A flat-8 or horizontally opposed-8 is an internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 in flat

Flat engine

A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...

 configuration, having 8 cylinders

Cylinder (engine)

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

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The most famous engine of this type is the Porsche

Porsche

Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....

 air-cooled flat-8 engine introduced in a 1.5 L version for Formula One

Formula One

Formula One, also known as Formula 1 or F1 and referred to officially as the FIA Formula One World Championship, is the highest class of single seater auto racing sanctioned by the Fédération Internationale de l'Automobile . The "formula" designation in the name refers to a set of rules with which...

 that grew up to a 3 L version used in the Porsche 908

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 under Ferdinand Piech....

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Another example is the Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

n-made Jabiru Aircraft

Jabiru Aircraft

Jabiru Aircraft Pty Ltd is an Australian aircraft manufacturer that produces a range of kit- and ready-built civil light aircraft in Bundaberg, Queensland. The company also designs and manufactures a range of light aircraft engines...

 5100. This is a 5.1-litre, air-cooled engine for aircraft use.

Continental and Lycoming

Lycoming Engines

Lycoming Engines is a U.S. aircraft engine company, known primarily for its general aviation engines. For most of its history Lycoming has been part of the AVCO group as AVCO Lycoming. In 1987 AVCO was purchased by Textron to become Textron Lycoming...

have both produced flat-8 aircraft engines as well. The Lycoming IO-720 and Continental IO-720 series of engines have been in production for several decades.