Estimate the effective intake-path length, engine RPM, or intake sound speed using a simplified quarter-wave model. Choose a 2-stroke or 4-stroke cycle and an odd intake-pulse harmonic. The result is not the physical stack length alone.

Ideal quarter-wave estimate of the effective intake path, including runner and stack. Valve timing, geometry and end corrections require engine-specific validation.

Related Calculators

Velocity Stack Length Formula

The following simplified quarter‑wave relation can be used to estimate a tuned effective intake length (stack/runner). It requires converting engine speed (RPM) into an intake‑event frequency and selecting a tuning order.

LS = (C × 60 × S) / (4 × N × O)

Variables:

  • LS is the estimated effective tuned intake length (meters, then convert to mm/cm/in as needed)
  • C is the speed of sound in the intake (m/s) (about 343 m/s at 20 °C; higher when the intake air is hotter)
  • N is the engine speed (revolutions per minute)
  • S is the engine‑cycle factor: S = 2 for a 4‑stroke engine (1 intake event per 2 revolutions) and S = 1 for a 2‑stroke engine (1 intake event per revolution)
  • O multiplies the intake-pulse excitation frequency (odd choices 1, 3, 5, 7…); larger values give a shorter length estimate in this convention. It is not the odd spatial mode number of a fixed pipe

To calculate LS, select S (2‑stroke vs 4‑stroke) and a tuning order O, then use the formula above. With C in m/s and N in RPM, the result is in meters and can be converted to millimeters (×1000) or other units. This is a rule‑of‑thumb starting point; real intake tuning depends on valve timing, temperature, runner geometry, and end corrections.

What is a Velocity Stack?

A velocity stack, also known as an air horn, is a trumpet-shaped device that is part of the air intake system of an internal combustion engine. It is designed to allow smooth airflow into the intake manifold and can improve the engine’s performance by optimizing the air pressure waves within the intake system.

How to Calculate Velocity Stack Length?

The following steps outline how to calculate the Velocity Stack Length:

  1. First, determine the engine speed (N) in revolutions per minute (RPM).
  2. Next, determine the speed of sound in the intake (C) in meters per second (m/s).
  3. Choose the engine‑cycle factor (S) and a tuning order (O) for the harmonic you want to target.
  4. Next, gather the formula from above: LS = (C × 60 × S) / (4 × N × O).
  5. Finally, calculate LS and convert the result to your desired length unit (for example, multiply meters by 1000 to get millimeters).
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Engine Speed (N) = 6000 RPM

Speed of Sound in Intake (C) = 343 m/s

With a 4-stroke cycle (S = 2) and pulse harmonic O = 7, LS = (343 × 60 × 2) / (4 × 6000 × 7) = 0.245 m = 245 mm. This is the effective runner-plus-stack path before engine-specific geometry and end corrections are resolved.