Straight six

The straight-six engine or inline-six engine (often abbreviated I6 or L6) is an internal combustion engine with the cylinders mounted in a straight line along the crankcase with all the pistons driving a common crankshaft (straight engine).

The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.

In automobiles, the straight-six design is used for engine displacements ranging from approximately 2 to 5 litres (120 to 310 cu in). It is also sometimes used for smaller engines but these, although very smooth running, tend to be rather expensive to manufacture in terms of cost-to-power ratio. Since the length of an engine is roughly proportional to the number of cylinders in one bank, the straight-six is necessarily longer than alternative layouts such as I4, V4, V6, or V8.

One of the smallest production straight-sixes was found in the Benelli 750 Sei motorcycle, displacing 747.7 cc (45.63 cu in) (0.7477 L). Honda and Mike Hailwood raced in the 1960s with the RC166 250 cc (15 cu in) (0.25 L) six-cylinder, 24-valve motorcycle engine. Pre-World War II engines could be quite large by modern standards — such as the Rolls Royce Silver Ghost's 7.4 L engine and the 824 cu in  (13.5 L) of the 1910s Peerless, Pierce, and Fageol.

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