The Word On CompressionWe cheat by using a Reher Morrison Racing Engine's computer program to quickly calculate any compression ratio. But even though the computer does our math, we still need to input critical data to get things right. Your compression ratio is actually just a comparison of the volume in the cylinders with the piston at TDC vs. BDC. Or, how much the air and fuel gets "squeezed" when the piston moves up the bore. If you've got a 10:1 compression ratio, your mixture is squeezed down to 1/10th its full size at TDC, yet it still contains the same mass, which gives it such explosive power.
The compression ratio formula involves calculating the Swept Volume (SV) of the cylinder, which is everything BELOW the piston top, and calculating the Total Chamber Volume (CHV), which is everything ABOVE the piston top. The Swept Volume formula looks like this: (bore x bore x stroke x 12.87 = SV). The 12.87 is the only magic number here. Within its mystical confines are the calculations to figure volume of a cylinder and the conversion for cubic inches into cubic centimeters, which is the only easy way to do this. All the other numbers are either measurements that you take, or figures supplied with the parts.
The accepted formula for calculating compression ratio is:CR = (SV + CHV) / CHV
In Example 1, the parts manufacturers have supplied key figures, making our lives much simpler.
Example 1:B = 4.030-inch (Bore)S = 3.48-inch (Stroke)CH = 70cc (Combustion chamber volume)G = 9cc (Head gasket volume)P = 10cc (Piston dish volume w/ 4 valve reliefs)DV = 0.020-inch (Deck Volume i.e. piston-installed height)
First calculate the Swept Volume: (B x B x S x 12.87)SV = 4.030 x 4.030 x 3.48 x 12.87 = 727.39cc
Next calculate Deck Volume, if there is any in your engine. DV is the small area that's left over at TDC when the piston does not reach the deck. In Ex 1 the pistons are 0.020-inch down the bore at TDC. The deck height measurement replaces the Stroke in the SV formula to calculate Deck Volume.
DV = 4.030 x 4.030 x 0.020 x 12.87 = 4.18cc
Now calculate your total CHV:(CH + G + DV + P)
*Using dished or domed pistons will add or subtract volume, respectively, to the CHV equation and the head gasket's volume, along with the piston's installed height, must be taken into account.
CHV = 70cc + 9cc + 10cc + 4.18cc = 93.18cc
To calculate the compression ratio, simply add Swept Volume (SV 93.18cc) + Total Chamber Volume (CHV 727.39cc) then divide the sum by Swept Volume (SV 93.18cc).
Ex 1's compression ratio is 8.8:1.(93.18 + 727.39) / 93.18 = 8.8
But in DM this month we installed domed pistons, which removes volume from the CHV figure, instead of adding to it like a dished piston would. Also, DM's Victor Jr. heads have a small CH volume; and its head gasket (G) volume is less than Ex 1 as well. DM's pistons are installed at 0.005-inch below deck, so DM - Part 23's equation for CHV looks like this:DM's CHV = 64cc + 8.6cc - 10cc + 1.05cc = 63.65cc
Swept volume does not change in this case, since it's all below the piston top. It would only change if we altered the bore or stroke of the engine, but we didn't, so we plug DM's figures into the rest of the CR equation and get:(63.65 + 727.39) / 63.65 = 12.43cr
See how easy that was?