High Quality Content by WIKIPEDIA articles! A flat-4 or horizontally-opposed-4 is a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase. The pistons are usually mounted on the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching their gloves together before a fight, which has led to it being referred to as a boxer engine. The configuration results in inherently good balance of the reciprocating parts, a low centre of gravity, and a very short engine length. The layout also lends itself to efficient air cooling. However, it is an expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for cruising motorcycles and aircraft than ordinary passenger cars.

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