What's Compression Ratio
Strictly defined, compression ratio is the total volume above the piston at bottom dead center (BDC), divided by the total volume above the piston at top dead center (TDC). First, let's look at the things that make up the total volume above the piston at TDC, which is the clearance volume.
1. Chamber Volume: This volume is measured in cc's, usually by checking the volume with a graduated burette. A burette is a long glass tube with cc's marked off on its length and a shut-off valve at the bottom. Many cylinder heads have typical cc numbers listed by the manufacturers or other references, but when it's important to know the true volume, measuring the heads is the only way to be absolutely sure.
2. Gasket Volume: This volume is also measured in cc's, and is published by the gasket manufacturer. The published numbers are generally accurate, but if no numbers are available, the cc's can be estimated by figuring 2.2 cc per .010 inch of thickness for small-blocks, and 2.5 cc per .010 inch for big-blocks.
3. Piston Dome Volume: The piston's net dome volume is typically published by the manufacturer, and is comprised of the dome-or material above the piston deck-minus the valve notches. Domed pistons lessen the clearance volume, since the net of the dome and notches takes up space above the piston's deck. With flat-top pistons, the volume considered is just the valve notch clearance, which will add to the total clearance volume. If the pistons have been massaged with custom cutting or profiling on the domes, about the only way to accurately get the cc volume is to measure the domes directly (see How to measure).
Deck clearance is easy to...
Deck clearance is easy to measure, using a deck bridge or dial indicator, as shown on this 318 short-block. Just zero the indicator at the deck, park the piston at TDC, and measure the distance from the piston to the deck as near to the piston's center as practical. Try rocking the piston in the bore in the axis of the piston pin and see how the measurement varies. If the rocking alters the reading, use an average number.
4. Deck Clearance Volume: This is the clearance created by the piston's position in the bore. If the piston is way down in the hole at TDC, there will be a loss in compression due to deck-clearance volume. To figure this volume, you can do the math, πr2(h) where r is half the bore size, and h is the piston deck clearance. Then, multiply the result by 16.387 to convert from cubic inches to cc's. Most handy online calculators let you just enter the piston-to-deck measurement in thousandths of an inch and will do the math for you.
5. Crevice Volume: This is the volume of the area between the top of the piston to the top of the ring. The volume here is fairly small, and the crevice volume is usually neglected when the ratio is calculated. You can add about 1/2 cc to account for it.
With the total volume above the piston at TDC calculated, getting the total volume above the piston at BDC is just a matter of adding in the swept volume. The swept volume can be obtained by again doing the math. By using the formula πr2(h), where r = 1/2 the bore and h = the stroke, and again multiplying by 16.387 to get cc's. To get the compression ratio, simply divide the total volume above the piston at BDC by the total clearance volume at TDC.
Get Real, Dulcich
Ok, some of the above may be a little more math-heavy than a lot of guys want to get into. Luckily, these days there are numerous Web sites with calculators that do the heavy number crunching for us, and it's free. These still need the basic numbers to punch in, but they do away with having to figure the volumes associated with deck clearance and swept area. One of our favorites is http://www.race-cars.net/calculators/compression_calculator.html. This handy calculator is easy to use and greatly simplifies calculating compression. Just plug in your bore, stroke, chamber cc's, piston dome cc's, and the gasket thickness in thousandths, plus or minus the amount the piston is in the hole. The calculator will spit out the compression ratio in an instant. It is especially handy in quickly comparing how various changes will affect the ratio, without all the longhand-math and slide-rule action.