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