Engine displacement


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Engine displacement

LenNH Occasionally we see questions about engine "size" (another way of saying "displacement"). The one thing I remember from high-school geometry is how to calculate the volume of a cylinder. The formula is Pi x r (squared) x h. In English, that is 3.1416 x radius of the cylinder, squared x height of the cylinder.
Example: 4" bore and 5" stroke: The radius is 1/2 the diameter, or 2". Square this to get 4. Multiply 4 x 3.1416 = 12.57. This result is the area of the circle. Multiply area x height (stroke in an engine) to get the total volume or displacement of the cylinder: 12.57 x 5 = 62.83. Multipy the volume of one cylinder to get the total displacement. A four cylinder engine would have a displacement of 62.83 x 4 = 251.3 cubic inches.
There is undoubtedly a formula for doing this directly in cubic centimeters or liters, or for converting cubic inches into liters, but I don't have this info at hand.

Try this with some of the engines we are all familiar with:
F-12: 3" bore x 4" stroke
10-20: 4.25" bore x 5" stroke
Chevvy Stovebolt Six: 3.5" bore x 3.75" stroke.

If you enjoy armchair engineering (it doesn't pay anything, but it doesn't require any degrees, either), you might like to compare the outputs of various engines depending on their size, rpms, etc. I have fun with silly questions like "Why does the McC-D 10-20, with the 4.25" x 5" cylinders, have a little less power than the F-20, with only 3.75" bore (same stroke)?"
Armchair engineer's answer: F-20 runs faster, may have a little more efficient carb/manifold.



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