Stroke ratio
Encyclopedia
In a reciprocating piston engine
, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term which is used to describe the ratio between the diameter of the cylinder bore and the length of the piston
stroke within its cylinder
s. This can be used for either an internal combustion engine
, such as a petrol-
or diesel 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: holding displacement
constant, lengthening the crank throw reduces the piston area by an exactly corresponding amount.
, and these are often mixed together causing confusion. These are: bore/stroke ratio, and stroke/bore ratio.
, Europe
, United Kingdom
, Asia
, Australia
, and some other countries.
The diameter of the cylinder bore is divided by the length of the piston
stroke to give the ratio.
for example.
The length of the piston
stroke is divided by the diameter of the cylinder bore to give the ratio.
Stroke/bore ratio is similar to the bore/stroke ratio with the following exception:
When stroke/bore value is over 1:1 the engine is long-stroke or undersquare and when the stroke/bore value is under 1:1 the engine is short-stroke or oversquare. The square engine has a value of 1:1 in both cases.
For example an engine with 110 millimetres (4.33 in) stroke and 80 millimetres (3.15 in) bore, stroke/bore value 1.375, is an undersquare or long-stroke engine. An engine that has 70 millimetres (2.76 in) stroke and 100 millimetres (3.94 in) bore, stroke/bore value 0.7, is oversquare or short-stroke.
Sportsbikes and their racing version use short-stroke engines.
stroke within the cylinders
of a piston engine.
when it has equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.
For example an engine which has 95 millimetres (3.74 in) bore, and an identical 95 millimetres (3.74 in) stroke, has a bore/stroke value of:
Usually engines that have a bore/stroke ratio of 0.95 to 1.04 are referred as square engines.
W16 engine as used in the Bugatti Veyron
is an example of a square engine - with an identical bore and stroke of 86 millimetres (3.39 in). Another example of a square engine is the 1970s Ford 400M with a 4.00" bore and stroke.
The BMW
M52
engine with a displacement of 2793 cubic centimeters is an example of a perfect square engine with a bore of 84 mm and a stroke of 84 mm.
The Mercedes-Benz
M117
engine with a displacement of 5547 cubic centimeters is an example of a nearly square engine with a bore of 96.5 millimeters and a stroke of 94.8 millimeters.
The Cadillac
500-V8 manufactured from 1970-1976 is a nearly square engine with a 4.300 inch bore and a 4.304 inch stroke.
Similarly, the BMW S70/2 used in the McLaren F1
is also a nearly square engine with a 86 mm (3.4 in) x 87 mm (3.4 in) bore/stroke.
Nissan's
SR20DE is a square engine, with a bore and stroke of 86mm.
Toyota's 2JZ-GE/2JZ-GTE is a square engine, with a bore and stroke of 86mm.
The 1973-1976 Kawasaki's Z-1 and KZ(Z)900 had a 66 mm bore and a 66 mm stroke, also making it a square engine.
1982 0nwards Honda
CB 250 Night Hawk and Honda CMX 250 Rebel have a 53mm bore and 53mm stroke also making it a square engine.
400M Ford is also square. 4.00 bore and 4.00 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 1968-1981 Buick
350
engine with a displacement of 349.3 cubic inches is an example of a nearly perfect square engine with a bore of 3.80 inches and a stroke of 3.85. Its design remained virtually untouched for 23 years.
For example an engine which has 100 millimetres (3.94 in) bore and 80 millimetres (3.15 in) stroke has a bore/stroke value of:
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). 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 than undersquare engines of similar engine displacement
but wider and longer (for engines with vertical cylinder axes).
By changing the crankshaft and modifying the connecting rod(s), piston(s) and/or engine block an engine can be "de-stroked". This reduces the displacement and consequently the torque of the engine, but can allow it to run at higher speeds and in fact develop greater peak power.
Most Boxer (horizontally-opposed) engines (such as those built by Volkswagen, Porsche, and Subaru) feature oversquare designs since any increase in stroke length would result in twice the increase in overall engine size.
This is particularly crucial in Subaru's front-engine layout, where the steering angle of the front wheels is limited largely by the size of the engine. Although oversquare engines have a reputation for being high-strung, low-torque machines, the Subaru EJ engine
develops peak torque at speeds as low as 3200 RPM.
Extreme examples of oversquare engine designs are found in Formula One
race cars, whose rules tightly limit displacement and thereby require 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.
For example an engine which has 90 millimetres (3.54 in) bore and 120 millimetres (4.72 in) stroke has a bore/stroke value of:
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.
