Throttle Body and Main Venturi Sizing

When deciding upon an appropriate throttle bore diameter for your carburetors it is easy to err by over-sizing them in the belief that a larger bore size will deliver more power.  It is true that larger throttle bores combined with larger venturis are needed to produce higher power outputs but only when the engine is designed to need these larger diameters.  The real answer is that main venturis are sized to provide the air flow required by your engine and that the throttle bores are sized to compliment the main venturi size.  Do not be tempted to select larger carburetors in the attempt to develop more power without engine modifications to support the larger carburetor sizing.

The relationship between main venturi diameter and throttle bore diameter is a balanced relationship that is optimized when kept within the guidelines recommended by the classical formulas provided below.  To simplify the selection of main venturi size a chart is provided which was developed from the classical formulas and then adjusted to fit known dyno data.  The chart shows the relationships of engine displacement, RPM at which peak horsepower is generated and the main venturi size associated for these two parameters.  Once the main venturi diameter has been selected then an optimum throttle bore diameter may be determined.

For most applications it is better to select a smaller main venturi diameter than what will provide maximum horsepower output.  This is true for road cars as well as for race cars.  It is therefore most important to be realistic with your intended application so an appropriate carburetor setup is achieved.  A large venturi and throttle bore will provide high RPM power (assuming the engine components are up to the task: cams, compression, ignition, exhaust, inlet port diameter, intake valve diameter,  etc.) but will do so at the expense of lower RPM tractability.  Smaller venturis for a given application will shift the peak horsepower so it will be developed at a lower RPM with the benefit of more useable power throughout the RPM band which also says you will have more TOTAL available power than a high RPM engine of similar displacement would produce.

Another benefit from downsizing your venturi/throttle bore size is the smaller venturi (when compared to a larger venturi selection) will generate greater vacuum for a sharper signal to initiate fuel delivery from the main circuit.  A higher vacuum results in sharper transition onto the main circuit and better atomization of the fuel resulting in decreased fuel consumption.

SUMMARY: Be realistic with your carburetor setup; although it is fun to have an engine that really pulls at high RPM it may also be demanding to the point that it is not as much fun to drive as one with a broader power band.

Classical venturi and throttle bore sizing formulas:

These formulas are based upon one throttle bore per cylinder:

  • Throttle bore diameter: D = (.8 to .9) x (V x n)^.5
  • Main venturi diameter: d = (.7 to .9) x D
  • Where:
    • D = Throttle bore diameter, mm
    • d = Main venturi diameter, mm
    • V = cylinder displacement, liters
    • n = engine speed, RPM

If you are designing your engine and want to select inlet port diameter and upstream carburetor components to support the inlet valve selection then the following information may be useful.  Typically the maximum airflow past an inlet valve may be taken as 80% of the valve diameter for normal camshafts and as much as 85% when high-lift, race camshafts are selected.  This means that an inlet port diameter in the cylinder head should not be larger than 80% to 85% of the intake valve diameter.  Once the inlet port diameter has been sized then the throttle bore size for the carburetor may be selected to provide the same flow area as that of the inlet port.  Of course the throttle bore diameter of the carburetor will be a bit larger in diameter than that of the inlet port due to the reduction in area due to the obstruction provided by the throttle shaft.  Also, a decrease in diameters from the intake air horn to  the  intake valve is good design as the velocity of the air/fuel mixture is always increasing.

After the throttle bore size has been determined, the main venturi size may be selected.  This approach is offered as food for thought and not as a definitive design tool.  Better to let your engine specialist provide final recommendations and not rely upon cookbook solutions.  However, it is illuminating to compare some popular Porsche engine configurations with the selection method presented such as:

1967 911S 2.0

  • OEM Inlet valve size = 42mm
  • OEM Inlet port diameter = 36mm
    • Calculated diameter: 42 x .85 = 35.7mm
  • OEM Weber throttle bore diameter = 40mm
    • Calculated throttle bore diameter (with adjustment for shaft blockage): 39.1mm
  • OEM Main venturi diameter = 32mm
    • Calculated venturi diameter (using .8 coefficient): 31.3mm

1966 906 2.0

  • OEM Inlet valve size = 45mm
  • OEM Inlet port diameter = 38mm
    • Calculated diameter: 45 x .85 = 38.3mm
  • OEM Weber throttle bore diameter = 46mm
    • Calculated throttle bore diameter (with adjustment for shaft blockage): 43.7mm
  • OEM Main venturi diameter = 42mm
    • Calculated venturi diameter (using .9 coefficient): 39.3mm

The following main venturi selection chart was generated using classical venturi sizing formulas and is based on a sizing coefficient of .80 for the venturi diameters.  Additional tailoring of the formulas was performed to help fit the data to agree with actual OEM production and race engine configurations.

Main venturi selection chart based upon cylinder displacement and peak horsepower RPM.

Selection criteria for throttle bore diameters

Although a 40mm bore throttle body may be used adequately for engine sizes from 2.0 liters through 3.0 liters and larger it is preferable to match the throttle body bore to the main venturi size that is selected based upon two criteria:

  • Cylinder displacement
  • RPM at peak horsepower

After the main venturi size has been identified the range of appropriate throttle bores are calculated using the Classical Sizing formula provided earlier:

D = d/(.7 to .9)

The formula suggests a main venturi size is acceptable for a range of .7 to .9 of the throttle bore.  This implies main venturi diameters from 28mm through 36mm are recommended for 40mm throttle bores with the mid-point main venturi size of 32mm.  The following data was generated using a coefficient of .8 for calculating optimum throttle bore sizes for a given venturi size and the range of throttle bores is recommended to be extended by 2mm either side of the optimum.

Throttle bore diameter recommendations for venturi diameter selections:

  • 30mm venturi: 37.5mm +/- 2mm
  • 32mm venturi: 40.0mm +/- 2mm
  • 34mm venturi: 42.5mm +/- 2mm
  • 36mm venturi: 45.0mm +/- 2mm
  • 38mm venturi: 47.5mm +/- 2mm
  • 40mm venturi: 50.0mm +/- 2mm
  • 42mm venturi: 52.5mm +/- 2mm

Sizing Example:

2.25 liter with "S" cams and 46mm intake valves:

  • Inlet port diameter: Dia = 46 x .8 = 37.8mm
    • Select 37mm as slightly conservative
  • Throttle bore diameter: Calculated Dia = 42.2mm (37mm is equivalent flow area once throttle shaft blockage is subtracted from the 42.2mm throttle bore diameter)
    • Select 42mm throttle bore
  • Venturi diameter: Based on optimum venturi size from above
    • Select 34mm venturi
  • RPM for peak horsepower for 2.25 liter engine with 34mm main venturis (from chart using 2250/6 = 375 for cylinder displacement): 6500 RPM
    • matches performance of 911S camshaft design