Flat-eight engine

A Flat-Eight or Horizontally-Opposed Eight is an internal combustion engine in flat configuration, having 8 cylinders.

A well-known engine of this type is the Porsche air-cooled flat-8 engine. It was introduced as a 1.5-litre engine for Formula One, and grew to a 3-litre version used in the Porsche 908.

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

A flat-8 is marginally smoother than a flat-6, but since both types are exceptionally smooth, it is normally simpler and cheaper to specify a flat-6. The main reason to use a flat-8 would be if the engine's cubic capacity is such that the cylinder size in a flat-6 would be greater than optimal. For instance, the Australian Jabiru Aircraft company makes three different boxer air-cooled aircraft engines: a 2.2-litre flat-4, a 3.3-litre flat-6, and a 5.1-litre flat-8. A 5.1-litre flat-8 has eight cylinders of 638cc, whereas a 5.1-litre flat-six would have cylinders of 850cc.

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