flatsix

A flat-six, also written flatsix or boxer-six, is an internal combustion engine with six cylinders arranged horizontally in two banks of three opposite each other. It is a variant of the boxer engine and shares the characteristic opposing pistons, which helps produce a compact, low-profile engine with naturally balanced firing.

In operation, the two banks share a single crankshaft; the opposing piston motion cancels much of the vibration, delivering smooth operation. The horizontal layout yields a low center of gravity and a relatively short engine length, which aids installation in front- or mid-engined vehicles. Cooling methods have varied: many early flat-sixes were air-cooled (notably in classic Porsche 911s); most modern flat-sixes are water-cooled.

Displacements for road-going flat-six engines typically range from about 2.0 to 4.0 liters, with power outputs rising significantly in higher-performance models and racing variants. Modern designs commonly use dual overhead camshafts and four valves per cylinder, though older or simpler configurations used different valvetrains.

Notable applications include the Porsche flat-six engines used in the 911 and related models since the 1960s, a configuration that has become closely associated with the brand. Outside Porsche, flat-sixes have appeared in a smaller number of models, such as certain Subaru vehicles that used horizontally opposed six-cylinder engines in the SVX and later Legacy/Outback variants. The flatsix is prized for smoothness and a low center of gravity, but its width and length can complicate engine bay packaging and maintenance compared with some alternative layouts.