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

The use of larger stroke limits the RPM of the engine for two reasons, first the total speed of the piston has a practical limit, second the burning of the fuel mixture proceeds at a fixed rate, and the use of a longer stroke dictates that at high RPM, the fuel cannot be fully burned in a larger cylinder.

Most engines manufactured before WWII were of undersquare design. Undersquare engines have their advantages, however they tend to be heavy and use more fuel than more modern designs.

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