbacks, the boxer layout remains prized in performance and rally applications for its low, wide mass distribution.