Undersquare
A
piston engine is
undersquare or
longstroke if its
cylinders have a smaller
bore (width, diameter) than
stroke (length of piston travel). This can be a negative trait, since a longer stroke usually means greater friction, a weaker
crankshaft, and a smaller bore means smaller valves which restricts gaseous exchange; however, with the aid of modern technology, these are not the large problems that they used to be. An undersquare engine usually has a lower
redline than an
oversquare one, but it may generate more low-end
torque. In addition, a longstroke or undersquare engine can have a higher
compression ratio with the same
octane fuel compared to a similar displacement engine with a much shorter
stroke ratio. This also equals better fuel economy and somewhat better emissions. An undersquare engine does not overheat as easily as similar oversquare engine. Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.
Undersquare engines are typically shorter in length, but heavier and taller than equivalent oversquare ones, which is one of the reasons why this type of engine is not generally used.
Many
British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore. Therefore, many of the most famous cars of that era use this design. This includes the
Austin A-Series engine and many
Nissan derivatives.
Willys also used mostly undersquare engines, in fact the L- and F-134 engines, with their fairly small 3.125 inch bore and outrageous 4.375 inch stroke, are probably the most undersquare engines ever built.
Nowadays, undersquare engines tend to be quite rare, but this form of engine is still used in some aplications. For example, a modern 8.4 liter Valmet 645
inline-6 tractor
diesel engine is a longstroke/undersquare engine, but has an
output of over 300
horsepower (224 kW) with
turbocharger and
intercooler.
*
Oversquare*
Power*
Square engine
