How boost affects compression
Forced induction packs more air in before the piston ever moves, so the effective compression ratio rises with the pressure ratio:
effective CR = static CR × (boost + atmospheric) ÷ atmospheric
Higher effective compression means more power — but also more heat and detonation risk, which is why boosted engines run lower static compression, intercooling and higher-octane fuel. The pressure ratio (absolute manifold ÷ ambient) is the number you use to read a turbo compressor map.
Boost in psi, bar and kPa
Three units, one pressure. The conversions are exact and worth knowing by heart:
1 bar = 14.5038 psi = 100 kPa · 1 psi = 6.8948 kPa
The trap is not the arithmetic, it is gauge versus absolute. A boost gauge reads the pressure above the air around it, so "1 bar of boost" means roughly 2 bar absolute in the manifold — twice the air, a pressure ratio of about 2.0. Get that wrong on a compressor map and you will pick a turbo for half the work you are actually asking it to do.
Scan tools make this worse by mixing conventions. A generic OBD-II MAP reading is absolute, usually in kPa: about 100 kPa with the engine off at sea level, 30–40 kPa at idle under vacuum, and 250 kPa when a European ECU is showing you 1.5 bar of boost. Some manufacturers report millibar absolute instead, so 2,000 mbar is 1 bar of boost, not 2. If a number looks like double what you expected, it is almost certainly absolute.
One more caveat this page already accounts for: boost is measured against ambient, and ambient falls with altitude. The same 15 psi on the gauge is a higher pressure ratio in Denver than at sea level, so the compressor is working harder and the charge is hotter for an identical gauge reading. The density altitude calculator puts a number on the air you are actually starting from.
Boost pressure conversion chart
Every row is produced by driving the calculator above at build time, so the chart and the tool cannot disagree.
psi, bar and kPa — and what each is worth
Gauge pressure on the left, absolute manifold pressure and pressure ratio on the right — at the sea-level ambient the calculator is set to. 14.7 psi is one atmosphere, the row where an engine is breathing twice as much air as it would on its own.
| Boost (psi) | bar | kPa gauge | Manifold (abs) | Pressure ratio | Air density |
|---|---|---|---|---|---|
| 1 | 0 | psi bar | +% | ||
| 2 | 0 | psi bar | +% | ||
| 3 | 0 | psi bar | +% | ||
| 4 | 0 | psi bar | +% | ||
| 5 | 0 | psi bar | +% | ||
| 6 | 0 | psi bar | +% | ||
| 7 | 0 | psi bar | +% | ||
| 8 | 0 | psi bar | +% | ||
| 9 | 0 | psi bar | +% | ||
| 10 | 0 | psi bar | +% | ||
| 11 | 0 | psi bar | +% | ||
| 12 | 0 | psi bar | +% | ||
| 14.7 | 0 | psi bar | +% | ||
| 16 | 0 | psi bar | +% | ||
| 18 | 0 | psi bar | +% | ||
| 20 | 0 | psi bar | +% | ||
| 22 | 0 | psi bar | +% | ||
| 25 | 0 | psi bar | +% | ||
| 29 | 0 | psi bar | +% | ||
| 30 | 0 | psi bar | +% | ||
| 35 | 0 | psi bar | +% | ||
| 40 | 0 | psi bar | +% |
FAQ
What effective CR is "safe"?
It depends on fuel, cooling and tuning — but very roughly, pump-gas builds often keep effective CR in the low-to-mid teens; E85 and race fuel allow more. Treat this as a planning number, not a guarantee.
Does intercooling change this?
This shows the pressure side. An intercooler lowers charge temperature, raising real air density further and cutting detonation risk for the same boost.
How many psi is 1 bar of boost?
14.5 psi. Because a gauge reads above ambient, 1 bar of boost is about 2 bar absolute in the manifold — a pressure ratio near 2.0, or roughly twice the air an engine would breathe on its own.
My scan tool shows MAP in kPa — how do I read it as boost?
MAP is absolute, so subtract ambient: about 100 kPa at sea level. A 250 kPa reading is 150 kPa of boost, which is 1.5 bar or 21.8 psi. Under vacuum at idle you will see 30–40 kPa, well below atmospheric.