Oversquare
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A piston engine is oversquare or shortstroke if its cylinders have a greater bore (width, diameter) than stroke (length of piston travel). This is generally considered to be a positive trait, since a longer stroke means greater friction and a weaker crankshaft. An oversquare engine is generally more reliable, wears less, and can be run at a higher speed; though with the aid of modern technology, the disadvantages of undersquare or longstroke engines have been overcome. In oversquare engines power does not suffer, but low-speed torque does to some degree, since torque is relative to crank throw (distance from the crank center to the crankpin)—the leverage, essentially. An oversquare engine cannot have as high a compression ratio as a similar engine with a much higher stroke ratio, and using the same octane fuel. This causes the oversquare engine to have poorer fuel economy, and somewhat poorer exhaust emissions. Engines can be modified by being "de-stroked", shortening the stroke to increase maximum rpms and top-end horsepower, at the expense of low-end torque.
Oversquare or shortstroke engines have a tendency to overheat, but modern designs usually compensate for this tendency. Oversquare engines are lighter and shorter than similar undersquare engines, except for their longer length. As the length is not a large problem, these engine types are highly favored by many manufacturers because of their power and compact size.
See also
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