Displacement formula

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Hauula Pan
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Displacement formula


Post by Hauula Pan » Thu Jan 27, 2011 7:44 pm

I understand that the formula for figuring cubic inch displacement is (#of cyl. X 0.7854 X Bore Squared X Stroke) However what I do not know is what the Stroke is when you use 80" flywheels & what your Bore is at .060" over? Can anyone tell me what the Bore & Stroke would be?... P.S. I would also like to know what the original stock bore & stroke for a 74" motor are? Thanks.

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Re: Displacement formula


Post by Bosheff » Thu Jan 27, 2011 7:55 pm

A .060" overbore 74 cu in cylinder is the same as a stock bore 80 cu in cylinder, therefore using a 60 over 74 cylinder or a stock bore 80 in cylinder with a set of 80 in wheels will give 80 cu in....bosheff

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Re: Displacement formula


Post by Cotten » Thu Jan 27, 2011 8:49 pm

Hauula Pan!

V= pi times radius squared times stroke times two cylinders.

3.4375" + .060" = 3.4975", divided by two gives a radius of 1.749"

Radius squared is then 3.058
3.1416 times 3.058 is 9.607
times a stroke of 4.250" = 40.830,
times 2 cylinders is 81.660 cubic inches.

Every little bit helps when bragging at the bar,

Somebody check my math,

PS: 74 model bore was 3.4375"
Stroke was 3.969" instead of 4.250"

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Re: Displacement formula


Post by FlatHeadSix » Thu Jan 27, 2011 9:15 pm

Your formula is good and so is Cotten's math. I never played with 80 cube stuff for the shovels but here is the spec sheet for the knuckles in both displacements and for the U & ULH (again, both displacements). As you can see the 80 cube side-valves had the same bore as the 74 cube OHV with a longer stroke: 4.28125".

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Re: Displacement formula


Post by Panacea » Fri Jan 28, 2011 3:49 am

Surprising to see the horsepower ratings are the same for the OHV and the big flatheads...Mike

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Re: Displacement formula


Post by RUBONE » Fri Jan 28, 2011 5:47 am

Those ratings are not BHP or Brake Horse Power but are merely based on displacement. N.A.C.C. was considered "Taxable" horsepower.No relevance to the actual power output!

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Re: Displacement formula


Post by kitabel » Fri Jan 28, 2011 3:40 pm

Taxable horsepower isn't based directly on displacement, since it ignores stroke completely.
The formula: bore^2 × # of cylinders ÷ 2.5,
for a H-D: bore^2 × 80%.
This is why the EL and UL have the same taxable - the only difference is the stroke.

The page shown also contains an error. The 74" OHV bore is 3-7/16", but the UH/ULH (1930-36 V also) isn't, it's 3-27/64" or 3.421875".

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