Enter the speed of sound (in the intake air) and the engine RPM into the calculator to estimate a tuned intake runner length using a simplified pressure-wave (acoustic) model. This is a starting-point estimate for intake tuning; real “optimal” length depends on valve timing, runner/port geometry, and other engine details.
Intake Length Formula
The following simplified quarter-wave relationship is commonly used to estimate a tuned intake runner length for a 4-stroke engine (selecting a harmonic order):
L = \frac{c}{4\left(\frac{N}{120}\right)H} = \frac{30c}{NH}Variables:
- L is the estimated tuned intake runner length (meters)
- c is the speed of sound in the intake air (meters per second)
- N is the engine speed (revolutions per minute)
- H is the harmonic order (dimensionless, e.g., 3, 4, 5…)
For a 4-stroke engine, each cylinder has one intake event every two crank revolutions, so the intake-event frequency is N/120 in events per second. A common starting point is to use a higher harmonic (shorter, more practical runner lengths). The Basic tab of the calculator uses H = 4, which simplifies the formula to L = 7.5c/N (with N in RPM).
What is Intake Length?
In this context, intake length (often called intake runner length) is the effective distance from the intake valve/port to the runner inlet (plenum entry, bellmouth, or open end) used for pressure-wave tuning. Changing runner length alters when reflected pressure waves arrive back at the intake valve, which can improve cylinder filling over a chosen RPM range. The actual effective length can differ from the physical length because of geometry and “end correction” effects.
How to Calculate Intake Length?
The following steps outline how to estimate a tuned Intake Length.
- Determine the speed of sound (c) in the intake air (m/s). (It depends mainly on air temperature.)
- Determine the engine speed (N) in RPM.
- Select a harmonic order (H). (Higher H gives a shorter runner.)
- Use the formula L = 30c/(NH) to estimate the runner length (L) in meters.
- Check your result with the calculator above (Basic tab uses H = 4).
Example Problem:
Use the following variables as an example problem to test your knowledge.
Speed of sound (c) = 343 meters per second
Engine RPM (N) = 3000 RPM (using H = 4, L ≈ 0.8575 m)
