Q&A for How to Calculate Compression Ratio

You cannot. There is really no relationship between cubic centimeters and horsepower. While bigger engines tend to have more power, there are a lot more factors contributing to power output. You could have a 100cc engine with the same amount of power as a 50cc engine because of the other factors.

First, measure the cylinder bore and then its height. The engine volume is as follows: number of engine cylinders x 3.1415 x (cylinder bore)^2. Another method is to inverse the cylinder block on a flat metal surface and use a graduated container to fill the cylinder space with a liquid (as you keep track of the amount of liquid you pour inside) until all engine cylinders are full. The volume of liquid required to fill up all engine cylinders amounts for engine capacity.

Diesel engines are designed to operate at higher compression ratios (usually between 15 to 20), while petrol engines have lower compression ratios (usually between 8 to 10).

Measure the cylinder pressure at BDC and TDC. Then, divide the TDC pressure by the BDC pressure; the result is the compression ratio.

BDC is the length of the engine stroke below TDC, i.e. if the stroke is 80 mm then BDC is 80 mm below TDC.

Taking proper measurements is critical to correctly calculate an engine's compression ratio. Start by using a bore gauge to measure the cylinder bore’s diameter (the width of the cylinder). Then, find the stroke measurement in centimeters. 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. After that, measure the distance between the top of the cylinder and the flat surface at the top of the piston. Finally, use the manufacturer specs to find the piston top's volume in cubic centimeters and the volume of the head gasket and the combustion chamber.

You can use a series of formulas to find the exact value. Assuming you have all of the correct measurements, use this formula: (cylinder diameter / 2)2 x π x stroke. First, 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. For example, if you have a cylinder diameter of 8 cm and your stroke is 9 cm, you can divide 8 by 2 to get 4 and then square that to get 16. Multiply that by 3.14 to get 50.24 and then multiply that by 9 (your stroke value) to get 452.16 cc.

You can use the formula "Vcombustion chamber + Vpiston + Vgasket + Vdeck clearance," which is actually pretty simple if you have all of your measurements. Add the volume of the combustion chamber, the piston top volume, the gasket thickness, and the deck height or clearance. If you already have your swept volume calculated, you can plug the clearance volume value and the swept volume value into the formula "CR = (Vsw + Vcl) / Vcl" to find your engine's compression ratio. Add the swept volume and cylinder volume together first. Then, divide the result by the cylinder volume to find the compression ratio.

Calculating a compression ratio (CR) is easy, with just a few formulas and some given information. Calculate the displacement volume (Vd) using the formula below. The displacement volume is the volume that the piston sweeps during its stroke. Vd = b^2 * s * pi / 4 ~ Vd = displacement volume ~ b = Piston Bore ~ s = Piston Diameter ~ Take that number, and input it into the following formula to calculate CR. CR = (Vd + Vc) / Vc ~ Vd = displacement volume (found in step 1) ~ Vc = compressed volume. This is the volume above the piston when it is at TDC (top dead centre) You can find this number, along with the bore and stroke in an engine rebuild guide or service manual.