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

A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-six engine, where each pair of opposed cylinders moves inwards and outwards at the same time. An alternative configuration for flat engines is a 180-degree V engine, where both cylinders move to the right then the left at the same time.

The advantages of the flat-six layout are good engine balance (for reduced vibration), a low center of gravity, short length (compared with an inline-six engine) and being well suited to air-cooling. The disadvantages are a large width (which can limit the maximum steering angle when used in a front-engined car), a large intake manifold being required when a central carburetor is used, and duplication of the inlet and outlet connections for water-cooled engines.[1]

The first production flat-six engine was in the 1904 Wilson-Pilcher 18/24 HP car. The most notable use of flat-six engines is the Porsche 911 sports car, which has used flat-six engines continuously since 1963. Several other car manufacturers, including Subaru, have produced flat-six engines at times. Flat-six engines have also occasionally been used in motorcycles, and commonly in general aviation aircraft, along with the flat-four engine.

Engine balance

[edit]
Further information: Flat engine

A boxer-style flat-six engine is able to have perfect primary and secondary balance. As in other six-cylinder engines, the overlapping of the power strokes of the different cylinders (with a firing interval of 120 degrees in a four-stroke engine) reduces the pulsating of the power delivery relative to that of similar engines with fewer cylinders.

In a boxer configuration, a flat-six engine does not have a rocking couple. The symmetry of the arrangement, in which one bank of three cylinders is mirrored by the other bank, means that there is no net force from any given mirrored cylinder pair along the axis of movement of the pistons. So not only is there no net primary or secondary reciprocating effect, there is no net turning force - moment - that would try to rotate the engine back and forth about its engine mountings. (The moment is not quite zero in practice because each cylinder pair is offset slightly from the other, or else they would clash at the crankshaft.) In this regard, the six cylinder boxer engine is the same as a flat-four boxer engine, which also does not experience any rocking couple.[2]

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

Use in aircraft

[edit]
1945–1975 Franklin O-335 air-cooled aircraft engine

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 their model codes for flat-layout engines as a designation for “opposed” cylinder layouts. 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, although the radial design offers improved cooling.

Some aircraft have used flat-six engines originally designed for cars. The Porsche PFM 3200 engine, produced from 1985 to 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]

A notable recent flat-six aero-engine is the 4-litre side-valve Belgian D-Motor LF39, a modular variant of the 2.7-litre flat-four D-Motor LF26. Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high engine rpm, the massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy (and bulky) complication of ohv valve-gear).[5]

Use in automobiles

[edit]

Porsche

[edit]
Further information: Porsche flat-six engine
1966 Porsche 911
1967 Porsche 911R
2006 Porsche Cayman S
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The flat-six engine is often associated with the Porsche 911, a rear-engined sports car which has used flat-six engines exclusively since 1963. The engines were air-cooled until 1999, when Porsche started using water-cooled engines.[6]

Other Porsche models that use flat-six engines are the 1970–1972 Porsche 914/6 (mid-engine), the 1986–1993 Porsche 959 (rear-engine), and the 1996–2021 Porsche Boxster/Cayman (mid-engine).

Other manufacturers

[edit]
1904 Wilson-Pilcher engine
2007–2019 Subaru EZ36 engine

The first car to use a flat-six engine was the 1904–1907 Wilson-Pilcher 18/24 HP,[7] which used an engine based on a flat-four engine with two cylinders added. The Wilson-Pilcher was a front-engined car with the crankshaft in-line with the chassis and the cylinders between the chassis rails. Reports on this car quote it as being "remarkably silent and smooth running" and "almost total absence of vibration".[8]

Two American manufacturers briefly produced cars with flat-six engines—the 1948 Tucker 48 (water-cooled, based on the Franklin O-335) and the 1960–1969 Chevrolet Corvair (air-cooled).[9] Both are rear-engined cars with rear-wheel drive. Chevrolet produced over 1.8 million Corvairs of various types and utilized a turbocharger on some models; one of the first uses of a turbocharger on a mass-produced automobile.

