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Square, Oversquare and Undersquare engines

Square, Oversquare and Undersquare are terms that describe the ratio of  it’s bore and stroke. Bore is the cylinder’s diameter, and Stroke is the piston’s travel from the very top, to the very bottom:

  • square engine has the exact bore and stroke dimensions
  • undersquare is when the stroke length is longer than the bore
  • oversquare is when the bore is larger than the stroke length

This is just a simple example of two different crank shafts rotating at the same RPM (revs per minute), and what happens up in the piston area is quite interesting and can teach us a few interesting facts and the fundamental difference between a hyper sport engine to a cruiser engine.

Although real undersquare engines are rare to be found on modern motorcycles, there is still a big difference in bore X stroke ratios between different engine types. That is because different engines are for different kind of motorcycles.

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These two engines above show two things:

  1. even though both turn at the same RPM, the undersquare piston speed is greater
  2. to modify stroke dimensions, you have to modify the crank shaft (and not the piston length itself, which can only alter compression ratio)

Pros and Cons:
If we compare the Harley Sportster 883 to the Suzuki 750R , we discover that the Harley’s piston speed is about twice as that of the Suzuki, in any RPM. This is because the piston tavel of the 883 is longer than the Suzuki. The higher piston speed of the undersquare of square engines, means more heat to get rid of, greater wear and tear and greater G forces that the piston copes with when going back and forth in the cylinder.

On the other hand, an engine like that of the 883, has relatively compact combustion chamber, and better intake turbulence, and along the long stroke (stroke equals leverage remember?) produces more engine torque in relatively low RPM.

On the other side, the hyper sport’s engines have big bore cylinders and large valves and ports, which are necessary for high end power but are not very optimal for low end torque.

Either engines win or lose. The choice of this engine or the other depends on the purpose of the motorcycle and even…fashion. If motorcycle and engine design was to be strictly practical , we wouldn’t have seen motorcycles like the Honda 1520 cc flat six Valkyrie or the 2.3-liter inline-3-cylinder-behemoth Triumph ROCKET III. I guess the old vibrating  Harleys including the sportsters were to disappear from history pages too. It would of been so BORING….


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