Exhaust Gas Velocity Calculator
Compute gas velocity in the exhaust (ft/s, m/s, Mach) from flow rate and pipe ID. Target the 250-290 ft/s scavenging window.
How to Use
- Enter exhaust volumetric flow (CFM hot, from the Flow Rate tool).
- Enter actual pipe inside diameter (ID), not OD.
- Check velocity against the 240-290 ft/s scavenging window.
Inputs
Velocity
Formulas
Area from ID
A = π · (ID/2)²
Velocity
v = Q / A
Sonic velocity (hot)
a = √(γ · R · T), γ≈1.33 for exhaust
Mach
M = v / a
Frequently Asked Questions
Why is 240-290 ft/s the target?
Classic Vizard finding — below 200 the slug loses inertia and scavenging stalls; above 300 friction ΔP dominates.
What about collectors?
Velocity drops across the collector as cross-section grows — that's the pulse energy converting to vacuum that sucks the next cylinder clean.
OD vs ID?
OD is the tube spec (e.g., 1-3/4"). ID = OD − 2×wall. 0.065" wall on 1.75" tubing gives 1.62" ID — a 16% smaller area.
Common Use Cases
Header primary sizing
Pick primary OD that keeps velocity in the scavenging window at peak torque.
Merge collector sizing
Design the cross-section step to get the desired velocity drop.
Diagnose a dog exhaust
Velocity > 400 ft/s means you're strangling the engine.
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