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Balance and smoothness

Straight-6 four-stroke cycle engines are in perfect primary and secondary balance and require no balance shaft. They are in primary balance because the two ends of the engines are mirror images of each other and the cylinders move up and down in pairs, one on the compression stroke and the other on the exhaust stroke. Piston #1 balances #6, #2 balances #5, and #3 balances #4, canceling the end-to-end rocking motion that would otherwise result. (This does not apply to two-stroke cycle engines.) Secondary imbalance occurs in straight-4 engines because the two pistons on the upper 180 degrees of the crankshaft rotation move faster than the two pistons on the lower 180 degrees, creating an unequal motion. Straight-6 engines have cranks at 120 degrees to each other, so the differences in speed on different parts of the crankshaft rotation are offset by the changing number of pistons on each portion.

The straight-6 is smoother than engines with a fewer number of cylinders because the power strokes of pistons partially overlap. Since each power stroke lasts 180 degrees of crankshaft rotation, while a new piston starts its power stroke every 120 degrees, there are 60 degrees of overlap on each stroke in which one piston is finishing while the next is starting. This results in a smooth delivery of power, unlike a four cylinder engine in which each piston must come to a complete stop before the next piston commences its power stroke. This makes the straight-6 engine's delivery of power much smoother than a four-cylinder engine. The fact that there are basically two straight sixes on the same crankshaft is the reason why 60 degree and 180 degree V12 engines are considered optimal for smooth power delivery; they allow for triple overlap of the power stroke between three cylinders at all times due to the overlap from their two component straight-6 banks. Eight cylinder engines have even more power stroke overlap than six cylinder ones, but the improvement in smoothness is not as pronounced as that of sixes over fours.