Why does compressor pressure ratio use absolute pressure?

Both Garrett Motion and BorgWarner technical standards define compressor pressure ratio as absolute discharge pressure divided by absolute inlet pressure (PR = P2_abs / P1_abs). Gauge pressure alone does not reflect atmospheric inlet pressure, which varies with altitude and weather.

How does an intercooler alter the horsepower calculation?

Compressing air heats it, reducing its density. Without intercooling, density ratio is significantly lower than pressure ratio. An intercooler drops charge temperature back toward ambient, increasing oxygen mass per unit volume according to Charles's and Boyle's gas laws.

What unit conversion factor is used between bar and PSI?

According to NIST standard conversion values, 1 bar equals exactly 100,000 Pa, which corresponds to 14.5037738 pounds per square inch (psi). Standard sea-level atmospheric pressure is 101.325 kPa (14.6959 psia).

Why is estimated horsepower labeled as a thermodynamic estimate?

The density ratio calculation models ideal gas behavior and intake air charge density. It omits engine volumetric efficiency variations across RPM, intercooler core and intake piping pressure drops, ignition timing retard, fuel octane limits, turbine exhaust backpressure, and mechanical friction losses.

What is typical compressor isentropic efficiency?

Modern radial flow centrifugal turbocharger compressors operate between 60% and 80% isentropic efficiency within their primary map islands, typically peaking around 72% to 78% depending on wheel trim and design.

Turbo Boost Calculator

Calculate compressor pressure ratio, discharge temperature, post-intercooler intake charge temperature, and density-corrected theoretical power gains.

Engine & Turbo Parameters

Target Boost Pressure (Gauge)

Gauge pressure above ambient inlet (1 bar = 14.5038 PSI).

Ambient Air Temperature

Atmospheric Pressure (psia)

Sea level standard = 14.696 psia (101.325 kPa). Use ~12.2 psia at 1,500 m.

Compressor Isentropic Efficiency (η_c): 70%

50% (low) 70% (typical) 90% (max)

Intercooler Effectiveness (η_ic): 70%

0% (None) 70% (Street) 100% (Ideal)

Baseline NA Engine Power (HP)

Naturally aspirated crank horsepower at ambient pressure.

Pressure Ratio (PR)

—

Absolute discharge / inlet pressure

Density Ratio (DR)

—

Post-IC air density vs. ambient

Compressor Outlet Temp (T2)

—

Discharge before intercooler

Intake Charge Temp

—

Air entering intake manifold

Estimated Thermodynamic Power Output

Simplified ideal-gas model

Baseline NA

—

Est. Power Gain

—

Est. Turbo HP

—

0 HP —

Unit Relationships & Model Limitations

Standard Conversions:

1 bar = 100 kPa = 14.5037738 psi. Standard sea-level atmosphere = 14.6959 psia (101.325 kPa). Absolute pressure ratio PR = (P_atm + P_boost) / P_atm.

Charge Density & Temperature Relations:

Compressor isentropic formula: T_2s = T₁ × (PR)^{(γ - 1)/γ} with γ = 1.4. Outlet temp: T₂ = T₁ + (T_2s - T₁)/η_c. Post-intercooler charge temp: T_charge = T_ambient + (1 - η_ic)(T₂ - T_ambient). Density ratio: DR = PR × (T_{ambient, K} / T_{charge, K}).

Limitations & Real-World Discrepancies:

The horsepower projection represents a theoretical upper-bound estimate scaled strictly by charge air density. Actual engine dynamometer output depends on volumetric efficiency changes across engine speed, intake manifold and intercooler core pressure drop, ignition timing, combustion knock limits, exhaust turbine backpressure (pumping losses), and mechanical drivetrain losses.

Summary

Calculate compressor pressure ratio, discharge temperature, post-intercooler intake charge temperature, and density-corrected theoretical power gains.

How it works

Enter gauge boost pressure in PSI or bar, along with baseline naturally aspirated engine horsepower.
Specify ambient air temperature and local atmospheric pressure to determine absolute inlet pressure (P1).
Set compressor isentropic efficiency (typically 65–80% on performance turbocharger maps) and intercooler effectiveness (0% for non-intercooled, 60–85% for street/competition cores).
The tool calculates absolute compressor pressure ratio: PR = (P_atm + P_boost) / P_atm.
Discharge temperature is derived from isentropic temperature rise: T2 = T1 + [T1 × ((PR)^((γ - 1)/γ) - 1)] / η_c, using γ = 1.4 for dry air.
Post-intercooler charge temperature is calculated as T_charge = T_ambient + (1 - η_ic) × (T2 - T_ambient).
Charge density ratio DR = PR × (T_ambient_K / T_charge_K) is applied to baseline horsepower as a theoretical upper-bound estimate.

Use cases

Evaluate compressor outlet heat and verify intercooling requirements for target boost pressures.
Convert between metric boost (bar) and Imperial boost (PSI) under standard or high-elevation atmospheric pressures.
Assess the thermodynamic impact of intercooler efficiency improvements on intake charge air density.
Compare theoretical density-ratio power scaling against empirical dynamometer measurements.
Examine charge heating effects across seasonal ambient temperature variations.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu