Flat-six engine

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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.

Balance and smoothness[edit]

The movement of the pistons in a horizontal engine is all in the same plane, so it creates less vibration than in a V-configuration engine; particularly one, like a V6, with an odd number of cylinders on each side of the engine. Unlike the V6 but like the inline-6, the flat-6 is a fully balanced configuration which is in perfect primary and secondary balance. The three cylinders on each side of the crankcase tend to have an end-to-end rocking motion, like a pair of inline-triple engines, but in the usual boxer engine configuration, the imbalances on each side cancel each other, resulting in a perfectly smooth engine.

The flat-6 is also smoother than the flat-4 or inline-4 because the power strokes of the cylinders overlap in a four-stroke cycle engine. In these four-cylinder configurations, pistons are 180 degrees apart in crankshaft rotation and start their power strokes every 180 degrees, so each piston must come to a complete stop before the next one commences its power stroke. In the flat-6, each power stroke begins 120 degrees after the previous one starts, resulting in 60 degrees of overlap between power strokes and a much smoother delivery of power to the flywheel.

Aircraft engines[edit]

Franklin O-335 335 cu in (5.49 l) air-cooled aircraft engine introduced in 1945

Lycoming is a major US manufacturer of civilian aircraft engines that power over one half of general aviation aircraft. They have developed and currently produce a series of flat-4 and flat-6 aircraft engines, and one flat-8 engine. The configuration is expressed in the engine's model numbers as the letter "O", standing for "opposed" in reference to the cylinder arrangement. Franklin produced horizontally opposed flat aircraft engines beginning in the 1930s. The Franklin O-335 was used in the Bell 47 light helicopter. Continental Motors continues to manufacture flat-4 and flat-6 air-cooled aircraft engines — their initial model, the Continental A-40, appeared in 1931. Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because they were less expensive to manufacture. Radial engines have inherently good cooling, but their large frontal area increases drag. It is not difficult to design flat-6 and flat-8 engines from flat-4 designs by adding more cylinder pairs with a new crankshaft. In an air-cooled opposed-cylinder layout engine there can be cooling problems on flat-six designs, with the middle cylinder pairs. The flat-6 is smoother running than a flat-four design. Porsche adapted their 911 6 cylinder horizontally opposed air-cooled engines to become the Porsche PFM 3200 aero engine in late 1985 but, after low sales and a US$75 million investment, production ceased in 1991 - "homebrew" conversions of the Corvair flat-six air-cooled automobile engine have been used for powering examples of the Pietenpol Air Camper homebuilt monoplane aircraft.[2]

Automotive use[edit]

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[3] 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.[4]

List of automobiles with flat-six engines[edit]


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.[6] This larger model lasted for 13 years before being replaced by the even-larger GL1800 in 2001.[7]

The Honda Valkyrie F6C (1997–2003) was a cruiser based on the GL1500 Gold Wing.[8] The limited edition 2004 Valkyrie Rune was based on the GL1800.[9]


  1. ^ Nunney, M J (2007). Light and Heavy Vehicle Technology. Butterworth-Heinemann. p. 13. ISBN 978-0-7506-8037-0.
  2. ^ "Pietenpol Official Pietenpol Air Camper Family Website". community.pressenter.net. 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".
  3. ^ "The Wilson-Pilcher Petrol Cars", The Automotor Journal, April 16th, 1904, pp463-468
  4. ^ "6H 358". Chtz-uraltrac.ru. Retrieved 2011-12-06.
  5. ^ "Subaru Cars, Sedans, SUVs - Subaru of America". Subaru of America, Inc.
  6. ^ Mitchel 2005, p. 170.
  7. ^ Mitchel 2005, p. 208.
  8. ^ Mitchel 2005, pp. 198, 218.
  9. ^ 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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