A flat-twelve engine, also known as a horizontally opposed-twelve, is a twelve-cylinder piston engine with six cylinders on each side of a central crankshaft. Flat-twelve engines are less common than

V12 engine A V12 engine is a twelve-cylinder piston engine where two banks of six cylinders are arranged in a V configuration around a common crankshaft. V12 engines are more common than V10 engines. However, they are less common than V8 engines. The f ...

s, but they have been used in various racing cars during the 1960s and 1970s, and in mid-engined Ferrari road cars from 1973 to 1996.

Design

Unlike most flat-twin, flat-four, and flat-six engines, flat-twelve engines typically use the crankshaft configuration of a 180°

V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...

. Instead of the

boxer engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, wh ...

design of each piston having its own crankpin, flat-twelve engines have each pair of pistons sharing a

crankpin A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the ...

, and thus are flat, but not boxers. Compared with a V12 engine, a flat-twelve has a lower

center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...

, but because a flat-twelve is wider it is rarely used in front-engined cars.

Racing cars

Formula One

The first known flat-twelve engine was built by Porsche in 1947 for the abandoned

Cisitalia Grand Prix The Cisitalia Grand Prix is a single-seater car for the postwar 1.5-litre supercharged Grand Prix class, built by Italian sports car manufacturer Cisitalia and introduced in 1949. It was designed on behalf of Cisitalia by Porsche between 1946� ...

racing car. The engine, known as the Porsche Typ 360, was supercharged and had a displacement of . One engine was built and the car conducted top speed testing, but it never competed in any races. In 1964–65, the

Ferrari 512 F1 The Ferrari 158 was a Formula One racing car made by Ferrari in 1964 as a successor to the V6-powered Ferrari 156 F1. Ferrari 158 The 158 was equipped with a 1.5-litre V8 engine, with a bore and stroke of . It was the first Ferrari Formul ...

competed in several Formula One races. The 512 F1 was powered by a flat-twelve engine and raced alongside the V8-engined Ferrari 158 upon which it was based. Ferrari returned to using flat-twelve engines in 1970, when the

Ferrari 312B The Ferrari 312B is a Formula One racing car designed and built by Scuderia Ferrari. It was the successor to the Ferrari 312 and was used from 1970 until early 1975. The original 312B was developed into the 312B2 and 312B3. History The early 19 ...

switched from the V12 engine used by its predecessor. Its successor, the

Ferrari 312T The Ferrari 312T was a Ferrari Formula One car design, based on the 312B3 from 1974. In various versions, it was used from 1975 until 1980. It was designed by Mauro Forghieri for the 1975 season, and was an uncomplicated and clean design tha ...

, was introduced in 1975 and won the Formula One constructors championships from 1975 to 1979. The success of the Ferrari 312T led other Formula One teams to build flat-twelve engines, such as the 1979 Alfa Romeo 177. In 1990, the Subaru 1235 flat-twelve engine was built for Subaru's unsuccessful attempt to compete in Formula One as an engine manufacturer.

Sports car racing

In 1969, the Porsche 917 sports prototype racing car introduced an air-cooled flat-twelve engine. This flat-twelve engine was based on the previous flat-eight engine, but it used a V12 crankshaft configuration instead of the boxer configuration used by the flat-eight. The domination of the Porsche 917 probably influenced Ferrari, because they switched from V12 engines to flat-twelve engines (using a boxer configuration) for the

Ferrari 312 PB The Ferrari 312 PB was a Group 6 Prototype-Sports Car introduced in 1971 by Italian carmaker Ferrari. It was officially designated the 312 P, but often known as the 312 PB to avoid confusion with a previous car of the same name. It was ...

, which competed from 1971 to 1973. Alfa Romeo also used flat-twelve engines in the 1973-1976 Alfa Romeo 33TT12 and Alfa Romeo 33SC12 sports prototype racing cars. For the 1991 sports-prototype racing category, Mercedes-Benz switched from a twin-turbo V8 engine to a naturally-aspirated flat-twelve engine for in the

Mercedes-Benz C291 The Mercedes-Benz C291 was a sports-prototype racing car introduced for the 1991 World Sportscar Championship season. It was Mercedes-Benz’ final car in the Group C category. Introduction The 1991 season marked the introduction of the FIA’s ...

racing car. 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. This was done to allow the engine to be installed lower in the chassis. The C291 was unsuccessful and Mercedes withdrew from sports-prototype racing after the 1991 season.

Production cars

From 1973 to 1996, Ferrari used flat-twelve engines in various production models: the 1973-1976 Ferrari 365 GT/4 BB, the 1976-1981

Ferrari 512 BB The Ferrari Berlinetta Boxer (BB) is series of sports cars produced by Ferrari in Italy between 1973 and 1984. The BB was designed by Leonardo Fioravanti at Pininfarina. The first BB model, the 365 GT4 BB, replaced the front engined Daytona and ...

, the 1981-1984 Ferrari 512 BBi, the 1984-1991

Ferrari Testarossa The Ferrari Testarossa (Type F110) is a 12-cylinder mid-engine sports car manufactured by Ferrari, which went into production in 1984 as the successor to the Ferrari Berlinetta Boxer. The Pininfarina-designed car was originally produced from ...

, the 1991-1994 Ferrari 512 TR and the 1994-1996 Ferrrari F512 M.

Other uses

During World War II, the British Covenanter tank was powered by a

Meadows A meadow ( ) is an open habitat, or field, vegetated by grasses, herbs, and other non-woody plants. Trees or shrubs may sparsely populate meadows, as long as these areas maintain an open character. Meadows may be naturally occurring or artificia ...

D.A.V flat-12, 340 hp (250 kW) and the Churchill by a 350 hp (261 kW) Bedford engine flat 12. Another military vehicle usage was the

Panhard EBR The Panhard EBR (Panhard ''Engin Blindé de Reconnaissance'', French: Armored Reconnaissance Vehicle) is an armoured car designed by Panhard for the French Army and later used across the globe, notably by the French Army during the Algerian War a ...

armored cars during the 1950s. During the 1940s, the

Franklin Engine Company The Franklin Engine Company was an American manufacturer of aircraft engines. Its designs were used primarily in the civilian market, both in fixed wing and helicopter designs. It was briefly directed towards automobile engines as part of the Tuck ...

in the United States produced a flat-twelve aircraft engine called the O-805-2.

References

{{Piston engine configurations 12 Piston engine configurations