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 In a piston engine, the bore (or cylinder bore) is the diameter of each cylinder. Engine displacement is calculated based on bore, stroke length and the number of cylinders: displacement = The stroke ratio, determined by dividing the bore b ...

diameter and

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas- ...

stroke length. This can be used for either an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

, where the fuel is burned within the cylinders of the engine, or external combustion engine, such as a

steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...

, where the combustion of the fuel takes place ''outside'' the working cylinders of the engine. A fairly comprehensive yet understandable study of stroke/bore effects was published in ''Horseless Age'', 1916.

Conventions

In a

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...

, there are two different ways of describing the ''stroke ratio'' of its

cylinders A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

, namely: ''bore/stroke'' ratio, and ''stroke/bore'' ratio.

Bore/stroke ratio

Bore/stroke is the more commonly used term, with usage in North America,

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an ...

, and Australia. The diameter of the

is divided by the length of the

stroke to give the ratio.

Square, oversquare and undersquare engines

The following terms describe the naming conventions for the configurations of the various bore/stroke ratio:

Square engine

A square engine has equal bore ''and'' stroke dimensions, giving a bore/stroke value of exactly 1:1.

Square engine examples

1953 –

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had Lampredi V12 with bore and stroke. 1967 – FIAT 125, 124Sport engine 125A000-90 hp, 125B000-100 hp, 125BC000-110 hp, 1608 ccm, DOHC, bore and stroke. 1970 – Ford

400 __NOTOC__ Year 400 ( CD) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Stilicho and Aurelianus (or, less frequently, year ...

had a bore and stroke. 1973 – Kawasaki Z1 and KZ(Z)900 had a bore and stroke. 1973 – British Leyland's Australian division created a 4.4-litre version of the Rover V8 engine, with bore and stroke both measuring 88.9 mm. This engine was exclusively used in the Leyland P76. 1982 - Honda Nighthawk 250 and Honda CMX250C Rebel have a bore and stroke, making it a square engine. 1983 –

Mazda , commonly referred to as simply Mazda, is a Japanese Multinational corporation, multinational Automotive industry, automotive manufacturer headquartered in Fuchū, Hiroshima (town), Fuchū, Hiroshima Prefecture, Hiroshima, Japan. In 2015, M ...

FE 2.0L inline four-cylinder engine with a perfectly squared bore and stroke. This engine also features the ideal 1.75:1 rod/stroke ratio. 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's SR20DE is a square engine, with an bore and stroke. 1990 – Maserati Shamal had biturbo

V8 engine A V8 engine is an eight- cylinder piston engine in which two banks of four cylinders share a common crankshaft and are arranged in a V configuration. The first V8 engine was produced by the French Antoinette company in 1904, developed and u ...

''AM 479'' with bore and stroke. This engine, after modifications, was later installed in Quattroporte IV and 3200 GT. 1990–2010 Saab B234/B235 is a square engine, with a bore and stroke. 1991 – Ford's 4.6 V8 OHC engine has a bore and stroke. It has been the backbone of Ford V8-powered cars and trucks in different power levels and head designs for two decades. 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 Mercedes-Benz (), commonly referred to as Mercedes and sometimes as Benz, is a German luxury and commercial vehicle automotive brand established in 1926. Mercedes-Benz AG (a Mercedes-Benz Group subsidiary established in 2019) is headquarter ...

4.0-litre () OM628 V8 diesel engine is an example of a square engine – with an bore and stroke. The

Volkswagen Group Volkswagen AG (), known internationally as the Volkswagen Group, is a German multinational automotive manufacturer headquartered in Wolfsburg, Lower Saxony, Germany. The company designs, manufactures and distributes passenger and commercial ...

's 2005 W16 engine as used in the

Bugatti Veyron The Bugatti Veyron EB 16.4 is a mid-engine sports car, designed and developed in Germany by the Volkswagen Group and Bugatti and manufactured in Molsheim, France, by French automobile manufacturer Bugatti. It was named after the racing driver ...

also using a bore and stroke. The Peugeot XU10 engine line – with a displacement of 1998 cubic centimeters – is an example of a perfect square engine with an bore and stroke. Toyota's 2JZ and 4U are square engines, with bore and stroke.

