Physics · 5 min read
How to Size Velocity Stacks for ITBs Without Treating Dyno Results as Optional
Same quarter-wave budget per throttle — the dyno is the measurement
Individual throttle bodies share a formula with a single carburetor trumpet, not a permission to skip the rollers. This tutorial uses the quarter-wave length as a per-ITB starting budget, calls out plenums and airboxes that break the open-end story, and treats a dyno pull as the result, not an optional extra. The Velocity Stack Calculator does not replace that pull.
Written by the My Calculator Stack editorial team. About our methods
This is a length budget per open tract, not a model of ITB flow, cam overlap, or a shared airbox. Dyno results are the check. The formula is the estimate.
1.What is the same on an ITB bank
Each throttle still has a valve (or a throttle plate close to the valve) and an open mouth. The teaching length is still valve to that mouth. You measure one cylinder’s centerline, pick the RPM you actually use in that gear, pick an odd order that fits the space between the cam cover and the hood, and you get Ls. Four identical stacks on a four-cylinder do not get four different formulas. They get four copies of one budget — unless the outer runners are longer, in which case you measure those too instead of assuming symmetry.
2.What the formula does not know about ITBs
A shared airbox, a ram-air duct, a filter lid, or a plenum volume adds reflections the quarter-wave tube ignores. Unequal runner lengths, secondary butterflies, and velocity-stack screens change discharge and the apparent mouth. Cam duration and exhaust scavenging move the real torque peak away from N in Ls = (C × 60 × S) / (4 × N × O). None of that appears on the Velocity Stack Calculator. The page will still print 571.7 mm at the defaults. That number is the start of a test plan, not a substitute for the test.
\[ L_s=\frac{C\times 60\times S}{4 N O} \]
3.A starting budget, then hardware
Example: four-stroke motorcycle ITBs, peak-torque target 6000 rpm, O = 3, C = 343 m/s → L_eff = 571.7 mm. If each head-plus-throttle centerline is 380 mm to the stack flange, the trumpet budget is about 192 mm before any kD. That is a parts starting point: a stack you can bolt on without hitting the tank. It is not a claim that 192 mm will peak at 6000 rpm on that cam. If the hood only allows 120 mm of stack, you do not “make it work” by pretending O is still 3 — you either raise target RPM, raise O, shorten the runner, or accept a different band.
\[ 571.7-380=191.7\,\mathrm{mm} \]
4.The dyno is the measurement
Change one length at a time. Pull the same gear, same weather note, same fueling strategy. A longer stack that moves peak torque down 400 rpm and a shorter one that moves it up are the experiment the formula cannot run. If the airbox is on for the street and off on the dyno, you did not test the same reflection. Treating the printed Ls as optional and the dyno as optional is how a bank of pretty trumpets misses the band you ride in. The estimate is cheap. The pull is the answer.
5.Use the calculator as the budget sheet
Type RPM, S, and O on the Velocity Stack Calculator, enter the measured existing length per runner, and read the stack remainder. Use the harmonic table when the hood caps how long the trumpet can be. Then go to the dyno with that hardware — do not stop at the web page. A quarter-wave identity is a starting estimate. On ITBs, the rollers are not a nice-to-have.
Try it yourself
Open the related calculator and put these formulas to work.