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A flat-12 is an internal combustion engine in flat configuration, having 12 cylinders.

The flat-12 is larger than a V12 and has no advantage in terms of vibrations. Thus the design is rarely used on production cars.

Most of the flat-12 engines are not true horizontally opposed engines (boxer) but rather 180° V engines. The real boxer has one crank pin per piston while in the 180° V engine two pistons share the same crank pin. With twelve cylinders both layouts are perfectly balanced.

It was used in Formula One and Endurance racing, the flat engine concept had the advantage of a low center of gravity. When wing-cars requiring air-flow venturies came along in the late 1970s, the wide flat-layout obstructed the airflow and became obsolete.

In 1964-65, at the end of the 1.5 litre F1 era, Ferrari introduced a flat-12 on the Ferrari 1512, but a more classical V12 was chosen for the new 3 litre F1.

The Porsche 917 endurance racing car (introduced in 1969, for the Sport category) was powered by an air-cooled flat-12. This engine was an evolution of the Porsche flat-8 boxer engine and in fact used identical cylinders to those found on the 908 but it did differ in that it used a V12 type crankshaft.

The domination of the Porsche 917 over the V12-powered Ferrari 512 probably influenced Ferrari, for they returned to the flat-12 in 3 litre water-cooled form for its prototypes and Formula One cars.

The Ferrari flat-12 design was successful and influential especially on Italian manufacturers, Alfa-Romeo was also successful in endurance with a flat-12 while the Tecno Formula One flat-12 was a failure.

A 4.4-5.0 L 180° V12 was later introduced by Ferrari in some of its production models, including the Berlinetta Boxer and Testarossa.

Piston engine configurations
Straight Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V 2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat 2, 4, 6, 8, 10, 12, 16, H
W 8, 9, 12, 16, 18
Other inline H, VR, Opposed, U (Square), X
Other Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)

Heat engines
Stroke cycles
Engine types
Gas turbinePistonJetRocket engineSteam engineStirling engineTschudiTwingle
Cylinder head portingD slideFour-strokeManifoldMultiPistonPoppetSleeve
Piston layouts
Single cylinderStraightOpposedFlatVWHDelticRadialRocket engine nozzleRotaryStelzerControlled CombustionBourke
Motion mechanisms
CamConnecting rodCoomber rotaryCrankCrank substituteCrankshaftLinkages (EvansPeaucellier-LipkinSector straight-lineWatt) • Double acting/differential cylinder
Thermodynamic cycle