Compression Ratio Calculator

Created by Bogna Szyk
Reviewed by Dominik Czernia, PhD and Jack Bowater
Last updated: Nov 17, 2022

Our compression ratio calculator is the perfect tool for all car 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 a 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 uncompressed state.

Compression ratio formula

The compression ratio can be easily found with the following formula:

$\textrm{CR} = \frac{V_d+V_c}{V_c}$

where:

• $V_d$ is the displacement volume - the volume that has been swept by a moving piston. You can calculate it according to the cylinder volume formula: $V_d = \tfrac 14 \cdot b^2 \cdot s \cdot \pi$.
• $V_c$ is the volume above the piston when it is at the top-dead-center (its topmost position), also called the compressed volume.
• $b$ is the cylinder bore (diameter).
• $s$ is 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.

\small \begin{align*} V_c = V_{\rm chamber} & + V_{\rm piston} \\ & + V_{\rm gasket} + V_{\rm clearance} \end{align*}

where:

• $V_{\rm chamber}$ is 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.
• $V_{\rm piston}$ - 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).
• $V_{\rm gasket}$ - the head gasket of thickness $t$ and bore (diameter) $g$ will occupy a volume of $V_{\rm gasket} = \frac 14 \cdot g^2 \cdot t \cdot \pi$
• $V_{\rm clearance}$ is equal to $V_{\rm clearance} = \frac 14 \cdot b^2 \cdot c \cdot \pi$, where $c$ is the deck clearance.

Our compression ratio calculator estimates the value of the CR and the total engine volume. You may calculate it 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 enter 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 discover more in Omni's Carnot efficiency calculator.

You can open the dynamic compression ratio calculator in our calculator'sadvanced mode It will calculate the DCR 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 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. However, if you're using race fuel, it can get much higher, up to 10:1 DSR and 15:1 static CR.

FAQ

How can I calculate compression ratio of a petrol engine?

To calculate the compression ratio, you may use the formula:

CR = (Vd + Vc) / Vc

So, let's suppose you are using the values of your petrol engine and follow the steps below.

1. Suppose the displacement volume(Vd) is 52 cc;
2. The compressed volume (Vc) is 8 cc;
3. Sum the values, and the result is 60;
4. Now divide 60 by 8 (Vc); and
5. The result is a compression ratio of 7.5:1.

What is a good compression ratio?

A high compression ratio is good as it is directly proportional to the engine's performance. But that doesn't imply you should take it to infinity.

Remember, if you want the best compression ratio, then your focus should be the dynamic compression ratio(DCR) instead of static. For traditional engines, the DCR ranges between 8:1 to 8.5:1. And if you're using race fuel, expect a much higher ratio up to 10:1 DSR.

Are static and dynamic compression ratio the same?

**No, they are different from each other. **

The static compression ratio doesn't take into account the closing of the intake valve, which impacts the practicality of the results.

The dynamic compression ratio(DRC), on the other hand, gives the actual cylinder pressure because it changes dynamically and hence provides information about how the engine will perform.

What happens if compression ratio is too high?

We know a high compression ratio is good, but we don't want it too high. As it causes the engine to knock, which is the same as detonation. This can, in turn, lead to engine damage and reduced efficiency.

Bogna Szyk
Engine characteristics
Number of cylinders
Bore diameter (b)
in
Piston stroke (s)
in
in
in
Deck clearance (c)
in
Combustion chamber volume
cu in
Piston volume
cu in
Compression ratio
Compressed volume
cu in
Displacement volume
cu in
Engine volume
cu in
Static compression ratio
: 1
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