Quickie 4-cylinder displacement calculation formula

tekenaar · 01-31-2005

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.

wayne70gt · 01-31-2005

Otto,

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

Wayne

tekenaar · 01-31-2005

Originally Posted by wayne70gt

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"!

GTJIM · 01-31-2005

Nice one, Otto!

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

nobody · 01-31-2005

Why not use the bore size?

wayne70gt · 01-31-2005

I quess I should have spent more time on the message and less on the formula!

GTJIM · 01-31-2005

Originally Posted by nobody

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!

namba209 · 01-31-2005

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.

tekenaar · 02-01-2005

Originally Posted by nobody

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.

nobody · 02-01-2005

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.

01-31-2005	#1 (permalink)
tekenaar Cunning Linguist Join Date: Mar 2002 Location: Plano, TX 75074 Posts: 4,441 Real Name: Otto	Quickie 4-cylinder displacement calculation formula 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. __________________ 1960: ♥ '61 Rekord PII 1.7 3S 3.9 ♥ '69 Kadett LS 'sprint' 1.9 3A 3.18 1970: ♥ '70 GT 1.9 4S 3.44 ♥ '72 GT 2.2SSD 5S 3.44 ♥ '72 GT 2.4FI 5S 3.44P ♥ '73 GT 1.9FI 4S 3.44 ♥ '75 1900 1.9FI 4S 3.44 1980: ♥ '85 Bitter SC 3.9FI 5S 3.44P 2000: ♥ '09 Solstice GXP Coupe 2.0 SIDI VVT Turbo 5S 3.73P Last edited by tekenaar; 04-26-2007 at 04:05 PM. Reason: change piston to bore

01-31-2005	#4 (permalink)
GTJIM Old Opeler Join Date: Jun 2003 Location: New Zealand Posts: 5,686	Nice one, Otto! D squared is FOUR times bigger than R squared .... so your formula quickly gives the displacement of a FOUR cylinder motor! __________________ GTJim Opel Owner since last Century! Copyright © 2000-2009 J D Henry All Rights Reserved

01-31-2005	#6 (permalink)
wayne70gt Opeler Join Date: Jan 2003 Location: Iowa Posts: 58	You humble me, Master! I quess I should have spent more time on the message and less on the formula! Last edited by Gary; 02-01-2005 at 07:44 AM.

01-31-2005	#8 (permalink)
namba209 6,000 Post Club Join Date: Aug 2002 Location: Imperial Beach, CA South of San Diego Posts: 6,054	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. __________________ Ron 72 GT 3.4L V-6/T-5/ZF posi - almost done - Just need AC installed. 75 Chevy monza 5.7L/TH350/Auburn 3.08 posi - Next

02-01-2005	#10 (permalink)
nobody No Access Join Date: Nov 2003 Location: in transit Posts: 3,873	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. Last edited by tekenaar; 04-26-2007 at 04:07 PM. Reason: sence?

01-31-2005	#2 (permalink)
wayne70gt Opeler Join Date: Jan 2003 Location: Iowa Posts: 58	Otto, Shouldn't that be pi x r squared x S, or pi x (D/2 squared) x S. Wayne

01-31-2005	#5 (permalink)
nobody No Access Join Date: Nov 2003 Location: in transit Posts: 3,873	Why not use the bore size?

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