Japanese manufacturer Subaru produced water-cooled flat-6 engines from 1988–1996 and 2000–2019.[10] Their first flat-6 engine, the Subaru ER27, was added to the model range of the Subaru XT coupe. In 1991, the Subaru EG33 engine was used in the Subaru Alcyone SVX, the replacement for the Subaru XT.[11] Following a hiatus of flat-6 engine production for four years, the Subaru EZ30 engine was introduced in the 2000 Subaru Legacy/Outback. The Subaru EZ36 engine was introduced in 2007 and was produced until 2019. All engines were used in front-engine cars with all-wheel drive, plus the XT and Alcyone SVX were also available with front-wheel drive.

Use in motorcycles

[edit]
1997–2004 Honda Valkyrie engine

Most motorcycles use engines with four or fewer cylinders, however the Honda Gold Wing touring motorcycle has used a water-cooled flat-six engine since 1988.[12] Initially, the engine had a displacement of 1.5 L (92 cu in), until it was enlarged to 1.8 L (110 cu in) in 2001.[13]: 208 

The Honda Valkyrie F6C (1997–2003) was a cruiser based on the GL1500 Gold Wing.[13]: 198, 218  The limited edition 2004 Valkyrie Rune was based on the GL1800.[13]: 218, 220 

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Flat-six engine

View on Grokipedia
from Grokipedia
A flat-six engine, also known as a boxer-six or horizontally opposed-six engine, is a six-cylinder configuration featuring two banks of three cylinders each, arranged flat and opposed at a 180-degree angle on either side of a central , with the pistons moving horizontally toward and away from each other. This design, a specific extension of the broader flat or boxer engine family, originated from Carl Benz's 1896 invention of the two-cylinder "contra engine," which laid the groundwork for horizontally opposed layouts to achieve inherent balance and reduced vibration. Flat-six engines have also been widely used in for their balance and reliability. The flat-six's technical design emphasizes compactness in height and length while being wider than inline-six equivalents, allowing for a low center of gravity that enhances vehicle stability, traction, and handling—particularly in rear-engine sports cars. Key characteristics include smooth operation without secondary vibrational forces due to the opposed pistons canceling each other's motion, high specific power output, and revving capability up to 9,000 rpm in modern variants, often paired with air- or water-cooling systems, turbocharging, and direct injection for efficiency and performance. Early flat-six engines were air-cooled, as seen in Porsche's implementations, but transitioned to water-cooling in the late 1990s to enable advanced valvetrain designs and emissions compliance without sacrificing dynamics. Historically, the first production flat-six appeared in the 1904 Wilson-Pilcher 18/24 HP automobile, a British design leveraging the layout's balance for luxury motoring, followed by innovative uses like the 1948 Tucker Torpedo's 5.5-liter air-cooled unit producing 166 horsepower. In the post-war era, Chevrolet adopted it for the 1960–1969 Corvair with displacements up to 164 cubic inches (2.7 liters) and outputs reaching 150 horsepower, though packaging challenges and cooling issues contributed to its discontinuation. Subaru introduced flat-six engines in production vehicles starting in 1991 with the EG33 in the SVX, followed by the EZ-series from 2000 in models like the Legacy and , offering all-wheel-drive integration until phasing out flat-six production in 2019 due to stricter emissions standards and a shift to four-cylinder turbo alternatives. Porsche remains the sole major automaker producing flat-six engines as of 2025, most iconically in the 911 sports car line since its 1963 debut with a 2.0-liter air-cooled unit delivering 130 horsepower, evolving to today's 3.0- to 4.0-liter twin-turbocharged water-cooled versions exceeding 500 horsepower in models like the 911 GT3 RS. This enduring choice stems from the engine's synergy with 's rear-engine philosophy, providing superior , braking performance, and a distinctive flat curve that improves at higher loads compared to V6 counterparts. Despite challenges like increased manufacturing complexity and wider packaging demands, the flat-six's blend of precision and driving thrill continues to define high-performance .