Toyota is a Japanese multinational automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on . Toyota is one of the largest automobile manufacturers in the world, producing about 10 ...

1G is a square engine with an bore and stroke. Honda's J30A engine has an bore and stroke. Suzuki AX100 engine has a bore and stroke. Yamaha YBR125 engine has a bore and 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, 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 Engine displacement is the measure of the cylinder volume swept by all of the pistons of a piston engine, excluding the combustion chambers. It is commonly used as an expression of an engine's size, and by extension as a loose indicator of t ...

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 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. Horizontally opposed, also known as "Boxer" or "flat", engines 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 The is a line of automobiles produced by the Japanese automaker Nissan from 1978 until 2000, when it was replaced by the Nissan Bluebird Sylphy in the Japanese market. Between 2000 and 2005, the name "Pulsar" has been used in Australia and N ...

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 Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship ...

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 A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used. Although some mop ...

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 The D-Motor LF26 is a lightweight liquid cooled side-valve four-stroke flat four, 2.7 litre petrol aircraft engine, produced by D-Motor in Deerlijk, Belgium. Design and development This direct-drive aero-engine is unusual in two respects: it i ...

aero-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.

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 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. Undersquare engines have become more common lately, as manufacturers push for more and more efficient engines and higher fuel economy.

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 Volkswagen (),English: , . abbreviated as VW (), is a German motor vehicle manufacturer headquartered in Wolfsburg, Lower Saxony, Germany. Founded in 1937 by the German Labour Front under the Nazi Party and revived into a global brand post ...

, Nissan,

Honda is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan. Honda has been the world's largest motorcycle manufacturer since 1959, reaching a producti ...

, and

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 bore and stroke of 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 The Mazda MX-5 is a lightweight two-passenger Roadster (automobile), roadster sports car manufactured and marketed by Mazda with a Front-engine, rear-wheel-drive layout#Front mid-engine, rear-wheel-drive layout, front mid-engine, rear-wheel-driv ...

) use an undersquare design. BMW's acclaimed S54B32 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 The Ferrari 458 Italia (Type F142) is a mid-engine sports car produced by Ferrari. The F458 is the successor of the F430, and was first officially unveiled at the 2009 Frankfurt Motor Show. It was succeeded by the 488 GTB (Gran Turismo Berl ...

. Many British automobile companies used undersquare designs until 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 Trojan Car used an undersquare, split piston, 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. The 225 cu in (3.7

litre The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3 ...

) 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 engine from the Australian

Ford Falcon Ford Falcon is an automobile nameplate applied to several vehicles worldwide. * Ford Falcon (North America), an automobile produced by Ford from 1960 to 1970. * Ford Falcon (Argentina), a car built by Ford Argentina from 1962 until 1991. * For ...

, 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

crosshead In mechanical engineering, a crosshead is a mechanical joint used as part of the slider-crank linkages of long reciprocating engines (either internal combustion or steam) and reciprocating compressors to eliminate sideways force on the piston ...

s. A

Wärtsilä Wärtsilä Oyj Abp (), trading internationally as Wärtsilä Corporation, is a Finnish company which manufactures and services power sources and other equipment in the marine and energy markets. The core products of Wärtsilä include technol ...

two-stroke marine diesel engine has a bore and stroke of , (bore/stroke ratio = 0.384:1). While most modern

s are square or oversquare, some are undersquare. The Kawasaki Z1300's

straight-six engine The straight-six engine (also referred to as an inline-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine bal ...

was made undersquare to minimise engine width, more recently, a new

straight-twin engine A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft. Straight-twin engines are primarily used in motorcycles; ot ...

for the Honda NC700 series used an undersquare design to achieve better combustion efficiency in order to reduce fuel consumption.

Notes

References

{{DEFAULTSORT:Stroke Ratio Engine technology Piston engines Engineering ratios