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Discussion Starter · #1 · (Edited)
Everyone knows how to calculate displacement of an engine by traditional means (volume of one cylinder times number of cylinders), but the formula for a 4-cylinder engine and the steps it takes to get there can be significantly simplified.

4-cyl. displacement = πD²S (pi x D-squared x S), where D is bore diameter and S is stroke length. :eek:
 

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Discussion Starter · #3 ·
Not quite, Grasshopper!

wayne70gt said:
Otto,

Shouldn't that be pi x r squared x S, or pi x (D/2 squared) x S.

Wayne
Not quite, Grasshopper! That would be correct for ONE SINGLE cylinder displacement, but I did say "displacement for a FOUR CYLINDER ENGINE"! :eek:
 

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Nice one, Otto!

D squared is FOUR times bigger than R squared .... so your formula quickly gives the displacement of a FOUR cylinder motor!
 

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D = Bore Size!

nobody said:
Why not use the bore size?
Otto was only making the MATH simpler - not the MEASURING! ;)

"piston size" is very close to bore size so makes very little difference to the answer BUT means you have to pull a piston to measure it instead of just getting the size of the bore! :D
 

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Nobody is correct in his thinking to use the bore size in this equation, but cylinder wear comes in to play also, so multiple measurements need to be taken and averaged out. Measuring the pistons, which is supposed to be done 2.5" below the oil ring groove 90 degrees from the wrist pin, could be a problem if the pistons were cam ground. Which means they are elliptical not completely round. Also, the rings do take up space between the piston and cylinder wall, although supposedly only .0012" it does add to the total displacement if only by a few CCs. Anything to throw garbage into the mix, and keep you guys thinking. :D
 

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Discussion Starter · #9 ·
nobody said:
Why not use the bore size?
OK, OK, diameter of BORE then!

Was really just trying to point out that you don't have to halve the bore (radius), square it and then multiply it by pi and times four to calculate the four cylinder total bore area. (D/2)² = (D²/4) x 4 (cylinders) = D²! I digress. :rolleyes:
 

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I agreed with the equasion and it makes perfect sense. I was just thinking and asked what seemed like a stupid question on my part. It was meant to be a good way to determine total displacement and it is. The D value piston vs. bore is roughly .012 so as motors are classed pretty roughly anyway it doesn't matter when you factor that in. It's only when you start splitting hairs to go from a say a 2.0 to a 1.997, (just random numbers). My last one CCs at 1.992 but I call it the 2.0. Anybody out there with a 1.897 should agree.
 
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