Engine balance

In a flat-six engine, the horizontally opposed layout ensures inherent balance by having pistons in opposing cylinders move symmetrically toward and away from the . This configuration cancels primary inertial forces—resulting from reciprocation—along the axes, producing no net horizontal vibration. Secondary forces, caused by the varying during the cycle (particularly near top dead center), are also neutralized due to the even number of cylinders and their paired opposition, eliminating the rocking couple present in flat-four engines. As a result, flat-six engines operate smoothly across their rev range without requiring balance shafts, similar to the inline-six but with a wider profile. Flat-six engines, known for their balance and low vibration, have been widely used in , particularly in and . Early examples include the Franklin 6A and 6V series, air-cooled horizontally opposed six-cylinder engines producing 215–320 horsepower, which powered like the Piper PA-23 Apache and from the 1940s to the 1960s. Continental Motors developed the O-300 family in the 1940s, a 145–160 horsepower flat-six engine that became popular in single-engine such as the and the early , valued for its reliability and direct-drive compatibility. Larger variants like the IO-520 and IO-550, with displacements up to 550 cubic inches (9.0 liters) and outputs reaching 300 horsepower, have been standard in higher-performance models including the and Piper Malibu since the 1960s, often featuring for improved efficiency. Lycoming Engines offers the O-435 and IO-540 series, six-cylinder air-cooled designs producing 190–360 horsepower, used in both like the Six and helicopters such as the Bell 47. The IO-540, introduced in the , remains in production as of 2025 for its robust performance in demanding environments. Modern and experimental applications include the , a 120-horsepower air-cooled flat-six for , and ULPower's UL520iS, a fuel-injected 200-horsepower variant with electronic ignition. In the 1980s, adapted its 911 flat-six into the PFM 3200 turbocharged engine (300–375 horsepower) for experimental kit planes like the Glasair III, though production was limited due to certification challenges.

Use in automobiles

Porsche

Porsche has been synonymous with the flat-six engine since the introduction of the 911 sports car in 1963, where it replaced the flat-four used in the predecessor 356 model. The engine's horizontally opposed cylinder layout, mounted at the rear, contributes to a low center of gravity and balanced weight distribution, enhancing the car's handling dynamics. This design principle, rooted in Ferdinand Porsche's early work on the Volkswagen Beetle's flat-four, was expanded to six cylinders for greater power and refinement while maintaining the boxer configuration's inherent smoothness and vibration-free operation. The original 911's air-cooled 2.0-liter flat-six produced 130 PS (96 kW), enabling a top speed of 210 km/h, and evolved through displacements up to 3.6 liters by the 993 generation in 1998, with outputs reaching 300 hp in naturally aspirated variants like the Carrera RS. Turbocharging was introduced in 1975 with the 911 Turbo's 2.7-liter engine delivering 260 PS, progressing to twin-turbo setups in later models for boosted performance, such as the 993 Turbo's 408 PS from a 3.6-liter unit. These air-cooled engines featured single overhead camshafts per bank, aluminum construction, and dry-sump lubrication, prioritizing lightweight construction and high-revving capability. The air-cooled era ended with the 993, celebrated for its iconic fan-driven cooling and raspy exhaust note. In 1998, the 996 generation marked the shift to for improved emissions compliance and , introducing four valves per cylinder and the Variocam system in the 3.4-liter base engine producing 296 hp. Subsequent 997 and 991 generations refined this with direct , as seen in the 991's 3.8-liter unit outputting up to 400 hp in Carrera S models, and advanced turbo systems like variable turbine geometry in the 997 Turbo's 3.6-liter engine achieving 500 hp. The flat-six's rear-engine placement continued to define Porsche's philosophy, with displacements stabilizing around 3.0 to 4.0 liters in modern variants, including high-performance GT3 models featuring naturally aspirated 4.0-liter engines exceeding 500 hp. Hybrid integration began in the 992 generation's 2024 models, pairing a 3.6-liter flat-six with a single and electric motors for outputs over 500 hp, blending traditional boxer character with electrification. As of 2025, the hybrid flat-six continues in the 911 lineup, with no other major automakers producing flat-six engines. Beyond the 911, Porsche applied flat-six technology in models like the 914/6 (1969–1972) with a 2.0-liter mid-engine variant producing 110 hp. The engine's enduring appeal lies in its engineering balance, providing superior refinement.

