Engine Compression Ratio Calculator
Enter the engine specifications below to calculate the compression ratio.
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How to Calculate Engine Compression Ratio
The compression ratio of an engine is the ratio between the swept volume and the compressed volume.
In other words, it is the ratio between the volume of the combustion chamber when the piston is at the bottom of its stroke and the air-fuel mixture is uncompressed (also known as bottom dead center or BDC) and the volume when the piston is at the top of its stroke and the mixture is fully compressed (top dead center or TDC).
A higher compression ratio allows the engine to output more power using the same amount of fuel, making it an important performance metric. Thus, higher compression ratios also allow engines to be more fuel efficient, but these engines may need fuel with a higher octane rating to operate properly.
To calculate the compression ratio of an engine, use the following formula:
compression ratio = Vd + Vc / Vc
Vd is the displacement volume of the cylinder, or the volume displaced by the piston from the start to the end of the stroke.
Vc is the clearance volume of the cylinder, or the remaining space left when the piston is at the end of the stroke.
How to Calculate Displacement
The displacement of a piston engine is the volume defined by the travel of its pistons as they travel from BDC to TDC. Displacement is also known as the “swept volume” of the engine.
You can calculate displacement using this simple formula:
displacement = π / 4 × b2 × s
b is the bore.
s is the stroke.
This formula will yield the displacement of one cylinder, so to find the total displacement of the engine, simply multiply the result by the number of cylinders in the engine.
Try our engine displacement calculator to find this.
How to Calculate Clearance Volume
The clearance volume of an engine is the space remaining in the cylinder when the piston is at TDC. It can also be defined as the part of the cylinder’s volume that is not swept by the piston.
You can find the clearance volume using the formula:
clearance volume = c + p + d + g
c is the chamber volume.
p is the volume of the piston dome or dish.
d is the deck volume.
g is the volume of space taken up by the gasket bore when compressed.