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The straight-4 is an internal combustion engine with four cylinders aligned in one row. This is the most common configuration used in carss with an engine size up to 2.0, in some cases up to 3 litres.

The straight-4 engine is not a balanced configuration and while this is tolerable in a small, low-displacement, low-power configuration the vibrations get worse with increasing size and power. Most straight-4 engines below 2 litres in displacement rely on the damping effect of their engine mountings, but most engineers consider the use of balancing shafts essential above that size. A 4-cylinder engine needs twin balancing shafts, rotating at twice the crankshaft frequency, to be smooth.

Use of balance shafts allowed Porsche to use a 3 litre straight-4 engine on the Porsche 944 and other vehicles.

See also: straight engine