Exhaust Scavenging Calculator
Design a tuned header — primary tube length and diameter for peak-RPM scavenging, using Vizard / Blair methods.
How to Use
- Enter cylinder displacement and peak-torque RPM.
- Enter exhaust cam duration at 0.050" lift.
- Tool returns ideal primary length and diameter, and collector sizing.
Engine
Header geometry
Formulas
Primary length (Vizard)
L = (850 · ECD) / RPM − 3
Primary ID (flow-based)
ID = √(CID_cyl · RPM / 88200)
Collector area
A_col ≈ 1.9 · A_primary (per tube)
Collector length
L_col ≈ 0.6 · L_primary
Frequently Asked Questions
Primary length — why does it matter?
The exhaust pulse reflects off the collector as a low-pressure wave. Time it right and it arrives at the valve during overlap to suck in charge. Wrong length = no scavenging.
Tuning for peak torque or HP?
Length tunes for the RPM you design around. Torque = shorter. HP = longer. Pick one.
Stepped primaries?
Stepping up (1.625 → 1.75 → 1.875) broadens the tuned range by ~500 RPM per step. Costs a little peak.
Common Use Cases
Header design
Compute primary length for your target RPM before ordering tubing.
Broadening powerband
Check stepped-primary sizes for mid-range gain.
Collector tuning
Size the collector cross-section step for pulse tuning.
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