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Continental O-470-13A air-cooled aircraft engine

A flat-six or horizontally opposed-six is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase. This should not be confused with the Opposed-piston engine. The pistons are mounted to the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching their gloves together before a fight, which has led to it being referred to as a boxer engine.

The configuration results in inherently good balance of the reciprocating parts, a low center of gravity, and a very short engine length. The layout also lends itself to effective air cooling. The shape of the engine suits it better for rear engine and mid-engine designs, where the low center of gravity is an advantage; in front engine designs the width interferes with the ability of the front wheels to steer. However, it is an intrinsically expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for luxury sports cars, cruising motorcycles, and aircraft.[1]

Only a few auto makers, including Porsche and Subaru, currently use horizontally opposed engines. Porsche continues to be the most prominent manufacturer of flat-6 engine luxury sports cars, while Subaru uses it in its all-wheel drive cars, where the difficulties of fitting the engine between the front wheels are offset by the efficiency of adding four-wheel drive to the layout. In the past a number of other manufacturers have used them, notably Preston Tucker in the 1948 Tucker Sedan and Chevrolet in the 1960s Corvair with the flat-6 air-cooled Chevrolet Turbo-Air 6 engine. The Citroën DS was originally intended to be fitted with an air-cooled flat-6, but this never materialised.


Engine balanceEdit

A flat-six engine is able to have perfect primary balance and secondary balance. As per other six-cylinder engines, the overlapping power strokes of the cylinders (with a firing interval of 120 degrees in a four-stroke engine) reduces the pulsating power delivery seen on engines with fewer cylinders. Also, when used in a boxer configuration, a flat-six engine does not have the rocking couple that is present on flat-four engines.[2]

These characteristics result in low vibration for flat-six engines (as per straight-six engines), especially when compared with the imbalances that are present in V6 engines with a 90 degree bank angle.

Use in aircraftEdit

1945-1975 Franklin O-335 air-cooled aircraft engine

One of the first flat-six aircraft engines is the Continental A40, which was produced from 1931-1942. Other early flat-six engines include the Franklin O-265 which began production in 1940, and the Lycoming O-435 which began production in 1942. Several manufacturers use the letter "O" in the model code for flat-six engines, as an abbreviation for the opposed cylinder layout. The Franklin O-335 was used in the Bell 47 light helicopter.

Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because they were less expensive to manufacture. The smaller frontal area compared with a radial engine also results in less drag.

Some aircraft have used flat-six engines originally designed for cars. The Porsche PFM 3200 engine, produced from 1985-1991, was based on the engine used in the Porsche 911 sports car. Several examples of the Pietenpol Air Camper homebuilt monoplane aircraft have used the air-cooled engine from the Chevrolet Corvair compact car.[3] Compared with the engines used in cars, flat-six engines used by helicopters have large displacements and are low revving, producing more torque and less power.[4]

Automotive useEdit

1904 Wilson-Pilcher

Possibly the earliest flat-six engined car was the Wilson-Pilcher car in 1900 (a flat-four version was also available). The engine was conventionally placed with the crankshaft in-line with the chassis and the cylinders between the chassis rails. Reports on this car[5] quote it as being "remarkably silent and smooth running" and "almost total absence of vibration". The car was produced by Wilson-Pilcher in London until 1904, then by Armstrong Whitworth & Co in Newcastle until c1907. The engine had equal bore and stroke of 95mm, and the cylinders were slightly offset so that each cylinder had its own crank-pin, with intermediate crankshaft bearings between each pair of cylinders. The engine was directly mounted to a 4-speed epicyclic gearbox employing helical gears in an oil bath, which was another reason for its quiet running.

Air-cooled 1966 Porsche 911

Flat-six engines are wide, and could restrict steering lock if placed in the conventional position for front-engine drivetrain layout in a modern car. As a result, most flat-six automobile designs have been rear-engine or rear mid-engine designs, and have been subject to the limitations and drawbacks of these designs. The longest surviving flat-six-engined model, the Porsche 911, is a rear-engined sports car with acceptably compromised interior room and a long history of suspension development to utilize the characteristics of the drivetrain layout.

Subaru, an automobile manufacturer with a history of making aircraft engines, has adapted a front-engine configuration for use with its flat engines. The engine is mounted longitudinally ahead of the front axle and the transmission is mounted longitudinally behind the front axle. Although this layout is intrinsically more expensive to manufacture, less compact, and less space efficient for front-wheel drive than a transverse V6, it allows the addition of four-wheel drive by taking power off both the front and back ends of the transmission, since the transmission is located between the front and rear axles. Commensurate with these properties of this layout, Subaru now specializes in all-wheel-drive vehicles.

Subaru's 2003-2009 EZ series EZ30 engine featured a Porsche designed and German manufactured Variable Valve Lift (VVL) system, that further enhances high range power and smoothness at idle. It was discontinued on the successor of the EZ30 engine, the EZ36 engine. Instead, it has dual AVCS, VVT on both inlet and exhaust cams.

The Chelyabinsk Tractor Plant 6H 358 is a 16.95 L flat-6 multi-fuel diesel engine.[6]

List of automobiles with flat-six enginesEdit


The flat-six of the Honda Valkyrie motorcycle

In 1988, after thirteen years of manufacturing the Honda Gold Wing with a flat-four engine, Honda introduced the GL1500 with a flat-six.[8] This larger model lasted for 13 years before being replaced by the even-larger GL1800 in 2001.[9]

The Honda Valkyrie F6C (1997–2003) was a cruiser based on the GL1500 Gold Wing.[10] The limited edition 2004 Valkyrie Rune was based on the GL1800.[11]

See alsoEdit


  1. ^ Nunney, M J (2007). Light and Heavy Vehicle Technology. Butterworth-Heinemann. p. 13. ISBN 978-0-7506-8037-0.
  2. ^ "The Physics of: Engine Cylinder-Bank Angles". Retrieved 9 September 2019.
  3. ^ "Pietenpol Official Pietenpol Air Camper Family Website". Pietenpol Aircraft Company. Retrieved May 6, 2017. As the years passed, Bernard Pietenpol taught himself how to repair televisions and radios. By 1960, Chevrolet introduced the six cylinder air-cooled Chevrolet Corvair. This would be the last engine Bernard would experiment with in powering his Air Camper. In 1966 Bernard completed his first Corvair powered Air Camper, then another in 1970 which was dubbed "the Last Original".
  4. ^ "The Lycoming O-540 Engine and Why We Use It". 18 November 2015. Retrieved 10 September 2019.
  5. ^ "The Wilson-Pilcher Petrol Cars", The Automotor Journal, April 16th, 1904, pp463-468
  6. ^ "6H 358". Retrieved 2011-12-06.
  7. ^ "Subaru Cars, Sedans, SUVs - Subaru of America". Subaru of America, Inc.
  8. ^ Mitchel 2005, p. 170.
  9. ^ Mitchel 2005, p. 208.
  10. ^ Mitchel 2005, pp. 198, 218.
  11. ^ Mitchel 2005, pp. 218, 220.


  • Mitchel, Doug (2005). Honda Motorcycles: Everything You Need to Know About Every Honda Motorcycle Ever Built. Krause Publications. ISBN 9780873499668.

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