engineering

Stroke ratio

Stroke ratio is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stroke ratio rather than just read about it. In short: Stroke ratio, today often 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 c…

Key takeaways

Stroke ratio belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Stroke ratio to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Stroke ratio from memory before moving on to harder problems.

Reference excerpt

Stroke ratio, today often 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.

Conventions The usual way to describe the stroke ratio of a piston engine‘s cylinders is its bore/stroke ratio. The diameter of the cylinder bore is divided by the length of the piston stroke to give the ratio. Stroke/bore ratio is an less popular expression, dating from the early days of internal combustion engine development.

Square, oversquare and undersquare engines The following terms are used to label 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

2025 – Modenas Kriss 125 FI malaysia with 54.0 mm × 54.0 mm (2.1 in × 2.1 in) bore and stroke. 1953 – Ferrari 250 Europa had Lampredi V12 with 68.0 mm × 68.0 mm (2.7 in × 2.7 in) bore and stroke. 1967 – FIAT 125, 124Sport engine 125A000, 125B000, 125BC000, 1608 ccm, DOHC, 80.0 mm × 80.0 mm (3.15 in × 3.15 in) bore and stroke. 1970 – Ford 400 had a 101.6 mm × 101.6 mm (4.00 in × 4.00 in) bore and stroke. 1973 – Kawasaki Z1 and KZ(Z)900 had a 66.0 mm × 66.0 mm (2.60 in × 2.60 in) bore and stroke. 1982 – Honda Nighthawk 250 and Honda CMX250C Rebel have a 53.0 mm × 53.0 mm (2.09 in × 2.09 in) bore and stroke. 1983 – Mazda FE 2.0L inline four-cylinder engine with a 86.0 mm × 86.0 mm (3.4 in × 3.4 in) bore and stroke. 1987 – The Opel/Vauxhall 2.0 L GM Family II engines are square at 86.0 mm × 86.0 mm (3.39 in × 3.39 in) 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 86.0 mm × 86.0 mm (3.39 in × 3.39 in) bore and stroke. 1990–2002 – Maserati's biturbo 3.2L V8 has a bore and stroke of 80.0 mm × 80.0 mm (3.15 in × 3.15 in), and was used in the Shamal, 3200 GT and Quattroporte IV. 1990–2010 Saab B234/B235 is a square engine, with a 90.0 mm × 90.0 mm (3.54 in × 3.54 in) bore and stroke. 1991 – Ford's 4.6 V8 OHC engine has a 90.2 mm × 90.0 mm (3.552 in × 3.543 in) 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 84.0 mm × 84.0 mm (3.31 in × 3.31 in) 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 (3996 cc; 243.9 cu in) OM628 V8 diesel engine is an example of a square engine – with an 86.0 mm × 86.0 mm (3.39 in × 3.39 in) bore and stroke.

Oversquare 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 lack of homogeneity 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.

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. Flat engines, also known as horizontally opposed or boxer 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. BMC produced the 1071 cc and the 970 cc A-series engines in the early 1960's, both being oversquare with shorter stroke crankshafts than the 1275 cc versions. The 970 S being the rarest of all A series engines was produced to win the 1000 cc touring car championship of the time, which it duly did. 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 86 millimetres (3.39 in) bore and 68.7 millimetres (2.70 in) stroke, giving it 175–200 horsepower (130–150 kW) but relatively small torque of 119–134 pound-feet (161–182 N⋅m; 16.5–18.5 kg⋅m). 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 aero-engine has a bore/stroke ratio of 1.4:1. Early Mercedes-Benz M116 engines had a 92 millimetres (3.62 in) bore and a 65.6 millimetres (2.58 in) stroke for a 3.5 litre V8.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Stroke ratio

Start with the simplest possible case. Write down what Stroke ratio claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stroke ratio before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stroke ratio ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stroke ratio

In research: Stroke ratio appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Stroke ratio in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Stroke ratio is common in secondary-school and first-year university syllabi. It links to neighbouring topics Engine technology, Engineering ratios, Piston engines, so understanding it makes those chapters shorter.
In everyday life: Look for Stroke ratio outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Stroke ratio in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stroke ratio means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Stroke ratio out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stroke ratio in simple terms?

Why does Stroke ratio matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stroke ratio?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stroke ratio.