Cubic inch formula

300jk · Nov 19, 2019

Will the following formula give correct engine cubic inches. Bore x bore x stroke x # of cylinders x .7854. Just wondering.

20MuleTeam · Nov 19, 2019

Yes. Area of circle is 3.1416 * diameter * diameter divided by 4. (3.1416/4 = 0.7854) Then * stroke * number of cylinders

mmfan55 · Nov 19, 2019

R2x pi x strokex# cylinders. radius is half of bore multiplied by itself, x 3,1416 ( pi) , stroke x cylinders. This is the formula for "area of a cylinder ".

Gary Mitchell · Nov 20, 2019

pi, (3.141), times r squared, (which is half the diameter times itself), times the stroke, times the number of cylinders = cubic measurement.

(pi)x(r squared)x(stroke)x(# of cyl) = cubic in, ltr, etc.

gm

Gary Mitchell · Nov 20, 2019

300jk, after running a couple of different bore sizes with the formula you presented and comparing it with the one more commonly presented in math books, etc. I found that the outcomes are basically the same. May I ask where you came up with this one? Thanks, gm

20MuleTeam · Nov 20, 2019

If you could measure and divide perfectly, you would get 3.141592653589793238..., or pi.

Only difference would be where pi is rounded to - 3.14 or 3.1416 or ??.

BT in NE · Nov 20, 2019

I have no idea where I heard this same formula either, but I've been using it for over 20 years. BT

Indiana Ken · Nov 21, 2019

Gary, They are the same formula: (pi x D^2)/4 = pi x R^2, both will give you the area of a circle. In working with engines I find it easier to use the diameter because my caliper reads out the diameter, not the radius. Also, pistons are listed in diameters rather than in radius. Bore, is by definition a diameter. However, the math works equally well using either formula.

Lets consider the formula using the diameter and convert it to using the radius:

Area of a Circle = (pi x D^2)/4

D = 2 x R and D^2 = (2R)^2 = 4 x R^2

Substituting 4 x R^2 in the first equation yields:

Area of a Circle = (pi x 4 x R^2)/4, dividing by 4 yields:

Area of a Circle = pi x R^2

Engine Displacement = pi x R^2 x stroke x # cylinders:

Clear as mud....? I hope not.

Ken

TVMAN · Nov 23, 2019

go to murphy website put in info and it will fig it for you

300jk · Dec 8, 2019

Sorry I dont use this forum all that often. I velieve it was on an engine builders web site. Cant recall which though !

Cubic inch formula

300jk

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20MuleTeam

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mmfan55

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Gary Mitchell

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Gary Mitchell

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20MuleTeam

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BT in NE

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Indiana Ken

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TVMAN

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300jk

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