What this calculator does
Boost is usually quoted as pressure above atmospheric, so 8 psi of boost means the manifold sits at 22.70 psi absolute. The pressure ratio is that absolute figure over atmospheric, here 1.544, and it is the number that actually describes what the compressor is doing.
Pressure ratio translates directly into effective displacement, because the engine is being fed denser air. A 2.0 litre engine at 1.544 pressure ratio breathes like a 3.1 litre one, before any losses. That is the whole point of forced induction, and it is why boost is quoted so proudly.
The formula
The pressure ratio is atmospheric plus boost, divided by atmospheric. Effective displacement multiplies actual displacement by that ratio. Compressing air also heats it, and hot air is less dense, so the calculator applies a temperature correction using the ratio of absolute temperatures where a charge temperature rise is entered.
| Term | Meaning |
|---|---|
| Boost | Pressure above atmospheric, the figure normally quoted. |
| Absolute manifold pressure | Boost plus atmospheric, which is what the engine actually sees. |
| Pressure ratio | Absolute manifold pressure over atmospheric. The compressor map axis. |
| Intercooler | A heat exchanger cooling the charge after compression, recovering density lost to heating. |
The inputs explained
| Field | What to enter |
|---|---|
| Boost pressure (psi) | Boost pressure in psi, above atmospheric. |
| Atmospheric pressure (psi) | Atmospheric pressure. 14.696 psi at sea level; less at altitude. |
| Engine displacement (cc) | Actual engine displacement in cc. |
| Intake charge temperature rise (°C) | Charge temperature rise above ambient after compression and any intercooling. Set 0 for the ideal case. |
When to use it
Reading a compressor map
Compressor maps are plotted against pressure ratio, not boost, so the conversion is needed before the map can be used.
Estimating the gain from boost
Effective displacement gives a first approximation of how much more air the engine breathes.
Understanding intercooling
Entering a charge temperature rise shows how much of the density gain heat gives back.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each boost level give?
A range of boost pressures on the same engine.
| Boost pressure | Pressure ratio | Effective displacement | Charge density gain |
|---|---|---|---|
| 5 psi | 1.340 : 1 | 2,680 cc | 34.0% over atmospheric |
| 8 psi | 1.544 : 1 | 3,089 cc | 54.4% over atmospheric |
| 15 psi | 2.021 : 1 | 4,041 cc | 102.1% over atmospheric |
| 25 psi | 2.701 : 1 | 5,402 cc | 170.1% over atmospheric |
Questions
What is the difference between boost and pressure ratio?
Boost is gauge pressure, measured above atmospheric. Pressure ratio is absolute manifold pressure divided by atmospheric. Compressor maps and turbo specifications use pressure ratio, so converting from boost is the first step in matching a turbo to an engine.
Does 14.7 psi of boost double the power?
It roughly doubles the air, which is not the same as doubling the power. Fuelling, heat, mechanical losses and the power drawn by a supercharger all take their share, and the engine has to be built to survive the extra cylinder pressure. Power gains in practice are meaningfully less.
Why does charge temperature matter?
Because compressing air heats it, and hot air is less dense. The density gain follows the ratio of absolute temperatures, so a 60 °C rise gives back about 17% of what the pressure gained. Intercooling exists specifically to recover that loss.
Does altitude change things?
Yes. Atmospheric pressure falls with altitude, so the same pressure ratio produces less absolute pressure and less air. A turbo can compensate by spinning faster to hold boost, which is why forced induction engines lose far less power at altitude than naturally aspirated ones.
For the compression ratio, see the compression ratio calculator. For wheel and speedometer effects, see the tire size calculator.