Our compression ratio calculator is the perfect tool for all car and racing fans. If you have no idea what's the compression ratio formula, or how to use it, you've come to the right place! We will explain all the basics of these calculations and answer the question of what is the best compression ratio. If you're already an dedicated petrolhead, you will surely appreciate the advanced part of our calculator, where we deal with the dynamic compression ratio.
If you like this tool, be sure to check out the car crash calculator, too!
What is the compression ratio?
The compression ratio is a characteristic of combustion engines (and, surprisingly, has nothing to do with file compression). It is the ratio of the volume of the cylinder and the combustion engine combined when the piston is at the bottom of its stroke, and when it's at the top of its stroke. You can easily imagine it as the proportion between the volume of the engine in its compressed and the uncompressed state.
Compression ratio formula
The compression ratio can be easily found with the following formula:
CR = (Vd + Vc) / Vc
Vdis the displacement volume - the volume that has been swept by a moving piston. It can be calculated according to the cylinder volume formula:
Vd = b2 * s * π / 4.
Vcis the volume above the piston when the piston is at the top dead-centre (its topmost position), also called the compressed volume.
bis the cylinder bore (diameter).
sis the piston stroke length.
In our compression calculator, we subdivide the volume above the piston into four components to help you make the calculations as precise as possible.
Vc = Vchamber + Vpiston + Vgasket + Vclearance
Vchamberis the volume of the combustion chamber at the top of the engine. It's typically measured by pouring a liquid into the chamber and measuring its volume. You can often find it in the manufacturer's description.
Vpiston- if the piston doesn't have a flat top, but is domed or dished, it will take a bit of space (or leave that space free).
Vgasket- the head gasket of thickness
tand bore (diameter)
gwill occupy a volume of
Vgasket = g2 * t * π / 4
Vclearanceis equal to
Vclearance = b2 * c * π / 4, where
cis the deck clearance.
Our compression ratio calculator estimates not only the value of the CR but also the total engine volume. It's calculated by multiplying the displacement volume by the number of cylinders.
Static vs. dynamic compression ratio
Using the compression ratio formulas above, you will be able to compute the static compression ratio. However, this number does not perfectly describe reality. Why? It doesn't take into account the closing of the intake valve.
In a combustion engine, the piston moves up and down. When it moves up, the volume inside the cylinder is compressed; when it moves down, fresh fuel and air enters the chamber through the intake valve. The piston reaches its lowest position (bottom dead centre) and starts moving up again. At that time, though, the intake valve is not yet closed, so no compression occurs - the air is "pushed out" through the valve. We can only speak about compression once the valve is completely closed. This is why the dynamic compression ratio (DCR) is dependent on the intake valve closing (IVC) point, which is expressed as an angle after the bottom dead center (ABDC).
You can open the dynamic compression ratio calculator in the advanced mode of our calculator. The DCR will be calculated based on the modified stroke length, which in turn is derived from the rod length and the IVC.
What's the best compression ratio?
The higher the compression ratio, the better the engine performs. However, you can't increase the compression ratio to infinity. At some point, the combustion chamber won't have enough space to accommodate the combustion, and detonation will occur.
The resistance of an engine depends on many factors, but one of the most important is the fuel octane. The higher the octane, the more compression the fuel is able to resist, and the higher the potential compression ratio.
As we mentioned before, the dynamic compression ratio is the one that actually happens inside the engine. That's why, when we talk about the best compression ratio, we should focus on the DCR and not on the static CR. Most conventional engines will have the DCR in the range from 8:1 to 8.5:1, which corresponds to 10:1 to 12:1 static CR. If you're using race fuel, however, it can get much higher, up to 10:1 DSR and 15:1 static CR.