Exhaust Flow Rate Calculator
Calculate exhaust gas flow (CFM, lb/min, m³/s) from displacement, RPM, VE, and exhaust temperature.
How to Use
- Enter engine displacement (CID or L) and peak-power RPM.
- Set volumetric efficiency — 85% stock, 95-105% NA race, 110%+ forced induction.
- Set expected exhaust gas temperature (EGT) at the measurement point.
- Result shows intake CFM, hot exhaust CFM, and mass flow.
Inputs
Flow
Formulas
Intake CFM
CFM_in = (CID × RPM × VE/100) / 3456
Exhaust CFM (hot)
CFM_ex = CFM_in × (T_egt + 460) / (70 + 460)
Air mass flow
ṁ_air (lb/min) ≈ CFM_in × 0.0765
Exhaust mass flow
ṁ_ex ≈ ṁ_air × (1 + 1/AFR), AFR ≈ 14.7
Frequently Asked Questions
Why does exhaust CFM exceed intake CFM?
Exhaust gas is 2-3x hotter and less dense than intake air. Volumetric flow scales with absolute temperature ratio.
What EGT should I use?
Naturally aspirated pre-cat: 1200-1500°F. Turbo pre-turbine: 1500-1800°F. Post-turbo: 900-1200°F.
Why include VE?
Mass in = mass out (approx). Whatever mass the engine inducts has to leave. VE > 100% means forced induction or tuned intake/exhaust.
Common Use Cases
Exhaust sizing
Match pipe diameter to flow for minimal backpressure.
Turbo spec
Size turbine housing to the expected exhaust mass flow.
Catalyst sizing
Pick a cat substrate volume that flows without restriction.
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