A flat-eight engine, also called a horizontally-opposed eight, is an eight-cylinder

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common featu ...

with two banks of four inline cylinders, one on each side of a central

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

, 180° apart. In a flat-eight engine, the connecting rods for corresponding pistons from the left and right banks may share a crankshaft journal. A

boxer Boxer most commonly refers to: * Boxer (boxing), a competitor in the sport of boxing *Boxer (dog), a breed of dog Boxer or boxers may also refer to: Animal kingdom * Boxer crab * Boxer shrimp, a small group of decapod crustaceans * Boxer snipe ee ...

-eight engine is a special case of a flat-eight where each piston's connecting rod has its own journal, and each pair of opposed pistons moves inwards or outwards at the same time. Flat-eight engines have been used in automotive, aircraft, and marine applications.

Design

The advantages of a flat-eight engine are its minimal length and low centre of mass. A disadvantage is its greater width compared to a V8 or inline-eight engine. A flat-eight engine is able to have perfect

primary balance Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary bala ...

and

secondary balance Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary ba ...

. A boxer-eight engine has a single piston per

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 ...

, which increases the linear offset between the cylinder banks. A boxer-eight with nine main bearings may be thought of as two boxer-four engines laid end-to-end with a 90° phase angle between their crankshafts. Alternatively, a flat-eight engine where corresponding pistons from the two opposing banks share a crankshaft journal is often called a "180 degree

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 ...

". One possible configuration for this design uses a two-plane

. Another configuration uses a 180° single-plane crankshaft with the leading and trailing crankpins in the same position, while the two central crankpins are in the opposite position.

Use in automobiles

One of the earliest flat-eight engines was used in the 1904 Buffum ''Model G Greyhound'' racing car, which used an engine based on two of Buffum's existing flat-four engines joined together. The Model G was built in the United States and was introduced one year after the

Winton Motor Carriage Company The Winton Motor Carriage Company was a pioneer United States automobile manufacturer based in Cleveland, Ohio. Winton was one of the first American companies to sell a motor car. In 1912 Winton became one of the first American manufacturers of ...

''Bullet No. 2'', which used a straight-8 engine. Around the same time, the first V8 engines were beginning to appear in Europe. Several racing cars have used bespoke flat-eight engines based on two inline-four engines and a custom crankshaft. These include the 1928 ''Anderson Specials Number 2'' built in Scotland using two Humber 9/20 engines. The car is on display at the Kelvingrove Art Gallery and Museum in Glasgow. Another example is a 1977 Eagle 72 chassis raced by Bruce Crower in the United States. This engine used the cylinder heads from two Chevrolet Cosworth Vega engines. In 1969, a one-off engine was built for racing driver Emerson Fittipaldi to compete in the Thousand Miles of Guanabara endurance race. This flat-eight engine was made by joining two 1.3 L Volkswagen flat-four crankcases with an elastomeric gasket and connecting the two crankshafts together. Several similar Volkswagen-based engines were built for Fittipaldi's racing cars in the early 1970s.

Porsche

Porsche used flat-eight engines in various racing cars throughout the 1960s. The first Porsche flat-eight was the Type 753. Work began on it in 1960, following the announcement of a displacement limit for the 1961 Formula One season. Designed by

Hans Mezger Hans Mezger (18 November 1929 – 10 June 2020) was a German automotive engineer well known for his work at Porsche. Career Mezger was born in Besigheim, in Württemberg, Germany. He graduated from the Stuttgart Technical University in 1956 wi ...

and Hans Honich, the engine includes features shaft-driven double-overhead camshafts, a two-piece magnesium crankcase casting, eight individual finned cylinder barrels, a solid crankshaft running in nine main bearings and Lucas electronic ignition. The version of the Type 753 used for its debut at the 1962 Dutch Formula One Grand Prix produced . At the 1962 French Grand Prix, the Type 753 engine delivered Porsche's only F1 race win as a constructor, in an 804 driven by Dan Gurney. The 753 also influenced the design of the flat-six engine used in the first-generation

Porsche 911 The Porsche 911 (pronounced ''Nine Eleven'' or in german: Neunelfer) is a two-door 2+2 high performance rear-engined sports car introduced in September 1964 by Porsche AG of Stuttgart, Germany. It has a rear-mounted flat-six engine and origin ...

