How to Calculate Velocity Stack Length for a Target RPM

1.What a velocity stack is

The stack is the flared mouth of the intake trumpet. The flare helps air enter the bore; the tube behind it is also part of the length a pressure wave travels from the intake valve to the open end. The number you want is usually that full tract, then the stack is whatever length remains after the runner you already have.

2.The formula

The teaching form is Ls = (C × 60 × S) / (4 × N × O). C is the speed of sound in the intake air (about 343 m/s at 20 °C). N is target RPM. S is 2 for a four-stroke and 1 for a two-stroke. O is the harmonic order — odd values 1, 3, 5, 7 are the usual intake set. The 4 is the quarter-wave split. Rearranged, N = (C × 60 × S) / (4 × Ls × O). This is the same identity as the Velocity Stack Calculator on this site.

\[ L_s=\frac{C\times 60\times S}{4 N O} \]

3.Pick RPM and order

Choose an engine speed you actually use — peak torque, a cruise, or a power-band ceiling. Then pick an odd order. Order 1 is a long tract and a mid-range bias; O = 5 or 7 is shorter and higher RPM. O = 3 at 6000 rpm is a common street starting point. You will still verify on a dyno; cam timing moves the real peak.

4.Worked example

Take C = 343 m/s, N = 6000 rpm, four-stroke S = 2, and O = 3 — the Velocity Stack Calculator defaults. Then Ls = (343 × 60 × 2) / (4 × 6000 × 3) = 41160 / 72000 = 0.5717 m, which is 571.7 mm or 22.51 in. A two-stroke at the same RPM and order is half that length (285.8 mm) because S = 1. If the runner to the stack base is already 350 mm, the stack itself is about 222 mm.

\[ L_s=\frac{343\times 60\times 2}{4\times 6000\times 3}=0.5717\,\mathrm{m} \]

5.RPM and order playground

Move target RPM and harmonic order to substitute into Ls = (C × 60 × S) / (4 × N × O) at C = 343 m/s and four-stroke S = 2. The sketch is a simple tract with a flared mouth — a teaching widget, not the full calculator. Open the Velocity Stack Calculator when you want end correction, existing runner length, or the other solve modes.

6.Open the full calculator

The Velocity Stack Calculator uses the same formula, the same 6000 rpm / S = 2 / O = 3 example, and How it works plus FAQ on one page. It can also solve for RPM or C, optional end correction kD, and a table of odd harmonics. This is not the Velocity Calculator and it is not a dyno.