Oversquare vs. Undersquare Engines: Which is better?

With each giving a unique engine characteristic, here are a few things you might not know about the engine in your car.

By Christopher Hurst - April 1, 2019
Oversquare vs. Undersquare Engines: Which is better?
Oversquare vs. Undersquare Engines: Which is better?
Oversquare vs. Undersquare Engines: Which is better?
Oversquare vs. Undersquare Engines: Which is better?
Oversquare vs. Undersquare Engines: Which is better?

I thought Engines Had Cylinders?

They do, the square terminology simply refers to your bore and stroke numbers. In engines where the bore is larger than the stroke, this is called “oversquare.” If bore and stroke are the same then you have what’s called a “square” engine. Finally, an engine where the bore is smaller than the stroke is called “undersquare.” Each type is inherently good for specific goals and as such, you find them used more in the environment they show the most benefit. Let’s look at each one and see what purpose they serve.  

Oversquare (short stroke)

Oversquare (or short stroke) engines are commonly found in things like Formula 1 cars, two-stroke racing engines and anything else where you want to rev the engine up to produce power. They keep piston speed down which helps reliability by putting less force into things like your crankshaft and connecting rods. This is actually a critical part of engine design that engine builders factor in, even when producing a road car. In general, these engines will produce more horsepower than torque and require gearing to take advantage of their top end pull. Common to find where high horsepower is required, but rules are restricted to small displacement. Examples include the C30A which powered the NSX.  

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Undersquare (long stroke)

Higher stress levels on components, but more torque…at least in theory. Many performance engines have a longer stroke than bore, but there are limitations to this design. Decreasing bore size limits the width of intake and exhaust valves which can decrease engine efficiency and horsepower. Longer travel also means more time spent dragging rings on cylinder walls. Clearly, there’s a lot more to it than saying one is better than the other. Examples of this engine include the F20C which came in the S2000.   

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Square Engines

Square engines are ones in which the bore and stroke are exactly the same. Perhaps the most famous engine is the K20 which many enthusiasts flock to for reliably easy power in their project cars. There are clearly many ways to skin a cat as the C30, F20 and K20 are all brilliant engines to use. This is likely the point where you’re saying: if they’re all good engines, which is the best?  

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The Best Design

There are a lot more critical things to worry about than if an engine is oversquare or undersquare when it comes to building a motor that is usable with a great power curve and torque curve to match. Each of the engines mentioned previously are capable of creating big numbers, so focusing on tolerances, materials, block strength, fuel type and boost amount all play a factor as well. Having said that, piston speed is one of the biggest factors in determining safe rpm limits as well as how much stress you’re placing on internals and a long stroke engine will naturally have higher piston speeds than a short stroke of the same design. Hope you enjoyed.  

 

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