An engine can be "stroked" by replacing the crankshaft with a so-called "stroker" crankshaft and modifying the connecting rod(s), piston(s) or engine block to accommodate the increased piston travel. This increases the displacement and therefore the torque of the engine, but may reduce the peak speed at which it is safe to run.
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 Chrysler Slant-6 engine, in its most common 225 cubic inch (3.7 litre
) version, is a massively undersquare engine, with a 86 millimetres (3.39 in) bore and a 105 millimetres (4.13 in) 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 revolutions per minute
(rpm); red line with aftermarket connecting rod
s 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 engine
-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 79.4 millimetres (3.13 in) bore and 111.1 millimetres (4.37 in) stroke, are probably the most undersquare engines ever built (for Jeeps).
The Dodge Power Wagon
, among other vehicles, used a straight-six Chrysler Flathead engine
of 230 cubic inches (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 stroke ratio of 0.70.
The 292 Chevrolet I6 is also under square, with a bore of 3.875 inches and a stroke of 4.125
Virtually all piston aircraft engines used in military aircraft were long-stroke engines. The PW R-2800, Wright R-3350, PW R-4360, Rolls-Royce Merlin (1650), Allison V-1710, and Hispano-Suiza 12Y-Z are only a few of more than a hundred examples.
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term which is used to describe the ratio between the diameter of the cylinder bore and the length of the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
stroke within its cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
s. This can be used for either an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
, such as a petrol-
Petrol engine
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....
or diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
, where the fuel is burned within the cylinders of the engine, or external combustion engine
External combustion engine
An external combustion engine is a heat engine where an working fluid is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work...
, such as a steam engine
Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
, 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: holding displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
constant, lengthening the crank throw reduces the piston area by an exactly corresponding amount.
Conventions
In a piston engine, there are two different ways of describing the stroke ratio of its cylindersCylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
, and these are often mixed together causing confusion. These are: bore/stroke ratio, and stroke/bore ratio.
Bore/stroke ratio
Bore/stroke is the most commonly used term, which is mainly used in North AmericaNorth America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and some other countries.
The diameter of the cylinder bore is divided by the length of the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
stroke to give the ratio.
Stroke/bore ratio
Stroke/bore ratio is generally more rare than bore/stroke ratio, but is used in some countries, like in FinlandFinland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
for example.
The length of the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
stroke is divided by the diameter of the cylinder bore to give the ratio.
Stroke/bore ratio is similar to the bore/stroke ratio with the following exception:
When stroke/bore value is over 1:1 the engine is long-stroke or undersquare and when the stroke/bore value is under 1:1 the engine is short-stroke or oversquare. The square engine has a value of 1:1 in both cases.
For example an engine with 110 millimetres (4.33 in) stroke and 80 millimetres (3.15 in) bore, stroke/bore value 1.375, is an undersquare or long-stroke engine. An engine that has 70 millimetres (2.76 in) stroke and 100 millimetres (3.94 in) bore, stroke/bore value 0.7, is oversquare or short-stroke.
Short-stroke engine uses
Formula 1 engines use short-stroke designs because it allows the engine to rev higher. Because of this, they are capable of generating greater horsepower.Sportsbikes and their racing version use short-stroke engines.
Square, undersquare and oversquare engines
The following terms describe the naming conventions for the various configurations of the relationship ratio between the diameter of the cylinder bore and the length of the pistonPiston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
stroke within the cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
of a piston engine.
Square engine
An engine is described as a square engineStroke ratio
In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term which is used to describe the ratio between the diameter of the cylinder bore and the length of the piston stroke within its cylinders...
when it has equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.
For example an engine which has 95 millimetres (3.74 in) bore, and an identical 95 millimetres (3.74 in) stroke, has a bore/stroke value of:
- 95 mm / 95 mm = 1.00
Usually engines that have a bore/stroke ratio of 0.95 to 1.04 are referred as square engines.
Square engine examples
The Volkswagen GroupVolkswagen Group
Volkswagen Group is a German multinational automobile manufacturing group. , Volkswagen was ranked as the world’s third largest motor vehicle manufacturer and Europe's largest....
W16 engine as used in the Bugatti Veyron
Bugatti Veyron
The Bugatti Veyron EB 16.4 is a mid-engined grand touring car. The Super Sport version is the fastest road-legal production car in the world, with a top speed of . The original version has a top speed of...
is an example of a square engine - with an identical bore and stroke of 86 millimetres (3.39 in). Another example of a square engine is the 1970s Ford 400M with a 4.00" bore and stroke.
The BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
M52
BMW M52
The BMW M52 is a Straight 6 piston engine. Displacement ranges from 2000 cc to 3200 cc. It is a DOHC 24-valve engine, originally introduced with VANOS Variable Valve Timing. It replaced the BMW M50 engine in 1994 and was replaced in 2001 with the BMW M54 engine...
engine with a displacement of 2793 cubic centimeters is an example of a perfect square engine with a bore of 84 mm and a stroke of 84 mm.
The Mercedes-Benz
Mercedes-Benz
Mercedes-Benz is a German manufacturer of automobiles, buses, coaches, and trucks. Mercedes-Benz is a division of its parent company, Daimler AG...
M117
Mercedes-Benz
Mercedes-Benz is a German manufacturer of automobiles, buses, coaches, and trucks. Mercedes-Benz is a division of its parent company, Daimler AG...
engine with a displacement of 5547 cubic centimeters is an example of a nearly square engine with a bore of 96.5 millimeters and a stroke of 94.8 millimeters.
The Cadillac
Cadillac
Cadillac is an American luxury vehicle marque owned by General Motors . Cadillac vehicles are sold in over 50 countries and territories, but mostly in North America. Cadillac is currently the second oldest American automobile manufacturer behind fellow GM marque Buick and is among the oldest...
500-V8 manufactured from 1970-1976 is a nearly square engine with a 4.300 inch bore and a 4.304 inch stroke.
Similarly, the BMW S70/2 used in the McLaren F1
McLaren F1
The McLaren F1 is a supercar designed and manufactured by McLaren Automotive. Originally a concept conceived by Gordon Murray, he convinced Ron Dennis to back the project and engaged Peter Stevens to design the exterior of the car...
is also a nearly square engine with a 86 mm (3.4 in) x 87 mm (3.4 in) bore/stroke.
Nissan's
Nissan Motors
, usually shortened to Nissan , is a multinational automaker headquartered in Japan. It was a core member of the Nissan Group, but has become more independent after its restructuring under Carlos Ghosn ....
SR20DE is a square engine, with a bore and stroke of 86mm.
Toyota's 2JZ-GE/2JZ-GTE is a square engine, with a bore and stroke of 86mm.
The 1973-1976 Kawasaki's Z-1 and KZ(Z)900 had a 66 mm bore and a 66 mm stroke, also making it a square engine.
1982 0nwards Honda
Honda
is a Japanese public multinational corporation primarily known as a manufacturer of automobiles and motorcycles.Honda has been the world's largest motorcycle manufacturer since 1959, as well as the world's largest manufacturer of internal combustion engines measured by volume, producing more than...
CB 250 Night Hawk and Honda CMX 250 Rebel have a 53mm bore and 53mm stroke also making it a square engine.
400M Ford is also square. 4.00 bore and 4.00 stroke.
Škoda
Škoda Auto
Škoda Auto , more commonly known as Škoda, is an automobile manufacturer based in the Czech Republic. Škoda became a wholly owned subsidiary of the Volkswagen Group in 2000, positioned as the entry brand to the group...
used square engines in some of their rear-engine models between 1964-1990, with a bore and stroke of 68mm and 72mm.
The 1968-1981 Buick
Buick
Buick is a premium brand of General Motors . Buick models are sold in the United States, Canada, Mexico, China, Taiwan, and Israel, with China being its largest market. Buick holds the distinction as the oldest active American make...
350
350
Year 350 was a common year starting on Monday of the Julian calendar. At the time, it was known as the Year of the Consulship of Sergius and Nigrinianus...
engine with a displacement of 349.3 cubic inches is an example of a nearly perfect square engine with a bore of 3.80 inches and a stroke of 3.85. Its design remained virtually untouched for 23 years.
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 ratio value of greater than 1:1.For example an engine which has 100 millimetres (3.94 in) bore and 80 millimetres (3.15 in) stroke has a bore/stroke value of:
- 100 mm / 80 mm = 1.25:1
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). 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 than undersquare engines of similar engine displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
but wider and longer (for engines with vertical cylinder axes).
By changing the crankshaft and modifying the connecting rod(s), piston(s) and/or engine block an engine can be "de-stroked". This reduces the displacement and consequently the torque of the engine, but can allow it to run at higher speeds and in fact develop greater peak power.
Oversquare engine examples
Oversquare engines are extremely common, including both Chevrolet and Ford small block V8s.Most Boxer (horizontally-opposed) engines (such as those built by Volkswagen, Porsche, and Subaru) feature oversquare designs since any increase in stroke length would result in twice the increase in overall engine size.
