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By Paul Williams

Also termed a “boxer,” “flat-four” for four-cylinder engines, and “flat-six” for six-cylinder versions, the horizontally-opposed engine arranges its cylinders horizontally, so they push into a centrally located crankshaft. This is in contrast to the much more common “inline” engine (four, five or six cylinders, which is referred to as an I4, I5 or I6), where the cylinders are arranged vertically in a line, and the crankshaft is below them. Or the “V” engine (six or eight cylinders, arranged in two banks at an angle symbolized by the “V” (as in V6, V8, V10 or V12).

Probably the most well-known vehicle that used a horizontally opposed engine is the original Volkswagen Beetle/Bug. Another classic vehicle that used this type of engine was the Chevrolet Corvair.

Only two manufacturers currently use horizontally opposed engines: Subaru and Porsche. Subaru uses four and six-cylinder versions; Porsche uses a six-cylinder version in its Boxster, Cayman and Carrera models, but V6 and V8 engines in other models.

The benefits of a horizontally opposed engine are that it can be mounted lower in the car, lowering the vehicle’s centre of gravity and potentially improving handling. Also, “flat” engines are reputed to be better balanced, producing less vibration, and are compact in size.

However, “flat” engines tend to be noisier when idling and accelerating, and have a reputation for consuming more fuel than inline and “V” engines, although both Subaru and Porsche have taken steps to deal with the latter issue by using technologies such as CVT transmissions (Subaru) and direct fuel injection and twin-clutch transmissions (Porsche).

Subaru suggests that another benefit of the horizontally opposed engine is safety-related, in that because of its compact size, a larger “crush zone” can be created, and the engine can more easily slide below the vehicle’s front seat occupants in the event of a collision

Likely the most convincing reason for Subaru and Porsche to continue using “flat” engines is because of heritage and brand identity. Indeed, Porsche has on several occasions tried to move on from the H6 engine, but customers object too strenuously. For Subaru, along with standard all-wheel drive, the flat engine pretty much defines the brand.

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