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How to figure out engine displacement
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Posted by: beerock

This is for everyone who want to figure out there actual displacement size.

In this example I used a 67.75mm piston.

Just plug in your piston size and there ya go.

divide piston size by 2 (67.75mm(my piston = 33.875mm)

then convert to centimeters(move the decimal point over a place) 3.3875mm

multiply by stroke converted to centimeters (72mm= 7.2mm)

3.3875mm x 7.2mm = 24.372mm x Pie(3.14) = 76.5846mm x 3.3875(displacement size = 259.4303325)CC

convert milimeters to inches divide milimeters by 25.4

convert thousanths to milimeters multiply by 25.4

Posted by: TX400ex-250r

this is way easier
http://www.bgsoflex.com/displacement.html

Posted by: beerock

computers arent always around.

Mechanics know this stuff and use calculators or paper.

Posted by: dober250R

I always just use some digital calipers and push the lil mm button.

Posted by: baseballplaya23

how do you know or figure out your stroke?

Posted by: bwamos

Legend:

b = Bore
s = Stroke
d = Displacment
^2 = squared
* = multimply
/ = divide
sqrt = square root

Use mm for the unit of measure.

=================================

Bore & Stroke to Displacement:
(0.5b^2)*3.14*s=d

Bore & Displacement to Stroke:
d / ((0.5b^2)*3.14) = s

Displacement and Stroke to Bore:
2 * sqrt (d / 3.14s) = b

=================================

To go from mm^3 to cm^3 (CC) you need to divide d by 1000 or multimply by 1000 (10x10x10) to go from cc's to mm^3 for d.

d in these equations need to be in mm^3. So if you want 330cc you need to use 330,000 mm^3

Posted by: beerock

quote:
Originally posted by baseballplaya23
how do you know or figure out your stroke?

usually the engine oem specs tell you your stroke.

forgive me guys I wrote bore where it should have said displacement, I edited it.

Posted by: slick250

If you are figuring your stroke from scratch and dont know the stock stroke or are just not sure if it is stock, find top dead center and mark the piston height in the cylinder. Rotate the crank to bottom dead center and measure the distance traveled. That is the stroke.

Posted by: 2004exrider

Heres another way...
http://www.freewebs.com/rideyellow04/techpagefaqs.htm

Jimmy