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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 true horizontally opposed engines (boxer), but rather 180° V-engines. A true boxer has one crank pin per piston, while in the 180° V-engine, two opposing pistons share the same crank pin. The engine also has a naturally lower center of gravity than a V12, but, with the exception of the Mercedes-Benz C291, is usually 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.
Eleven 917s pull away at Goodwood Festival of Speed 2009
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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 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 3 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 their 3.5 litre M-291 flat-12 engine for endurance racing. This engine employed a clever cylinder-head design in which the exhaust ports are located where the intake ports would normally be (on top of the engine, pointing upwards). The intake ports are then re-located between the intake and exhaust camshafts, just above the spark-plugs, pointing at an outward angle from the vertical. By locating the entire exhaust system on top, rather than underneath the engine, a much lower mounting height was achieved for this engine in the engine-bay of their racing car, the C-291 (and later C-292). The engine suffered from oil scavenging problems early on in its development, and although the problem was eventually solved, changes in the rules by FIA and ACO effectively doomed the future prospects of what could otherwise have been a competitive engine.
Henry Meadows built a flat-12 petrol engine in World War II for the British Covenanter tank. In 1954, Panhard fitted a flat-12 (based on its earlier air-cooled two-cylinder) in the EBR armored car.
Franklin 12 series
- https://www.stuttcars.com/about-porsche/museum-gmund/ Retrieved 6th April, 2017.
- http://www.ultimatecarpage.com/car/139/Ferrari-1512-F1.html Retrieved 6th April, 2017.
- https://www.formula1.com/en/championship/drivers/hall-of-fame/Niki_Lauda.html Retrieved 6th April, 2017.
- https://www.formula1.com/en/championship/drivers/hall-of-fame/Jody_Scheckter.html Retrieved 6th April, 2017.
- Ogorkiewicz, R. M. AFV Weapons Profile 039 Panhard Armoured Cars (Windsor, Berks: Profile Publications).