. A version of the Porsche flat-eight, designated the Type 771, was developed alongside the Type 753. This engine was intended for sports car and endurance racing, and was used in the Porsche 718 W-RS, Porsche 904/8),

Porsche 906 The Porsche 906 or Carrera 6 is a street-legal racing car from Porsche. It was announced in January 1966 and 50 examples were subsequently produced, thus meeting the homologation requirements of the FIA's new Group 4 Sports Car category to the ...

Porsche 907 The Porsche 907 is a sportscar racing prototype built by Porsche in 1967 and 1968. 1967 The 907 was introduced at the 1967 24 Hours of Le Mans. Following a suggestion by Ferdinand Piëch, the position of the driver was moved from the tradition ...

Porsche 909 The Porsche 909 "Bergspyder" was a spyder sports car designed and built by Porsche in 1968. Its purpose was to compete in hillclimbing competitions, specifically the EHCC Sports Car (Gr. P) class of the European Hillclimb Championship. It was a sh ...

and

Porsche 910 The Porsche 910 or Carrera 10 was a race car from Porsche, based on the Porsche 906. 29 were produced and were raced in 1966 and 1967. The factory name for the 910 was the 906/10. The 910 was considered the next sequence in the 906 line. Histor ...

racing cars between 1962 and 1968. A version of the Type 771 enlarged to was designated Type 771/1. In 1968, a new flat-eight engine called the Type 908 was introduced in the

Porsche 908 The Porsche 908 was a racing car from Porsche, introduced in 1968 to continue the Porsche 906-Porsche 910-Porsche 907 series of models designed by Helmuth Bott (chassis) and Hans Mezger (engine) under the leadership of racing chief Ferdinand Piëc ...

, which competed in the Group 6 Prototype-Sports Cars category. The Type 908 was not based on the Type 771, but instead the Type 916 flat-six racing six-cylinder engine with two additional cylinders. Porsche also installed Type 908 engines into two Porsche 914/8 mid-engined road cars. The first of these was a concept study built by the head of the racing department, and the second was built as a 60th birthday present for the company chairman. Although Porsche never produced any road cars with a flat-eight engine, a prototype of such an engine was built in the late 1960s and early 1970s. During this time, Posche had been commissioned by Volkswagen to develop "Project EA266", a replacement for the Volkswagen Beetle. Project EA266 was powered by a water-cooled inline-four engine which was mounted longitudinally under the rear passenger seat with the cylinders oriented horizontally. Porsche investigated using the Project EA266 chassis as the basis for a mid-engined replacement for the Porsche 911 called the "Typ 1966". As part of the Typ 1966 development process, a prototype water-cooled flat-eight engine was built. However, Project EA266 was cancelled by Volkswagen, all materials relating to the Typ 1966 were destroyed, including the prototype flat-eight engine.

Use in aircraft

Several flat-eight aircraft engines have been produced over the years: *1948-1961

Lycoming GSO-580 The Lycoming GSO-580 is a family of eight-cylinder horizontally opposed, supercharged, carburetor-equipped aircraft engines for both airplanes and helicopters, manufactured by Lycoming Engines in the late 1950s and early 1960s. The family incl ...

Air-cooled, 9.47 L (578 cu in) * 1961-present

Lycoming IO-720 The Lycoming IO-720 engine is a large displacement, horizontally opposed, eight-cylinder aircraft engine featuring four cylinders per side, manufactured by Lycoming Engines. There is no carburated version of the engine, which would have been d ...

. Air-cooled, * 1969-1975 Continental Tiara 8-380 (O-540) and T8-450 (O-540). Air-cooled, * 1997-200?

Jabiru 5100 The Jabiru 5100 is a lightweight four-stroke horizontally opposed flat-eight air-cooled aircraft engine, manufactured by Jabiru Aircraft. Design and development The engine is direct-drive and is fitted with alternators, silencers, vacuum p ...

. Air-cooled, * 2013-???? Engineered Propulsion Systems ''Vision 350''. Turbo-diesel, water-cooled,

Use in marine vessels

Flat-eight engines are seldom used in marine applications, with the 1957 Fageol VIP 88 outboard engine being a rare example of a production engine. The VIP 88 consisted of two

Crosley Crosley was a small, independent American manufacturer of subcompact cars, bordering on microcars. At first called the Crosley Corporation and later Crosley Motors Incorporated, the Cincinnati, Ohio, firm was active from 1939 to 1952, inter ...

engine blocks joined on a common crankshaft. Another marine engine was the one-off Miller 148 marine engine which was built in 1928 and used in the racing boat ''Miss Rioco III''. The Miller 148 had dual overhead camshafts and a displacement of .

References