Other manufacturers

The Wilson-Pilcher Motor Carriage Company of England produced the first known production flat-six engine in 1904, a water-cooled 4.0-liter unit offering 24 horsepower in their 18/24 HP model. This horizontally opposed design provided inherent balance and a low center of gravity, influencing later engineering, though only a handful of vehicles were built before the company's closure in 1907. In the United States, the sedan of 1948 featured a rear-mounted, water-cooled 5.5-liter flat-six engine derived from a Franklin helicopter powerplant, producing 166 horsepower and enabling a top speed of around 120 mph. Adapted by Air Cooled Motors for automotive use, this 335-cubic-inch engine emphasized safety and innovation in the short-lived Tucker Corporation's output of just 51 cars before financial collapse. Chevrolet introduced the Turbo-Air 6, an air-cooled aluminum flat-six engine, in the rear-engined starting in 1960. Displacing 140 cubic inches (2.3 liters) initially and producing 80 horsepower, it evolved to 164 cubic inches (2.7 liters) by 1964 with outputs up to 140 horsepower in naturally aspirated form. A turbocharged variant in the 1962-1965 models delivered 150 horsepower, marking the first production turbocharged passenger car engine in the U.S. and highlighting the layout's potential for despite cooling challenges. Over 1.8 million Corvairs were produced until 1969, after which discontinued the engine due to reliability issues and shifting market preferences. Subaru, known for boxer engines, developed its first flat-six in 1987 with the water-cooled ER27, a 2.7-liter unit producing 145 horsepower for the Alcyone XT6 coupe and SVX sports car. This evolved into the EZ series in 2000, including the 3.0-liter EZ30 (up to 250 horsepower) used in Legacy sedans, Outback wagons, and Tribeca SUVs through 2019, offering naturally aspirated and turbocharged options for improved refinement and all-wheel-drive integration. The EZ36, a 3.6-liter variant with 256 horsepower, powered higher-trim models like the 2008-2014 Tribeca until Subaru phased out flat-six production in favor of four-cylinder turbo engines for efficiency.

Use in motorcycles

Flat-six engines are rare in motorcycles due to packaging constraints, but they have been used in high-capacity touring models for their smoothness and balance. The , introduced in 1988, was the first production to feature a flat-six engine, with the GL1500 model employing a 1,520 cc water-cooled unit producing 100 (75 kW) and 110 lb⋅ft (149 N⋅m) of . This engine remained in use until 2000. In 1997, introduced the (F6C), a derived from the GL1500's flat-six engine, detuned to 100 bhp (75 kW) and 87 lb⋅ft (118 N⋅m) of , and produced until 2003. The Gold Wing continued with the flat-six layout in the GL1800 model from 2001 onward, featuring a larger 1,833 cc engine delivering 125 bhp (93 kW) and 125 lb⋅ft (169 N⋅m) of as of the 2024 model. As of 2025, the Gold Wing remains in production with this configuration, emphasizing luxury touring with low vibration and a low center of gravity.

References

  1. https://newsroom.porsche.com/en_US/technology/porsche-flat-engine-sports-car-tradition-design-principle-internal-combustion-engine-lightweight-power-dynamic-15767.html
  2. https://www.enginebuildermag.com/2014/09/boxer-engine-novel-technology-dying-fad/
  3. https://www.carthrottle.com/features/7-flat-six-powered-vehicles-arent-porsches
  4. https://carbuzz.com/only-one-automaker-producing-flat-six-engine-porsche-911/
  5. https://motorwerke.ca/porsche-flat-six-engine-explained/
  6. https://www.autozine.org/technical_school/engine/Smoothness2.html
  7. https://www.flyingmag.com/photo-gallery-photos-50-amazing-aircraft-engines/
  8. https://www.aeroexpo.online/aeronautic-manufacturer/6-cylinder-piston-engine-5411.html
  9. https://www.roadandtrack.com/car-culture/travel/a18196664/when-porsche-put-the-911s-engine-into-airplanes/
  10. https://ulpower.com/en/engines/ul520/ul520is
  11. https://newsroom.porsche.com/en/2024/history/porsche-drive-system-911-36059.html
  12. https://www.caranddriver.com/news/a15366914/flat-sixy-the-evolution-of-porsche-911-engine-size-technology-and-output-in-the-u-s/
  13. https://newsroom.porsche.com/en/2024/products/porsche-new-911-world-premiere-hybrid-36322.html
  14. https://www.petersen.org/vehicle-spotlights/1948-tucker-48
  15. https://www.audrainautomuseum.org/shining-bright/1948-tucker-model-48
  16. https://www.hemmings.com/stories/corvair-turbo-air-6/
  17. https://carbuzz.com/chevrolet-corvair-monza-worlds-first-turbocharged-flat-six/
  18. https://motofutura.com/2023/03/19/flat-six-engine-history/
  19. https://carbuzz.com/what-the-last-subaru-flat-six-engine-model-costs-today/
  20. https://www.visordown.com/features/top-10s/top-10-six-cylinder-production-bikes
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