Determining Compression Ratio
The next step is to move on to calculating your compression ratio. We'll call that "CR" and-not to belabor the obvious-the symbol for displacement will be "D." Compression ratio is simply the volume of the cylinder and the 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.

Of course, for all the importance given to compression ratio, the number itself is a bit theoretical. It assumes that the valves close exactly at TDC or BDC, and that never happens in a race engine, because high-performance cams have way too much duration for that. It also does not take into account volumetric efficiency, which is the rate that the intake ports flow air and fuel, and that changes throughout the rpm range. Still, compression ratio is useful whenever you are working with an engine package to make sure you are legal.

Before you can determine an engine's compression ratio, several factors must first be nailed down. These include piston volume, deck clearance, gasket thickness, and combustion chamber volume. You might not think piston volume (PV) is important since most stock car racing classes require flat top pistons. But you must also take into account the negative area created by the valve pockets. The piston manufacturer will usually give you this area on your spec sheet, but you can also easily determine this for yourself-which is important if you have cut your own valve reliefs or fly cut the top of the pistons.

Deck clearance (DC) is the volume of the bore between the top of the piston at TDC and the top of the deck of the engine block. The thickness of the head gasket (G) essentially acts as an extension of the deck clearance, just make sure to include the compressed thickness of the gasket which is quite different from the thickness when it first comes out of the box. And finally, you will need to know the volume of one of the cylinder head's combustion chambers (CC).

The formula for calculating compression ratio is just a bit more complex than the one for displacement. Still, as long as you keep a calculator handy it should be no big deal. Here it goes:

(D + PV + DC + G +CC) / (PV + DC + G +CC) = CR

When working this equation, make sure that each variable is the same unit of measure. In other words, don't mix cubic inches and cubic centimeters or you will get some completely useless (and confusing) numbers. This can often be an issue since combustion chamber and piston volumes are often referenced in cubic centimeters. Since your displacement figure is probably already in cubic inches, we'll go with that. Also, it's easy enough to convert cubic centimeters into cubic inches if you need to. Simply divide the number by 16.386.

Each of the volume variables is easy enough to determine with a little patience and the correct tools. Displacement, like we've already mentioned, is for one cylinder only. If you don't have access to the manufacturer's spec sheets, or anything has been modified, you will need to determine the volumes yourself. We've demonstrated the best procedures for cc'ing both your cylinder heads and the volume of the cylinder with the piston at TDC. When you do this, the cylinder volume equals both the piston volume and deck clearance.