Intake Runner Length Calculator
Compute tuned intake-runner length via quarter-wave resonance for each harmonic (2nd, 4th, 6th) at your target RPM.
How to Use
- Enter target peak-power RPM and intake air temperature.
- Pick a harmonic — 2nd tunes long runner for low-end, 4th/6th shorter for top-end.
- Subtract port length (~3-6") from total runner — the port counts toward tuning.
Inputs
Runner lengths by harmonic
Formula
Sound speed in air
V_s (ft/s) = 49.02 · √(T_R), T_R = T_°F + 459.67
Intake event frequency
f = RPM · H / 120, H = 2 · harmonic
Quarter-wave runner
L = V_s / (4 · f) - L_port
IVC correction
Effective intake duration: 180° + IVC
Frequently Asked Questions
What is a harmonic?
The intake pulse bounces back and forth. At certain lengths, the return wave arrives as a positive pressure during valve open. H=2 gives the longest tune; H=4 or H=6 give shorter runners.
How does IAT affect this?
Sound speed scales with √T. Cold air intake = slower wave = slightly longer ideal length.
Should I include the head port?
Yes — the pulse tunes from the valve, not the plenum. Measure from the plenum floor to the back of the valve.
Common Use Cases
Individual-runner intake design
ITBs or trumpets for peak VE at a target RPM.
Plenum-fed intake redesign
Swap spacer heights to hit the 2nd or 4th harmonic.
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