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Old 09-02-2009, 06:52 PM   #10 (permalink)
some_other_dave
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Math time:

94mm bore X 71mm stroke, 1mm deck height, 55cc combustion chamber, flat-top pistons.

Deck volume = 94 / 2 * 94 / 2 * 3.14159 * 1 ~= 6939.77 mm^3, or 6.9 cc.
Swept volume = 94 / 2 * 94 / 2 * 3.14159 * 71 ~= 492723.83 mm^3, or 492.7cc

Fixed volume = 55 + 6.9 = 64.9
Fixed + swept = 64.9 + 492.7
Compression ratio = 492.7 / 64.9 = 7.591 : 1

Change the "94" in the above to "96"--
96mm bore X 71mm stroke, 1mm deck height, 55cc combustion chamber, flat-top pistons.

Deck volume = 96 / 2 * 96 / 2 * 3.14159 * 1 ~= 7238.22 mm^3, or 7.2 cc.
Swept volume = 96 / 2 * 96 / 2 * 3.14159 * 71 ~= 513913.86 mm^3, or 513.9cc

Fixed volume = 55 + 7.2 = 67.2
Fixed + swept = 67.2 + 513.9
Compression ratio = 513.9 / 67.2 = 7.647 : 1

So an increase of 2mm bore (0.080") on a very mild two-liter four-cylinder (in this case going from 1971cc to 2056cc) would raise compression by one-twentieth of a point. The effect should be larger if the CC volume is lower, giving a higher initial compression ratio.

It's not a huge effect, but there is an effect.

-soD
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