Stroke ratio
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Stroke ratio, bore/stroke ratio and stroke/bore ratio are terms that are used to describe the form of a piston engine's cylinder when the piston is at the bottom dead center (bdc) 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 top dead center (tdc) and bottom dead center positions are compared, which is twice the crank throw.
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[edit] Two ratios
There are two different types of stroke ratios that are often mixed together causing confusion: Bore/stroke ratio and Stroke/bore ratio
[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 short-stroke 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 long-stroke 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.
[edit] Oversquare
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 (all things equal) a shorter stroke results in less friction and less stress on the crankshaft. An oversquare engine is generally more reliable, wears less, and can be run at a higher speed. In oversquare engines low-speed torque tends to be lower since torque is relative to crank throw (distance from the crank center to the piston pin center) as well as the lowering pressure of the combustion gases as the piston descends. Engines can be modified by being "de-stroked", shortening the stroke to increase maximum rpms and top-end power, at the expense of low-end torque.
Oversquare engines tend to be lighter and shorter than similar undersquare engines along the direction of piston travel, but they are wider in directions perpendicular to piston travel. As the length is not a large problem, these engine types are highly favored by many manufacturers because of their power and compact size.
Some Formula One engines have a bore to stroke ratio of approximately 2.5:1 and are capable of revving to 19,000 rpm.
[edit] 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 results in greater friction, more stress on the crankshaft, and a smaller bore requires smaller valves which restrict gaseous exchange. An undersquare engine usually has a lower redline than an oversquare one, but it generates more low-end torque. Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.
Undersquare engines typically are, proportionally, shorter in length, 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.
The Chrysler Slant-6, in the most common 225 cubic inch (3.7 L) version, is a massively undersquare engine with a 3.40" (86 mm) bore and a 4.125" (105 mm) stroke, producing most of its power right on the peak of its torque curve. The achilles heel of this engine, otherwise known for its exceptional durability, is being over-revved by inexperienced drivers. Red line for a factory engine is under 4,500 rpm; red line with aftermarket connecting rods is about 5,500 rpm. On the other hand, a well-maintained Slant-6 can be made to idle as low as 75 rpm (though this is *not* a recommended speed, neither the alternator nor the oil pump will function adequately). In some circles, the Slant-6 is nicknamed "The Stump-Puller" for its diesel-like low-speed torque. Appropriate gearing and driving skill is required for performance use.
Willys also used mostly undersquare engines, in fact the L134 and F134 engines, with their fairly small 3.125 inch (79.4 mm) bore and 4.375 inch (111.1 mm) stroke, are probably the most undersquare engines ever built (for Jeeps).
The Dodge Power Wagon, among other vehicles, used a Straight-6 Chrysler Flathead engine of 230 cubic inches (3.8L) with a bore of 3.25" (83 mm) and a stroke of 4.625" (117 mm) yielding a substantially under-square stroke ratio of 0.70.
Undersquare engines tend to be less common that oversquare, but this form of engine is still used in some applications. 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. Numerous Volkswagen engines are undersquare. The popular Mazda Miata also uses an undersquare engine.
