Stroke ratio
From Wikipedia, the free encyclopedia
Stroke ratio, bore/stroke ratio and stroke/bore ratio are terms that are used to describe the form of a piston engines cylinder when the piston is at the lowmost point. The stroke ratio also gives a general direction of the engines characteristics.
The bore is the diameter of the cylinder and the stroke is the length that piston travels, which is generated when pistons upmost and downmost positions are compared, which is twice the crank throw.
There are two different types of stroke ratios that are often mixed together causing confusion: Bore/stroke ratio and Stroke/bore ratio
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[edit] Bore/stroke ratio
Bore/stroke is the most commonly used term, which is mainly used in the USA, Great Britain, Australia and some other countries.
An engine that has wider bore vs. stroke has a bore/stroke value of over 1.
For example an engine which has 100 mm (millimeter) bore and 80 mm stroke has a bore/stroke value of:
100 mm / 80 mm = 1.25
Such an engine is referred as oversquare or shortstroke engine.
An engine that has shorter bore vs. stroke has a bore/stroke value of under 1.
For example an engine which has 90 mm bore and 120 mm stroke has a bore/stroke value of:
90 mm / 120 mm = 0.75
This type of engine is called an undersquare or longstroke engine.
An engine that has equal bore and stroke has a bore/stroke value of 1.
For example an engine which has both 95 mm bore and stroke has a bore/stroke value of:
95 mm / 95 mm = 1.00
This type of engine is called a square engine. Usually engines that have a bore/stroke ratio of 0.95 to 1.04 are referred as square engines.
[edit] Stroke/bore ratio
Stroke/bore ratio is generally more rare than bore/stroke ratio but is used in some countries, like in Finland for example. Stroke/bore ratio is similar to the bore/stroke ratio with the following exception:
When stroke/bore value is over 1 the engine is longstroke or undersquare and when the stroke/bore value is under 1 the engine is shortstroke or oversquare. However the square engine has a value of 1 in both cases.
For example an engine with 110 mm stroke and 80 mm bore, stroke/bore value 1.375, is an undersquare or longstroke and an engine that has 70 mm stroke and 100 mm bore, stroke/bore value 0.7, is oversquare or shortstroke.
