How do you calculate turbo Pressure Ratio (PR)?

Pressure Ratio (PR) is calculated as absolute compressor discharge pressure (P2c) divided by absolute compressor inlet pressure (P1c): PR = (Ambient Pressure + Boost Pressure + Intercooler Loss) / (Ambient Pressure - Intake Loss).

How is mass airflow (lb/min) calculated from engine specs?

Using the equation: Wa (lb/min) = (MAP * Displacement_CID * RPM * VE) / (639.6 * (Intake_Temp_F + 460)), where MAP is Manifold Absolute Pressure in psia.

How much horsepower is produced per lb/min of turbo airflow?

On standard gasoline engines, 1.0 lb/min of airflow produces approximately 10 to 11 flywheel horsepower.

Engineering Calculation Engine

Turbo Airflow & Pressure Ratio Calculator

Calculate compressor mass airflow (lb/min & CFM), Pressure Ratio (PR), and required boost pressure to properly match and size turbochargers on compressor maps.

Mode:

Quick Preset:

Engine Specifications

Displacement (Liters / CID) *

CID

Peak Power RPM *

RPM

Spool / Mid RPM

RPM

Volumetric Efficiency (VE) 92%

Target Manifold Boost Pressure *

PSI

Bar

Fuel Type / BSFC

Intake Temp (°F)

°F

Elevation Patm

IC Drop (psi)

Inlet Drop (psi)

Calculation Formulas:

PR = (Patm + Boost + ΔPic) / (Patm - ΔPinlet)

Wa = (MAP × CID × RPM × VE) / (639.6 × (T + 460))

Calculated Turbo Requirements

Mass Airflow (Wa)

42.4 lb/min

0.321 kg/s

Pressure Ratio (PR)

2.55

34.7 psia

Flywheel Power

424 HP

~360 WHP

Volumetric Flow 614 CFM

Density Ratio 2.28 DR

Spool Airflow 27.3 lb/min

Fuel Flow Req 212 lb/hr

Compressor Map Operating Line

PR vs Flow (lb/min)

Recommended Turbo Sizing:

Based on 42.4 lb/min at 2.55 PR: Garrett G25-550, Garrett GTX2867R Gen II, BorgWarner EFR 6758, or Precision 5558.

Engine Airflow & Boost Reference Matrix

Standard 4-Stroke Baseline

Displacement	Boost (PSI / Bar)	PR	Airflow (lb/min)	Airflow (CFM)	Crank HP	Popular Turbos
1.6L – 1.8L (4-Cyl)	14.5 PSI (1.0 bar)	2.08	26 – 32	380 – 460	270 – 330 HP	Garrett GT2560R, BorgWarner EFR 6258
2.0L (EA888 / K20)	22.0 PSI (1.5 bar)	2.62	45 – 52	650 – 750	460 – 530 HP	Garrett G25-660, BorgWarner EFR 7163
2.5L – 2.6L (5-Cyl / RB26)	25.0 PSI (1.7 bar)	2.83	62 – 72	900 – 1040	630 – 740 HP	Garrett G30-770, Precision PTE 6266
3.0L (2JZ / B58)	28.0 PSI (1.9 bar)	3.04	80 – 92	1160 – 1330	820 – 950 HP	Garrett G35-900, BorgWarner EFR 8474
5.0L – 6.2L (Twin Turbo)	15.0 PSI (1.0 bar)	2.12	95 – 125	1370 – 1800	1,000 – 1,300 HP	Twin Garrett G30-660 / Precision 6466s

Sizing Questions & Answers

What is Pressure Ratio (PR) and why is it critical?

Pressure Ratio (PR) is the ratio of absolute compressor discharge pressure to absolute compressor inlet pressure: PR = P2c / P1c. It defines how much the compressor must compress ambient air to produce the target manifold pressure.

How do I convert between CFM and Mass Flow (lb/min)?

Under standard temperature and pressure (68°F, 29.92 inHg), air density is ~0.0765 lb/cu ft. As a working rule: 100 CFM ≈ 6.9 to 7.5 lb/min.

What causes compressor surge?

Compressor surge occurs when the compressor builds high pressure at low engine flow (to the left of the surge line). The blades stall aerodynamically, causing destructive backward airflow pulses through the intake.