Differentiating engines as oversquare, square, and undersquare might sound too technical. However, the basis and the differences are mostly basic and simple to understand.
While there are some engineering technical terms, we will keep the description simple for easier understanding.
How are oversquare and undersquare engines differentiated?
If the piston stroke length is larger than the bore diameter, the engine is called an undersquare engine. And if the bore diameter is larger than the stroke length, the engine is called an oversquare engine.
If both the stroke and the bore are of the same (or around the same) length, then it is called a square engine.
Before we dive into the specifics and differences of these engines, it is vital to understand the bore-stroke ratio.
Bore Stroke Ratio
The stroke length and the bore diameter of the engine form the basis of differentiation for undersquare and oversquare engines.
The stroke length, or in short – stroke, is the distance traveled by the piston from top to bottom or vice versa along the cylinder lining.
The bore, or cylinder bore, is the diameter of the engine cylinder.
The Bore Stroke ratio is defined as the ratio between the bore diameter and the stroke length.
The bore divided by stroke gives you the bore-stroke ratio.
Defining Square, Undersquare, Oversquare Engines
Now, if the bore stroke ratio equals one, i.e., bore diameter and stroke length are the same, then the engine is called a square engine.
If the bore-stroke ratio is less than one, i.e., the stroke length is larger than the bore diameter, the engine is called an undersquare engine.
If the bore-stroke ratio is greater than one, i.e., the bore diameter is larger than the stroke length, the engine is called an oversquare engine.
Undersquare Engine: Bore/Stroke ratio < 1
Square Engine: Bore/Stroke ratio = 1
Oversquare Engine: Bore/Stroke ratio > 1
Square engines are considered to be the most reliable engines since the wear and tear will be comparatively lower than the other engine types.
This is because the stroke and bore are of the same length and as a result, the bearings of the crank will experience much lesser stress. The reduced stress on the bearings will decrease the wear and tear consequently.
Square engines are usually found in conventional motorcycles and petrol engines in the cars.
The much higher reliability that square engines offer is desirable in these vehicles since the vehicles themselves offer higher reliability and durability.
Undersquare engines are also called long-stroke engines since their stroke length is larger than the bore diameter.
A key characteristic of undersquare engines (long-stroke engines) is their thumping sound at low RPMs. You can hear a ‘duk-duk-duk’ thumping noise coming from motorcycles with undersquare engines.
The thumping noise is generated by the longer strokes produced by the piston.
The same thump cannot be heard in oversquare engines because the piston will not travel such large distances.
The main advantage of undersquare or long-stroke engines is that they produce larger torque in the engine at lower RPMs.
The stroke is longer in these engines. And torque is the cross product of force and perpendicular distance. The longer stroke length in these engines makes them produce higher torque at low-end rpm when compared to oversquare engines.
Since undersquare engines produce such high torque at low-end RPMs, they are used in heavy-duty vehicles like trucks, and extensively in diesel engines, and also in heavy motorcycles, tourers, and cruisers.
Almost all diesel engines are undersquare engines.
Other advantages are high compression and thermal efficiency.
The long strokes in undersquare engines give higher piston displacement. Leading to better and higher compression in the engines.
Plus, the long stroke provides sufficient time for complete combustion and lower chamber heat loss. Increasing their thermal efficiency.
The disadvantage of undersquare engines is that the bore will be smaller.
Longer stroke and smaller bore is the hallmark of undersquare engines.
However, a small bore means a small cylinder head. And a small cylinder head means smaller valves – both inlet and exhaust.
As a result, the entry fuel mixture and the exhaust gases are restricted in their flow. Because of such smaller valves.
The end result:
Undersquare engines do not produce high power and acceleration.
That’s why the undersquare engines produce large torque but not high acceleration.
The fuel entry is restricted. The fuel combustion is limited. The acceleration capabilities are capped off.
Oversquare engines are also called short-stroke engines since their stroke length is smaller than the bore diameter.
The main characteristics of oversquare engines are they produce higher power and acceleration. Especially at higher RPMs.
On the flip side, they produce lower torque, especially at low-end rpms.
Short-stroke or oversquare engines are considered to have low wear and tear.
The piston wear and the stress on bearings – are significantly lower when compared to undersquare engines. The piston has to travel shorter distances. And the bearing bear (;)) less load because of the shorter stroke.
Second advantage. More power and acceleration at high RPMs.
Since the bore diameter is larger, the fuel intake is far higher since the valves are designed larger as well. As a result, more fuel is burnt in each combustion cycle and hence, more power and acceleration are generated in each cycle.
Another advantage of short-stroke engines is that they are compact.
The lower stroke gives the provision to design the engine compactly.
Add in the reliability and low wear and tear that oversquare engines provide, they make an attractive type for manufacturing performance motorcycles.
Oversquare or short-stroke engines are used mostly in performance motorcycles.
The downside of short-stroke or oversquare engines is their low torque at low-end RPMs.
In addition, the short stroke means the engine compression is slightly lower as well. Making them less desirable for diesel engines.
Lastly, the thermal efficiency is lower. The short strokes make way for shorter but more combustion cycles. Paving the way for incomplete combustion (sometimes) and higher heat loss.
Comparison of Oversquare and Undersquare Engines
Here is a brief comparison of oversquare and undersquare engines across different parameters – Wear and tear, Torque, Power, and Acceleration.
|Bore Stroke Ratio
|Wear and Tear
|Torque (especially at low RPMs)
|Power & Acceleration (especially at high RPMs)
|Mostly used in
|Power and acceleration (especially at high RPMs)
Before you go…
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