What is the difference between gauge boost and absolute pressure?

Boost gauges read relative to ambient (gauge pressure). Absolute pressure (MAP) is gauge plus ambient: MAP = P_gauge + P_ambient. At sea level (14.696 psi / 1.01325 bar) 15 psi gauge = 29.7 psi absolute. Pressure ratio (PR = MAP / P_ambient) is always based on absolute values.

What is pressure ratio and why does it matter?

Pressure ratio is the dimensionless number that characterises compressor work: PR = P_absolute_out / P_absolute_in. It appears on every compressor map and determines adiabatic temperature rise, efficiency islands and surge/choke boundaries. Two cars running "15 psi boost" at different altitudes have different PRs and therefore different power outputs.

What is charge-air density ratio and how does it affect power?

Density ratio = PR × (T_ambient / T_outlet). Because power scales with air mass rather than air volume, density ratio directly predicts the power multiplier relative to naturally-aspirated. A density ratio of 1.7 means roughly 70 % more air mass per intake stroke — and therefore roughly 70 % more potential power before other limits (knock, fuelling, exhaust) come into play.

What injector duty cycle is safe?

The standard rule is to keep static duty cycle at or below 80 % at the RPM and load point you are targeting. This leaves headroom for ECU short-term fuel trims (typically ±15–25 %), injector-to-injector variation and gradual flow degradation over time. Above 85 % the ECU may clip fuelling and the mixture leans out unpredictably — a serious risk of detonation and engine damage.

How do I convert between psi, bar and kPa?

1 psi = 0.0689476 bar = 6.894757 kPa = 0.068046 atm. The calculator converts all inputs and outputs in real time, so you can enter bar and read psi or vice versa without touching a spreadsheet.

Boost Pressure Calculator

Q: How is post-compressor charge temperature calculated?

The isentropic (ideal) temperature rise is T₂_isen = T₁ × PR^((γ−1)/γ), where γ ≈ 1.4 for air and T₁ is the inlet temperature in Kelvin. A real compressor is less than 100 % efficient so the actual outlet temperature is T₂_real = T₁ + (T₂_isen − T₁) / η_c, where η_c is isentropic efficiency (typically 65–80 %). Hot charge air is less dense, so a good intercooler recovers density even after the pressure rise.

A boost pressure calculator built for anyone tuning or planning a turbocharged or supercharged engine. Whether you are reading a boost gauge, sizing a turbocharger on a compressor map, estimating charge-air temperature before specifying an intercooler, or checking whether your injectors have enough headroom at peak power, this tool covers all five calculations in one place — with no unit conversion fuss and nothing uploaded to a server.

How it works

Pressure ratio from gauge boost

The foundational conversion is simple but easy to get wrong:

MAP = P_ambient + P_gauge

PR = MAP / P_ambient

Boost gauges read gauge pressure — the excess above whatever the local atmospheric pressure is. Manifold absolute pressure (MAP) is what the engine actually sees. Pressure ratio (PR) is the dimensionless figure that appears on compressor maps and determines how hard a compressor is working. Two engines running 15 psi gauge boost at different altitudes have different PRs and different power outputs.

Standard sea-level ambient is 14.696 psi (1.01325 bar / 101.325 kPa). At 1,500 m altitude ambient drops to roughly 84.5 kPa — so the same gauge reading produces a meaningfully lower PR and less power.

Charge-air temperature and density

Compressing air heats it. The isentropic (ideal frictionless) temperature rise is:

T₂_isen = T₁ × PR^((γ−1)/γ)

where γ ≈ 1.4 for air and temperatures are in Kelvin. Real compressors are 65–80 % isentropic efficient, so the actual outlet temperature is:

T₂_real = T₁ + (T₂_isen − T₁) / η_c

Because power depends on mass flow, not volume flow, the useful metric is density ratio:

ρ_ratio = PR × (T_ambient / T_outlet)

A good intercooler drops T_outlet back toward ambient, recovering density. This tool lets you enter a measured manifold temperature to compute the post-intercooler density ratio — the real multiplier on naturally-aspirated power.

Injector duty cycle

Once you have the density ratio you can estimate fuel demand:

Air mass (g/min) = displacement × air_density × VE × RPM/2

Fuel mass = air_mass / AFR

Duty cycle (%) = (fuel_per_injector / injector_size) × 100

Standard air density at 25 °C is 1.2041 g/L; the calculator scales this by the density ratio at your manifold conditions. Keep duty cycle under 80 % for reliable fuelling.

Worked example

A 2.0-litre turbocharged engine at sea level (14.696 psi ambient, 25 °C ambient) running 18 psi gauge boost with a 75 % efficient compressor:

MAP = 14.696 + 18 = 32.7 psi absolute (2.253 bar)
Pressure ratio = 32.7 / 14.696 = 2.22
Isentropic post-compressor temp: 298 K × 2.22^(0.286) = 382 K (109 °C)
Real temp at 75 % efficiency: 298 + (382 − 298) / 0.75 = 410 K (137 °C)
Density ratio before IC: 2.22 × (298 / 410) = 1.61 — 61 % more air than naturally aspirated
After a quality intercooler dropping charge to 45 °C (318 K): density ratio = 2.22 × (298/318) = 2.08 — 108 % more air

With 550 cc/min injectors ×4, 95 % VE and a target AFR of 11.8 at 6,500 RPM, duty cycle works out to approximately 67 % — safely within the 80 % limit.

Formula reference

Formula	Meaning
MAP = P_amb + P_gauge	Manifold absolute pressure
PR = MAP / P_amb	Pressure ratio
T₂_isen = T₁ × PR^((γ−1)/γ)	Isentropic outlet temperature
T₂_real = T₁ + (T₂_isen − T₁) / η_c	Real outlet temperature
ρ_ratio = PR × (T_amb / T_out)	Charge-air density ratio
DC = (fuel_req / inj_size) × 100	Injector duty cycle (%)

All pressures must be absolute when computing PR. The calculator handles the ambient offset automatically regardless of which unit system you choose.