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Stroke ratio, today universally defined as bore/stroke ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke length in a reciprocating piston engine. 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.
Stroke/bore ratio is an obsolete expression dating to the early era of reciprocating engine development.
1967 – FIAT 125, 124Sport engine 125A000, 125B000, 125BC000, 1608 ccm, DOHC, bore and stroke.
1970 – Ford 400 had a bore and stroke.
1973 – Kawasaki Z1 and KZ(Z)900 had a bore and stroke.
1982 - Honda Nighthawk 250 and Honda CMX250C Rebel have a bore and stroke.
1983 – Mazda FE 2.0L inline four-cylinder engine with a bore and stroke.
1987 – The Opel/Vauxhall 2.0 L GM Family II engines are square at bore and stroke; example as C20XE C20NE C20LET X20A X20XEV X20XER Z20LET Z20LEH Z20LER A20NHT A20NFT.
1989 – Nissan Motors SR20DE is a square engine, with an bore and stroke.
1990-2002 - Maserati's biturbo 3.2L V8 has a bore and stroke of , and was used in the Maserati Shamal, 3200 GT and Quattroporte IV.
1990–2010 Saab Automobile is a square engine, with a bore and stroke.
1991 – Ford's 4.6 V8 OHC engine has a bore and stroke.
1995 – The BMW M52 engine with a displacement of 2793 cubic centimeters is an example of a perfect square engine with an bore and stroke.
1996 – Jaguar's AJ-V8 engine in 4.0-litre form has an 86.0 mm bore and stroke.
2000 – Mercedes-Benz 4.0-litre () OM628 V8 diesel engine is an example of a square engine – with an bore and stroke.
An oversquare engine allows for more and larger valves in the head of the cylinder, higher possible rpm by lowering maximum piston speed, and lower crank stress due to the lower peak piston acceleration for the same engine (rotational) speed. Because these characteristics favor higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed.
Due to the increased piston and head surface area, the heat loss increases as the bore/stroke ratio is increased. Thus an excessively high ratio can lead to a decreased thermal efficiency compared to other engine geometries. The large size/width of the combustion chamber at ignition can cause increased inhomogeneity in the air/fuel mixture during combustion, resulting in higher emissions.
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.
Examples include both Chevrolet and Ford small-block V8s; the GMC 478 V6 has a bore/stroke ratio of 1.33. The 1.6 litre version of the BMW N45 gasoline engine has a bore/stroke ratio of 1.167.
Flat engine 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. The Subaru EJ181 engine develops peak torque at speeds as low as 3200 rpm.
Nissan's RB, VQ, VK, VH and VR38DETT engines are all oversquare. Additionally, SR16VE engine found in Nissan Pulsar VZ-R and VZ-R N1 is an oversquare engine with bore and stroke, giving it but relatively small torque of
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 2.5:1 are allowed, enabling engine speeds of 18,000 rpm while remaining reliable for multiple races.
The Ducati Panigale motorcycle engine is extremely oversquare with a bore/stroke ratio of 1.84:1. It was given the name "SuperQuadro" by Ducati, roughly translated as "super-square" from Italian.
The side-valve Belgian D-Motor LF26 aircraft engine has a bore/stroke ratio of 1.4:1.
Early Mercedes-Benz M116 engines had a bore and a stroke for a 3.5 litre V8.
At a given engine speed, a longer stroke increases engine friction and increases stress on the crankshaft due to the higher peak piston acceleration. The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number.
Undersquare engines often exhibit peak torque at lower rpm than an oversquare engine due to their smaller valves and high piston speed limiting their potential to rev higher.
BMW's acclaimed S54B32 M54 engine was undersquare with a bore and stroke of ), offering a world record torque-per-litre figure () for normally aspirated production engines at the time; this record stood until Ferrari unveiled the 458 Italia.
Many United Kingdom automobile companies used undersquare designs until the 1950s, largely because of a motor tax system that RAC horsepower. This includes the BMC A-Series engine, and many Nissan derivatives. The Trojan Car used an undersquare, split-single, two stroke, two-cylinder inline engine; this was partly for this tax advantage and partly because its proportions allowed flexing V-shaped connecting rods for the two pistons of each U-shaped cylinder, which was cheaper and simpler than two connecting rods joined with an additional bearing.
Their French and German competitors at the time also used undersquare designs even in absence of the tax reasoning, e. g. Renault Billancourt engine and Opel straight-6 engine.
The 225 cu in (3.7 litre) Chrysler Slant-6 engine is undersquare, with a bore and stroke of stroke (bore/stroke ratio = 0.819:1).
The Ford 5.4L Modular Engine features a bore and stroke of , which makes a bore/stroke ratio of 0.852:1. Since the stroke is significantly longer than the bore, the SOHC 16V (2-valve per cylinder) version of this engine is able to generate a peak torque of 350 lb·ft as low as 2501 rpm.
The Willys Jeep L134 and F134 engines were undersquare, with a bore and stroke of stroke (bore/stroke ratio = 0.714:1).
The Dodge Power Wagon used a straight-six Chrysler Flathead engine of 230 cu in (3.8 L) with a bore and stroke of , yielding a substantially undersquare bore/stroke ratio of 0.709:1.
The 4-litre Barra Inline 6 and Intech engines from the Australian Ford Falcon, uses a bore and stroke of stroke, which equates to a 0.929:1 bore-stroke ratio.
The 292 Chevrolet I6 is also undersquare, with a bore and stroke of in (bore/stroke ratio = 0.939:1).
Mitsubishi's 4G63T engine found primarily in many generations of Mitsubishi Lancer Evolution is an undersquare engine with a bore and stroke of .
The Jaguar XK6 engine, used in all 6-cylinder Jaguars from 1949 to 1987 was undersquare. For example, the 4.2 litre engine had a bore and stroke of , providing a bore/stroke ratio of 0.869:1.
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, usually using . A Wärtsilä two-stroke marine diesel engine has a bore and stroke of , (bore/stroke ratio = 0.384:1).
While most modern motorcycle engines are square or oversquare, some are undersquare. The Kawasaki Z1300's straight-six engine was made undersquare to minimise engine width, more recently, a new straight-twin engine for the Honda NC700 series used an undersquare design to achieve better combustion efficiency in order to reduce fuel consumption.
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