Straight-eight engine

The straight-eight engine or inline-eight engine is an eight-cylinder internal combustion engine with all eight cylinders mounted in a straight line along the crankcase. The type has been produced in side-valve, overhead-valve, sleeve-valve, and overhead-cam configurations.

A straight-8 can be timed for inherent primary and secondary balance, with no unbalanced primary or secondary forces or moments. However, crankshaft torsional vibration, present to some degree in all engines, is sufficient to require the use of a harmonic damper at the accessory end of the crankshaft. Without such damping, fatigue cracking near the rear main bearing journal may occur, leading to engine failure.

Although an inline six-cylinder engine can also be timed for inherent primary and secondary balance, a straight-8 develops more power strokes per revolution and, as a result, will run more smoothly under load than an inline six. Also, due to the even number of power strokes per revolution, the straight-8 does not produce unpleasant odd-order harmonic vibration in the vehicle's driveline at low engine speeds.