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A flat-12 is a 12-cylinder internal combustion engine in a flat configuration. Rarer, wider, and less tall than a V12, the flat-12 design was used in Formula One and endurance racing and some exotic sports cars.
Flat-12 engines are generally not horizontally opposed engines (boxers), but rather 180° V-engines. A true boxer has one crankpin journal per piston, while in the 180° V-engine, two opposing pistons share the same crankpin journal. The engine also has a naturally lower center of gravity than a V12, but, with the exception of the Mercedes-Benz C291, is mounted somewhat higher in the engine bay to provide clearance for the exhaust system.
Porsche 360 / Cisitalia Grand Prix
In 1964-65, Ferrari introduced a flat-12 in the Ferrari 1512 F1 car  and won the F1 world championship with a flat-12 layout in the mid- and late 1970s with the Ferrari 312T. However, when wing-cars requiring air-flow venturis came along in the late 1970s, the wide flat shape obstructed the airflow and was considered obsolete. A 4.4-5.0 L 180° flat-12 was also introduced by Ferrari in some of their production models, including the Berlinetta Boxer and Testarossa.
The success of the flat-12 design had an influence on Italian manufacturers, including Alfa Romeo. They were unsuccessful with their return to Formula 1 with the flat-12 powered 177, but succeeded in endurance racing with the 33TT12 and 33SC12.
The Porsche 917 endurance racing car (introduced in 1969, for the sport category) was powered by an air-cooled flat-12. This was an evolution of the Porsche flat-8 boxer engine and used identical cylinders to those found on the 908, but differed in that it used a V12 type crankshaft. The domination of the Porsche 917 over the V12-powered Ferrari 512 probably influenced Ferrari, because they returned to the flat-12 in three-litre water-cooled form for their prototypes and Formula One cars.
Motori Moderni designed a 3.5-litre flat-12 engine for Subaru, the 1235, which would be used in the 1990 Coloni Formula One car. The Coloni−Subaru was not competitive and did not start a Grand Prix. The Motori Moderni flat-12 engine also appeared in detuned form in the Jiotto Caspita supercar. This only appeared in a single prototype and was not put into production.
The Tecno Formula One flat-12 was a failure.
In the early 1990s, Mercedes-Benz built a 3.5-litre M-291 flat-12 engine for endurance racing. This engine employed a cylinder-head design with exhaust ports where the intake ports would normally be (on top of the engine, pointing upwards). The intake ports are between the intake and exhaust camshafts, just above the spark-plugs, pointing at an outward angle from the vertical. Putting the exhaust system on top, rather than underneath, allowed the engine to ride much lower in the engine-bay of the C-291 (and later C-292).
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