Velocity Stack Length Calculator

Enter the engine speed and the speed of sound in the intake to estimate an effective tuned intake length (velocity stack/runner) using a simplified quarter‑wave model. Results depend on assumptions such as engine cycle, tuning order, valve timing, geometry, and end corrections.

Velocity Stack Length Calculator

Enter any 2 of the 3 numeric values below to calculate the missing one. Choose an engine cycle and tuning order (higher order gives a shorter length).

Engine Cycle

Tuning Order (odd harmonic)

Engine Speed

Speed of Sound in Intake

Velocity Stack Length

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 \cdot 60 \cdot S) / (4 \cdot N \cdot 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 is the tuning order (odd harmonic, such as 1, 3, 5, 7…); larger values give a shorter length estimate

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:

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

Follow-Up Calculations

If your stack length comes out around 7 inches for a target of 6,500 rpm, the next question is whether the drivetrain will actually keep the engine in that band. You can set the gearing to match it so the car stays closer to the RPM range where the intake resonance is doing its best work.

A 7-inch velocity stack tuned for about 6,200 rpm can lose some of its advantage if a tire change drops highway RPM by 10% or more, such as going from a 25-inch tire to a 28-inch tire. Use the effect of tire diameter on RPM to check whether a taller tire will move your engine out of the sweet spot you designed for.

If your setup makes roughly 230 lb-ft at the RPM where the stack is tuned, that number becomes much more meaningful after it passes through the transmission and differential. You can translate that into wheel torque to see how much force actually reaches the pavement after gearing and drivetrain losses.