Combustion-chamber volume also has a significant effect on the compression ratio. The same is true with piston-dome volumes. Obviously, as chamber volume decreases, compression will increase and vice versa. Pistons with a dish effectively increase combustion chamber size, while domed pistons reduce chamber volume. All these volumes are generally expressed in cubic centimeters (cc) and measured with a calibrated burette. The computer programs automatically convert cc's into cubic inches so that all the values are the same, but if you're doing the math on paper, the conversion is to multiply cc times 0.0610237 to equal cubic inches. For example, a 100cc chamber would be 6.10 cubic inches. Conversely, multiply cubic inches by 16.387 to get cubic centimeter.
The easiest way to measure piston dome or dish volume is to cc the piston in the cylinder. Seal the rings with grease, accurately place the piston 0.100 inch down in the cylinder and then measure the cc volume by filling up the cylinder. Next, compute the volume of a standard cylinder (bore x bore x height x 0.7854). For example, a 4.00-inch bore and a 0.100-inch height would be: 4 x 4 x 0.100 x 0.7854 = 1.256 ci x 16.387 = 20.59 cc. If you are measuring a piston with a dome, the measured volume will be less than the computed volume with the difference being the effective dome volume. For a dished piston, the measured volume will be more with the difference being the effective dish volume.
The real beauty of these computer programs is you can experiment with different values to quickly home in on a combination that will produce the ideal compression ratio. You can juggle combustion-chamber volume, deck height, head-gasket thickness, piston-dome (or dish) volume, and even bore and stroke and how these variables will affect compression ratio. Experiment for yourself-it's fun if you're into engine blueprinting to come up with the best engine combination.
Piston-to-wall clearanceSo you're having your engine bored 0.030-inch over and you order a set of 0.030-inch-over pistons, and then it hits you: If the bore is 4.030 inch and the piston is 4.030 inch, how will they fit together? The piston is not 4.030 inch, but rather, slightly smaller to create the desired fit between the piston and the bore, which should be right around the specified 4.030 inch. We say "around" because this is one of those areas where attention to detail will yield an optimum fit.
The first step is measuring the pistons that are to be used, but simply straddling one with a caliper is not accurate because pistons are not perfectly cylindrical despite what your eye may tell you. In reality, pistons are barrel shaped, having tapered ends that are actually narrower in diameter than the piston is in the middle. But the amount of taper and where it begins varies from manufacturer to manufacturer, and can vary among piston designs within the same manufacturer. For this reason it is critical that you consult with the manufacturer to determine the proper place to measure the piston at its widest point. This is usually located at a point just under the wristpin, but again-don't guess.