This is particularly crucial in Subaru's front-engine layout, where the steering angle of the front wheels is limited largely by the size of the engine. Although oversquare engines have a reputation for being high-strung, low-torque machines, the Subaru EJ engine
Subaru EJ engine
The Subaru EJ engine is a series of automotive engines manufactured by Subaru and introduced in 1989, intended to succeed the previous Subaru EA engine. The EJ series is the mainstay of Subaru's engine line, with all engines of this series being 16-valve flat-4 horizontal, with configurations...
develops peak torque at speeds as low as 3200 RPM.
Extreme examples of oversquare engine designs are found in Formula One
Formula One
Formula One, also known as Formula 1 or F1 and referred to officially as the FIA Formula One World Championship, is the highest class of single seater auto racing sanctioned by the Fédération Internationale de l'Automobile . The "formula" designation in the name refers to a set of rules with which...
race cars, whose rules tightly limit displacement and thereby require 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.
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.For example an engine which has 90 millimetres (3.54 in) bore and 120 millimetres (4.72 in) stroke has a bore/stroke value of:
- 90 mm / 120 mm = 0.75:1
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.
An engine can be "stroked" by replacing the crankshaft with a so-called "stroker" crankshaft and modifying the connecting rod(s), piston(s) or engine block to accommodate the increased piston travel. This increases the displacement and therefore the torque of the engine, but may reduce the peak speed at which it is safe to run.
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. Some rear-wheel-drive cars that borrow engines from front-wheel-drive cars (such as the Mazda Miata) 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
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
BMC A-Series engine
Austin Motor Company's small straight-4 automobile engine, the A-Series, is one of the most common in the world. Launched in 1951 with the Austin A30, production lasted until 2000 in the Mini. It used a cast-iron block and cylinder head, and a steel crankshaft with 3 main bearings...
, and many Nissan derivatives.
The Chrysler Slant-6 engine, in its most common 225 cubic inch (3.7 litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
) version, is a massively undersquare engine, with a 86 millimetres (3.39 in) bore and a 105 millimetres (4.13 in) stroke, producing most of its power right on the peak of its torque curve. The Achilles heel
Achilles Heel
Achilles Heel may refer to:* Achilles' heel, a metaphor for a fatal weakness in spite of overall strength* Achilles Heel , music by Pedro the Lion* Achilles Heel , off Antarctica...
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 revolutions per minute
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(rpm); red line with aftermarket connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....
s 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
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
nor the oil pump
Oil pump (internal combustion engine)
The oil pump in an internal combustion engine circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine...
will function adequately). In some circles, the Slant-6 is nicknamed "The Stump-Puller" for its diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
-like low-speed torque. Appropriate gearing and driving skill is required for performance use.
Willys
Willys
Willys was the brand name used by Willys-Overland Motors, an American automobile company best known for its design and production of military Jeeps and civilian versions during the 20th century.-Early History:In 1908, John Willys bought the Overland Automotive Division of Standard Wheel Company...
also used mostly undersquare engines; in fact the L134
Willys Go Devil engine
The Willys L134 was a straight-4 automobile engine made famous in the Jeep produced in World War II. The engine displaced with a bore and stroke, a very undersquare design. It was an L-head design, with valves parallel to the cylinders...
and F134
Willys Hurricane engine
The Willys F4-134 Hurricane was an inline-4 piston engine and powered the famous Jeep CJ in the CJ-3B, CJ-5, and CJ-6 models. It was also used in the Willys 473 and 475 pickups, wagons, and sedan deliveries...
engines, with their fairly small 79.4 millimetres (3.13 in) bore and 111.1 millimetres (4.37 in) stroke, are probably the most undersquare engines ever built (for Jeeps).
The Dodge Power Wagon
Dodge Power Wagon
The Dodge Power Wagon was a four wheel drive pickup truck produced from 1945 through 1980. This early version was based on a military truck and is a predecessor to the many four wheel drive pickups in use today.-History:...
, among other vehicles, used a straight-six Chrysler Flathead engine
Chrysler Flathead engine
The Chrysler Flathead engine is an automotive engine manufactured by the Chrysler Corporation.-Straight-4:Chrysler introduced a straight-4 derivative of their new Flathead straight-6 in 1926. Initially used by all Chrysler divisions, the straight-4 became Plymouth-only in 1929, lasting in...
of 230 cubic inches (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 stroke ratio of 0.70.
The 292 Chevrolet I6 is also under square, with a bore of 3.875 inches and a stroke of 4.125
Virtually all piston aircraft engines used in military aircraft were long-stroke engines. The PW R-2800, Wright R-3350, PW R-4360, Rolls-Royce Merlin (1650), Allison V-1710, and Hispano-Suiza 12Y-Z are only a few of more than a hundred examples.