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In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke. This can be used for either an internal combustion engine, where the fuel is burned within the cylinders of the engine, or external combustion engine, such as a steam engine, where the combustion of the fuel takes place outside the working cylinders of the engine.
While the stroke ratio can provide insight into the goals of an engine's designer, it has no direct effect on the speed at which an engine reaches maximum torque.
- 1 Conventions
- 2 Square, oversquare and undersquare engines
- 3 Notes
- 4 References
Stroke/bore ratio is less common than bore/stroke ratio, but is used in some countries, such as Finland.
Square, oversquare and undersquare engines
The following terms describe the naming conventions for the configurations of the various bore/stroke ratio:
A square engine has equal or very nearly equal bore and stroke dimensions, giving a bore/stroke value of exactly or almost exactly 1:1.
Square engine examples
Alberto Ascari's Lancia D50 Grand Prix car in 1954-55 had a DOHC V8 engine with near square dimensions with a 73.6 mm bore and a 73.1 mm stroke.
The 1970s Ford 400M with a 4.00" bore and stroke is another example of a square engine.
The 1973-1976 Kawasaki's Z-1 and KZ(Z)900 had a 66 mm bore and a 66 mm stroke.
Škoda used square engines in some of their rear-engine models between 1964–1990, with a bore and stroke of 68mm and 72mm.
The 2009 single cylinder Yamaha Fazer 150 (launched in India), is a nearly square engine with 58.0 mm bore and 57.9 mm stroke.
Oversquare or short-stroke engine
An engine is described as oversquare or short-stroke if its cylinders have a greater bore diameter than its stroke length, giving a bore/stroke ratio greater than 1:1.
An oversquare engine allows for more and larger valves in the head of the cylinder, lower friction losses (due to the reduced distance travelled during each engine rotation) and lower crank stress (due to the lower peak piston speed relative to engine speed). Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively. Because these characteristics favor higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed.
The reduced stroke length allows for a shorter cylinder and sometimes a shorter connecting rod, generally making oversquare engines less tall but wider than undersquare engines of similar engine displacement.
Oversquare engine examples
Oversquare engines (a.k.a. "short stroke engines") are very common, as they allow higher rpm (and thus more power), without excessive piston speed.
Examples include both Chevrolet and Ford small-block V8s. The BMW N43 gasoline engine has a bore/stroke ratio of 1.083.
Boxer engines (such as those built by Volkswagen, Porsche, and Subaru) typically feature oversquare designs since any increase in stroke length would result in twice the increase in overall engine width. This is particularly so in Subaru's front-engine layout, where the steering angle of the front wheels is constrained by the width of the engine. Although oversquare engines have a reputation for being high-strung, low-torque machines, the Subaru EJ engine EJ181 develops peak torque at speeds as low as 3200 rpm.
Nissan's SR16VE engine found in Nissan Pulsar VZ-R and VZ-R N1 is an oversquare engine with 86 mm bore and 68.7 mm stroke, giving it an impressive 175-200 hp but relatively small torque of 119 lb.ft-134 lb.ft
Extreme oversquare engines are found in Formula One racing cars, where strict rules limit displacement, thereby necessitating that power be achieved through high engine speeds. Stroke ratios approaching 1:2.5 are allowed, enabling engine speeds of 18,000 RPM while remaining reliable for multiple races.
Early Mercedes-Benz M116 engines have a 92 mm bore and a 65.6 mm stroke for a 3.5 L V8. 
Undersquare or long-stroke engine
An engine is described as undersquare or long-stroke if its cylinders have a smaller bore (width, diameter) than its stroke (length of piston travel) - giving a ratio value of less than 1:1.
At a given engine speed, a longer stroke increases engine friction (since the piston travels a greater distance per stroke) and increases stress on the crankshaft (due to the higher peak piston speed). The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Because these factors favor lower engine speeds, undersquare engines are most often tuned to develop peak torque at relatively low speeds.
An undersquare engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel.
Undersquare engine examples
Many inline engines, particularly those mounted transversely in front-wheel-drive cars, utilize an undersquare design. The smaller bore allows for a shorter engine that increases room available for the front wheels to steer. Examples of this include many Volkswagen, Honda, and Mazda engines. The 1KR-FE-engine used in the Toyota Aygo, Citroën C1 and Peugeot 107 amongst others is an example of a modern long-stroke engine widely used in FF layout cars. This engine has a 71 mm bore and 84 mm stroke giving it a bore/stroke ratio of 0.845:1. Some rear-wheel-drive cars that borrow engines from front-wheel-drive cars (such as the Mazda MX-5) use an undersquare design.
Despite their reputation as low-speed torque machines, some undersquare engines are designed for quite high speeds. The Honda Integra Type R's B18C5 engine has one of the highest redlines (8,400 rpm) of any production engine, yet features an undersquare design. The 2011 Ford Coyote engine is a modern undersquare engine with a 7,000 rpm redline.
Many British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore. This includes the BMC A-Series engine, and many Nissan derivatives.
The Dodge Power Wagon used a straight-six Chrysler Flathead engine of 230 cu in (3.8 litre) with a bore of 83 millimetres (3.27 in) and a stroke of 117 millimetres (4.61 in), yielding a substantially under-square bore/stroke ratio of 0.709:1.
The 292 Chevrolet I6 is also under square, with a bore of 3.875 in and a stroke of 4.125 in (bore/stroke ratio = 0.939:1).
Mitsubishi's 4G63T engine found primarily in many generations of Mitsubishi Lancer Evolution is an undersquare engine at 85 mm bore x 88 mm stroke.
Virtually all piston engines used in military aircraft were long-stroke engines. The PW R-2800, Wright R-3350, Pratt & Whitney R-4360 Wasp Major, Rolls-Royce Merlin (1650), Allison V-1710, and Hispano-Suiza 12Y-Z are only a few of more than a hundred examples.
All diesel-powered ships have massively undersquare marine engines. A Wärtsilä two-stroke marine diesel engine has a cylinder bore of 960 mm (37.8 in) and stroke of 2500 mm (98.4 in), (bore/stroke ratio = 0.384:1).
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