Several factors must be included if you are going to accurately determine your compression
We all know the age-old maxim, "There's no replacement for displacement." It's the truth. That's why so many sanctioning bodies use displacement and compression rules to limit engine power. Of course, they sometimes do it in wily ways. For example, NASCAR's Late Model Stock rule book has no specific rule on compression, but it limits bore, stroke, and minimum combustion chamber volume. Many other sanctions use this method. So if you want to maximize compression-and thus power-you have to know what you are doing when determining combustion chamber volume.
Other sanctions may choose to limit overall cubic inches and compression. You have more options in bore, stroke, and chamber volume size, but if you are cutting your own valve reliefs in the piston tops, experimenting with different head gaskets, or relieving the combustion chamber walls to reduce valve shrouding, are you sure that your compression ratio is still within the limits? Or just as important, that you haven't lost critical compression?
Jeff Dorton of Automotive Specialists demonstrates the proper procedure for cc'ing the com
Thankfully, determining compression isn't difficult with a few simple tools and a calculator.
First things first-you've got to be able to determine displacement exactly. Displacement is simply the area swept by the top of the piston as it moves up or down the bore one time. It does not include any of the area above top dead center (TDC) such as the combustion chamber. The conventional formula for determining displacement is as follows:
Bore x bore x stroke x 0.7854 x number of cylinders = cubic inches
For example, consider a standard Chevrolet 350 with a 4-inch bore and 3.48-inch stroke. Plugged into the formula, it would look like this:
4 x 4 x 3.48 x 0.7854 x 8 = 349.849
For this formula, both bore and stroke are measured in inches. The third number (0.7854) is a constant used to convert to cubic inches. Bore size can be determined simply with a dial caliper (although an engine builder's dial bore gauge is usually more exact), and stroke can be pulled right off the crankshaft spec sheet. For calculating compression ratio, you need only the displacement of a single cylinder, so to determine that, all you have to do is strike the number of cylinders multiplied.
Now that we have displacement nailed, it's time to move on to compression ratio (CR). The CR is simply the volume of the cylinder and combustion chamber when the piston is at bottom dead center (BDC), divided by the volume of the cylinder and combustion chamber when the piston is at TDC. For all the importance given to compression ratio, the number is, at best, theoretical. It assumes that the valves close exactly at TDC or BDC, which will never happen with a performance cam. It also makes no considerations for volumetric efficiency, either for the good or bad. Still, compression ratio is useful for engine builders putting together an engine package and knowing the CR is a must to ensure legality.
Install the plug you plan to run in the completed engine. This may not seem important, but
Several factors must be determined before calculating CR, including piston volume, deck clearance, gasket thickness, and combustion chamber volume. Piston volume is the negative area on the top of the piston. This is usually the valve reliefs or dish area. The manufacturer will give you this as part of the spec sheet. If you cut your own valve reliefs, you will need to make your own measurements. Deck clearance is the volume of the bore between the piston top at TDC and the top of the deck. Gasket thickness acts essentially as additional deck height, but when working the gasket into your calculations, you must use the compressed thickness.
The formula for calculating CR is slightly more complex than the formula for displacement, but is still straightforward. It is as follows:
(D + PV + DC + G + CC) (PV + DC + G + CC) = CR
CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume
It is important to make sure each variable is the same unit; do not mix cubic inches and cubic centimeters. Since your displacement figure is probably already in cubic inches, it's easy enough to convert cubic centimeters to inches. Simply divide the number by 16.386.