Calculating comp ratio ?
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Not sure what you mean. It's just the ratio of the volume of the combustion space in the cylinder at the bottom of the piston stroke (BDC) to the volume at the top of the stroke (TDC). There are on-line calculators all over the place that allow you to enter the different engine specs to derive the ratio, but that's all it is. Just geometry. You need to know the following to get pretty close.
bore and stroke (ie displacement)
piston dish/dome volume
combustion chamber volume
head gasket thickness and bore size
piston deck (how far the piston is below (or above) the block deck at TDC)
Just calculate the volumes and make a ratio.
Rich Krause
bore and stroke (ie displacement)
piston dish/dome volume
combustion chamber volume
head gasket thickness and bore size
piston deck (how far the piston is below (or above) the block deck at TDC)
Just calculate the volumes and make a ratio.
Rich Krause
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Posts: 1,462
From: smog zone adjacent to a great lake
Maybe I'm as confused (or unclear) as Rich is. For others that are unable to find an online calc, maybe they can view it from a different perspective. As the phrase states, it is a ratio. A ratio of the space above the piston at BDC, to the space above the piston at TDC. Maybe there are those that would rather use the calc. 
The calculation is basic 3rd grade math, composed of addition and division, with a little multiplication thrown in. Frankly, I consider it harder (and much more time consuming) to obtain the (accurate) figures to 'plug' in the calc, than to do the actual math by hand. For the piston at BDC, take the all the figures that Rich noted, and add them all up. Now for the piston at TDC, take the same previous figure, but now substract the area displaced by the stroke. Now take the second figure or volume, and divide into the first figure obtained. Your answer is the ratio of one figure to the other.
Ah, maybe this is where some get stymied. To figure the volume a piston displaces, requires the formula to determine area of a circle, which is pi (3.1416) multiplied by the radius of circle (which is 1/2 of the bore) squared, which is radius multiplied by itself, and then multiplied by the stroke. IOW, if radius is 2.002, it would be 2.002 multiplied by 2.002, multiplied by 3.1416. This figure, the area of a circle (bore), would then be multiplied by the stroke, to determine the volume of a cylinder. That's it!
Did I make any sense here?
Ya know, maybe this wasn't such a good idea after all. Sorry for the optimism.
The calculation is basic 3rd grade math, composed of addition and division, with a little multiplication thrown in. Frankly, I consider it harder (and much more time consuming) to obtain the (accurate) figures to 'plug' in the calc, than to do the actual math by hand. For the piston at BDC, take the all the figures that Rich noted, and add them all up. Now for the piston at TDC, take the same previous figure, but now substract the area displaced by the stroke. Now take the second figure or volume, and divide into the first figure obtained. Your answer is the ratio of one figure to the other. Ah, maybe this is where some get stymied. To figure the volume a piston displaces, requires the formula to determine area of a circle, which is pi (3.1416) multiplied by the radius of circle (which is 1/2 of the bore) squared, which is radius multiplied by itself, and then multiplied by the stroke. IOW, if radius is 2.002, it would be 2.002 multiplied by 2.002, multiplied by 3.1416. This figure, the area of a circle (bore), would then be multiplied by the stroke, to determine the volume of a cylinder. That's it!
Did I make any sense here?
Ya know, maybe this wasn't such a good idea after all. Sorry for the optimism.
Last edited by arnie; Feb 9, 2004 at 06:18 PM.
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