The model

The intake-sizing tool gives a geometric estimate of how well each commercial venturi flows for a given engine, by computing the peak gas velocity through it. It is a comparative tool to support carburetor selection, not a validated jetting recommendation. Tune at your own risk.

Gas velocity is the primary signal

For any commercial venturi diameter at any RPM, the peak gas velocity is:

v = Vt × VE × RPM / (10 × N × π × D²)   (m/s)

Where:

• v — peak gas velocity at the venturi throat (m/s)
• Vt — total displacement (cm³)
• VE — peak volumetric efficiency (0.5 to 1.15)
• RPM — engine speed
• N — pulse divisor, a function of cylinders, carburetors, barrels and firing interval (see below)
• D — venturi diameter (mm)

Two boundary effects matter:

• Too low → fuel atomizes poorly, mixture distribution suffers.
• Too high → the venturi itself restricts flow and the engine cannot breathe at the top.

Each commercial line on the chart is colored by where its velocity lands at each RPM.

Target velocity from the application profile (K)

The application profile (stock, sport, competition — one constant K per option) encodes which peak velocity the build is aimed at. Substituting the classic Bernoulli-derived venturi-sizing equation into the velocity equation cancels every engine variable and leaves a clean relationship:

v_target = 100 × P_abs / (π × K²)   (m/s)

Where P_abs = 1 + boost (bar). So K is, in effect, a velocity-target selector:

• Stock motorcycle (K=0.70): ~65 m/s
• Sport motorcycle (K=0.72): ~61 m/s
• Race motorcycle (K=0.75): ~57 m/s
• Stock car (K=0.60): ~88 m/s
• Race car (K=0.70): ~65 m/s

The healthy band on the chart is centered on v_target with a fixed half-width of ±30 m/s. Changing the application profile shifts the band and re-colors the chart.

The pulse divisor

The pulse divisor N converts total engine demand into the peak demand seen by one venturi:

N = max(venturis, cylinders / concurrent)
concurrent = max(1, 240 / (firing_interval × venturis))

The concurrent factor handles pulse overlap when multiple cylinders share a carburetor. If the firing interval per venturi is shorter than the intake duration (~240° of crank rotation), pulses overlap and the effective peak demand rises.

For typical 1-carburetor-per-cylinder setups, N = venturis = carburetors × barrels.

What is NOT calculated

• Flow rate (vazão) as actually measured on a flow bench. The formula uses geometric and breathing assumptions, not discharge coefficients.
• Transient effects (gas inertia in the manifold, intake tract resonance).
• Fuel atomization quality, mixture distribution, or AFR.
• Carburetor body losses outside the venturi (slide cutaway, throat shape).

Known approximations

• Intake duration is assumed to be ~240° of crank rotation. Real cams vary from 200° to 280°.
• Pulse overlap is modeled with a simple linear scaling — overlapping pulses share the carburetor proportionally to their duration overlap. Real engines have more complex pressure waves.
• Firing interval is a manual field defaulting to 180° (even firing for a 4-cylinder). It does not derive from the cylinder count: for other cylinder counts or uneven-firing engines (270° twins, cross-plane V8), set the interval manually.
• VE is taken as the peak value (single slider input). The chart evaluates velocity at this peak; real engines see lower VE outside peak-power RPM.

Sources

The K factor presets are drawn from common carburetion literature: David Vizard, Graham Bell, Dellorto's official tuning guides. Real-world carburetor sizes used to validate the model (Honda CG 125, VW Fusca, Ford Maverick V8, Harley-Davidson Evo 1340, and several others) come from manufacturer service manuals and aftermarket racing references.