How to Calculate Compression Ratio

Taking Measurements

Measure the bore in centimeters. Use a bore gauge to measure the cylinder bore’s diameter. Remember that the diameter refers to the width of the cylinder. Write this number down so you can refer back to it later.Tip: Many of the measurements you need are provided in the manufacturer specs. These numbers are more precise than the ones you’d get by measuring by hand, so check the specs first and only measure for the variables not listed.

Find the stroke in centimeters. The stroke refers to how far the piston travels within the cylinder. If you don’t have the specs, measure this using a deck bridge and dial calipers. Position the dial calipers on the deck bridge so the jaws are upward. Move the piston to top dead center, then position the bridge over the cylinder. Zero out the calipers, then put the piston at bottom dead center by rotating the crankshaft. Open up the calipers until the depth rod is touching the piston deck, then read the number.

Determine the deck height in centimeters. Ensure the piston is at top dead center, then measure between the top of the cylinder and the flat surface at the top of the piston. If your piston is above deck, it takes away from the clearance volume. If your piston is below deck, it adds to the clearance volume.

Figure out the piston top volume in cubic centimeters. Use the manufacturer specs for this one or look up the part number online to find the volume. Keep in mind that a piston with a dome takes away from the clearance volume, while a piston with a dish adds to the clearance volume. Note that cubic centimeters are abbreviated as cc.

Measure the head gasket volume in cubic centimeters. You may be able to find this information online or in the specs. If not, measure the sealing ring distance in inches and divide that by 3.1416. Square the answer then multiply it by the compressed gasket thickness in thousandths of inches. Take your answer and multiply it by 12.87 to find the head gasket volume in cubic centimeters. For example, say the head gasket is 13 inches long and 0.041 inches thick. Divide 13 by 3.1416, which is 4.138. Square this number to get 17.123. Multiply this by 0.041, which is 0.702, then multiply that by 12.87. The head gasket volume is 9.04 cubic centimeters.

Determine the combustion chamber volume in cubic centimeters. Use the specs from the manufacturer to find this measurement. If you need to convert from cubic inches to cubic centimeters, multiply the number by 16.387.

Doing the Calculations

Use the formula (cylinder diameter / 2) x π x stroke to find the swept volume. Divide the cylinder diameter by 2. Then, square the result and multiply it by π, which is 3.14. Finally, multiply the result by the stroke to determine the swept volume of the engine.Tip: If you don’t want to do the math by hand, search online for a compression ratio calculator. For instance, if your cylinder diameter is 8.1 cm and your stroke is 8.9 cm, divide 8.1 by 2, which is 4.05. Square 4.05, which is 16.4025. Multiply this by 3.14, which is 51.50385, then multiply that by 8.09. The answer is 458.38 cc.

Find the clearance volume using the formula Vcombustion chamber + Vpiston + Vgasket + Vdeck clearance. Simply add the volume of the combustion chamber, the piston top volume, the gasket thickness, and the deck height or clearance. For example, if the volume of the combustion chamber is 38.6, the piston volume is 9.0, the gasket volume is 4.5, and deck clearance is 1.6, the clearance volume is 53.7 cc.

Plug your numbers into the formula CR = (Vsw + Vcl) / Vcl. Now that you know the swept volume and clearance volume, simply insert those numbers into the formula and solve it. Add the swept volume and cylinder volume together first. Then, divide the result by the cylinder volume to find the compression ratio. For instance, if the swept volume is 458.38 and the clearance volume is 53.7, start by adding 458.38 and 53.7, which is 512.08. Divide 512.08 by 53.7, which is 9.5359. So, the compression ratio is 9.54:1.