An engine where the ratio of bore:stroke is greater than 1.0 is referred to as oversquare. Generally oversquare engines operate at higher RPM and lower torque.

An oversquare engine, due to its relatively short stroke length also uses a crankshaft with a smaller lever arm radius between the main and connecting rod bearings. The resulting smaller leverage and the larger cross section area of the piston results in higher direct loads being transmitted through the piston, rod and bearings.

Higher RPM and therefore total horsepower ranges are possible in oversquare designs for a few reasons. The combustion of the fuel mixture happens at a constant rate, therefor the use of a shorter cylinder allows all of the fuel to be consumed even at relatively high RPM. Perhaps more important, in the shorter stroke engines, the distance from the valves to the piston is shorter and therefore turbulence induced by the mixture entering through the relatively small intake valve port(s) reaches more of the combustion chamber. This results in better uniformity of the fuel:air mix and faster combustion rate or flame front rate.

Most engines manufactured after WWII are of oversquare design, some exceptions would include diesels or tractor engines. However even many modern diesel engines are now oversquare in design.

In aviation, oversquare refers to using a power setting with a manifold pressure in inches higher than the engine RPM in hundreds.

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