A flat-eight engine, also called a horizontally-opposed eight, is an eight-cylinder piston engine with two banks of four inline cylinders, one on each side of a central crankshaft, 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-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.


See also: Flat engine

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 and secondary balance.

A boxer-eight engine has a single piston per crankpin, 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.[1]: 156, 166, 171–173 

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

Buffum Model G Greyhound

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.[2] The Model G was built in the United States and was introduced one year after the Winton Motor Carriage Company 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.[3] The car is on display at the Kelvingrove Art Gallery and Museum in Glasgow.[4] 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.[5][6][7] In 1969, a one-off engine was built for racing driver Emerson Fittipaldi to compete in the Thousand Miles of Guanabara endurance race.[8] 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.[9][10][11][12]


Main article: Porsche flat-eight engines

Porsche 771 engine

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 1.5 L (92 cu in) displacement limit for the 1961 Formula One season. Designed by Hans Mezger 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.[13]: 98 [14]: 324  The version of the Type 753 used for its debut at the 1962 Dutch Formula One Grand Prix produced 138 kW (185 hp).[15] 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.[16]: 26 

A 2.0 L (122 cu in) version of the Porsche flat-eight, designated the Type 771, was developed alongside the Type 753.[14]: 324  This engine was intended for sports car and endurance racing, and was used in the Porsche 718 W-RS, Porsche 904/8[16]: 33 ), Porsche 906,[17] Porsche 907, Porsche 909 and Porsche 910 racing cars between 1962 and 1968. A version of the Type 771 enlarged to 2.2 L (134 cu in) was designated Type 771/1.

In 1968, a new flat-eight engine called the Type 908 was introduced in the Porsche 908, 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.[18] 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.[19]

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, Porsche had been commissioned by Volkswagen to develop "Project EA266", a replacement for the Volkswagen Beetle.[20] 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".[21] As part of the Typ 1966 development process, a prototype water-cooled flat-eight engine was built.[22]: 59, 68  However, Project EA266 was cancelled by Volkswagen, all materials relating to the Typ 1966 were destroyed, including the prototype flat-eight engine.[23]: 73, 86 

Use in aircraft

Jabiru 5100 aircraft engine

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

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 0.7 L (44 cu in) Crosley engine blocks joined on a common crankshaft.[27][28]

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 2.4 L (148 cu in).[29][30]

See also


  1. ^ Smith, Phillip H. (1977). The Design and Tuning and Competition Engines. Robert Bentley Inc. ISBN 0-8376-0140-1.
  2. ^ Evans, Steve (12 July 2013). "1904 Central Greyhound 8-cylinder racer by H. H. Buffum Co". vintagemotoring.blogspot.com. Archived from the original on 24 May 2021. Retrieved 9 May 2019.
  3. ^ B., W. (October 1962). "The Anderson Specials". MotorSport magazine. p. 26.
  4. ^ Barry, Maggie (17 January 2018). "The Anderson Special - a race car built in Scotland far ahead of its time". www.dailyrecord.co.uk.
  5. ^ "Ingenious Powerplant Wins Engineer Award". The Indianapolis Star. 21 May 1977. p. 24.
  6. ^ "Crower History". www.crower.com.
  7. ^ Brown, Allen (25 March 2019). "Eagle 1972 Indy car-by-car histories". www.oldracingcars.com.
  8. ^ Nishihata, Leo (8 May 2011). "Emerson Fittipaldi's double-engined Volkswagen Beetle". jalopnik.com.
  9. ^ "Prototype "Jamaro"". www.lorenagt.com (in Portuguese).
  10. ^ Nicoliello, Felipe (12 November 2010). "Reportagens - Oficina 1972 - Motor 8 cilindros" [News - Workshop 1972 - 8 cylinder engine]. www.pumaclassic.com.br (in Portuguese).
  11. ^ Nicoliello, Felipe (1 November 2014). "História Puma - Motor boxer 8 cilindros" [Puma History - 8-cylinder boxer engine]. www.pumaclassic.com.br (in Portuguese).
  12. ^ Biela, Chico (18 October 2009). "Preparação de motores VW AR e fuscas" [Preparation of VW AR engines and fuscas]. chicobiela.nafoto.net (in Portuguese).
  13. ^ Long, Brian (15 October 2008). Porsche Racing Cars: 1953 to 1975. Veloce Publishing. ISBN 978-1904788447.
  14. ^ a b Whitelock, Mark (10 August 2006). 1 1/2-litre Grand Prix Racing: Low Power, High Tech. Veloce Publishing. ISBN 978-1845840167.
  15. ^ Söhnke, Michael (April–May 2005). "A Relic of the Sixties" (PDF). Christophorus. Vol. 313. Porsche AG. pp. 38–44. Archived from the original (PDF) on 2019-05-08. Retrieved 2019-05-09.
  16. ^ a b Long, Brian (21 November 2003). Porsche 911, 1963 to 1971. Veloce. ISBN 978-1903706282.
  17. ^ "Alan Hamilton, Australian Champion: His Porsche 904/8 and two 906's…". www.primotipo.com. 20 August 2015. Retrieved 29 September 2019.
  18. ^ Smale, Glen (19 January 2018). "Porsche twin-cam 6-cylinder engine". www.porscheroadandrace.com.
  19. ^ "VW-Porsche 914 turns 40" (PDF). Zeitschrift. Club Veedub Sydney Inc. June 2009. pp. 15–16.
  20. ^ Torchinsky, Jason (29 December 2014). "This Fascinating Stillborn VW Prototype Would Have Been Revolutionary". jalopnik.com.
  21. ^ "Porsche 911 history: the complete timeline from 901 to 992 - Porsche 911: 1964 - 1989". www.evo.co.uk. Retrieved 29 September 2019.
  22. ^ Leffingwell, Randy (30 July 2019). The Complete Book of Porsche 911: Every Model Since 1964. Motorbooks. ISBN 978-0760365038.
  23. ^ Leffingwell, Randy (10 November 2005). Porsche 911: Perfection by Design. Motorbooks. ISBN 978-0760320921.
  24. ^ "Jabiru 8 Cylinder Engine". 4 March 2009. Archived from the original on 2 October 2009. Retrieved 17 April 2019.
  25. ^ "EPS Launches Diesel Aero-Engine Following Successful Propeller Vibration Testing at Hartzell Propeller". eps.aero. 24 January 2013. Archived from the original on 24 May 2021. Retrieved 9 May 2019.
  26. ^ "An aerodiesel engine". patents.google.com.
  27. ^ "Fageol". www.fiberglassics.com.
  28. ^ "Crosley Engine Family Tree - The Later Years". crosleyautoclub.com.
  29. ^ "The Miller 148 Flat-Eight Marine". www.milleroffy.com.
  30. ^ "Vintage Pornography: the naked Millers". www.prewarcar.com. 26 March 2013.