When building an engine, the terms over-square and under-square come up.
What do these terms mean?
How do they apply to engine building?
Which is better for what purpose?
Can you give examples of each and their usage?
These terms are related to the bore and stroke measurements of a pistons travel inside a bore of an internal combustion engine. Bore is the diameter of the cylinder. Stroke is the distance a piston travels within a bore.
Under-square refers to a piston diameter measurement that is larger than the stroke length measurement. Over-square refers to the opposite. A stroke length measurement that is larger than a piston diameter measurement.
They are essential components to accomplishing a particular goal in engine building. A larger bore allows a larger piston. A larger piston allows for a larger combustion chamber. A larger combustion chamber allows for larger valves which will effect the quantity of gasses that can potentially flow through the ICE.
In pushrod engines a longer bore can create more valve reciprocating weight due to the longer pushrods.
In DOHC engines a shorter stroke and a larger piston can allow for a higher RPM and more efficiency at higher RPM's due to less reciprocating weight related to the connecting rods, pistons and crankshaft.
Typically, a long stroke will result in an engine with higher torque at lower RPM due to the higher reciprocating mass.
A short stroke can result in greater engine performance if the engine is designed for high RPM running. Engines typically become more efficient at higher RPM's, to a point.
In motorcycles the Ducati Desmosedici powerplant has an over-square configuration. A short stroke with a very large piston. This is a high RPM 989cc engine. The Harley Davidson Twin Cam 98B powerplant is 1.8 liters is an under-square configuration with a very long stroke V-Twin.
Ducati Desmosedici Powerplant
Two Cylinder V-Twin BORE X STROKE 86mm x 42.56mm, 998cc, an over-square configuration. This engine produces 200 HP at 10,600 RPM's. Redline is 13,600 RPM's
Harley Davidson 98B Powerplant
Two Cylinder V-Twin BORE X STROKE 98.4 mm x 111.3 mm, 1800cc an under-square configuration. This engine produces approximately 95hp & 110 ft lbs of torque. Redline is 5,800 RPM's
The Harley with it's long stroke, high reciprocating mass and resulting low RPM has lot's of bottom end power and torque at low RPM's.
The Ducati with it's high revving, over-square bore Desmosedici powerplant get's deep into high RPM efficiency and puts out a massive 200 HP.
Completely different applications of under and over square bore designed engines and the results of these types of configurations.
Undersquare means the stroke is bigger than the bore. Oversquare means the bore is bigger than the stroke.
It doesn't necessarily mean anything in terms of RPM capability, torque curve or anything like that. Those are more dependent on things like rod ratio, head flow, camshaft profile, etc.
From a purely theoretical standpoint, you would think that an engine with lots of stroke but relatively little bore would be poor at making power up top due to less room for air to flow through the head (the size of the bore places an upper limit on the size of the valves) and more extreme rod angles as the crank turns (the more stroke, the farther out to the side the big end of the rod goes)... but this isn't necessarily true in the real world.
First, consider the honda B series engines... all significantly undersquare. But they have decently high decks (the measurement from the crank center and the top of the block) and thus good rod ratios. They also have good flowing heads and thanks to variable valve timing, they have excellent high rpm camshaft profiles. From the factory they typically make peak power past 8000 rpm.
Compare to the relatively less underquare mazda b series engines that have less stroke and more bore. Despite this, they don't make good power at high rpms- this is because they tend to have poor flowing heads, hydraulic lifter valvetrains and very mild camshaft profiles. The mazda b series also have short decks for their stroke and thus poor rod ratios which further hamper high rpm use. The early years tended to make peak power just above 6000 rpm.
As for what purpose undersquare and oversquare have, they are mainly just side effects of the automotive development process. The more bore an engine has, the more space the block and head take up in the engine bay in terms of length and width. This can allow a head with bigger valves and bigger ports, but there's more to head flow than raw size. The more stroke an engine has, the less RPM the engine is capable of for a given deck height, which means that more stroke means more deck height or less RPM. And the more the deck height rises, the longer and potentially heavier the rods have to get, which introduces its own set of constraints.
Basically oversquare and undersquare are a simple representation of the shape of the cylinder, but this doesn't really tell you anything at all about the engine.
Oversquare and undersquare have been described by others .If your car is heavy in relation to the cubic inches then go for Stroker Undersquare .You need the torque down low .If your car is light then oversquare will make more HP .If you are wanting to hotrod a motor that means more RPM then over square is less likely to blow up .This is why smallblock chev of all generations is popular .Under square gives better fuel economy so life is a trade off .In Great Britian they had a RAE horsepower tax that favoured long stroke engines so many cars were not optimum .When Ford came to Britian they made small cars like cortinas and escorts but did not go crazy undersquare like most local manufacturers did .This is why it was much easier to hot up a cortina than a morris.This is also why the chev 305 is not as good to hotrod as a chev 327.Remember that peak piston speed is more likely to blow up a motor than rpm .The short stroke that is expected on an under square engine gives lower piston speeds for the same rpm.
Thank you for your interest in this question.
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