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

Standard venturi selections based upon engine displacement and cam selection based upon sizing calculations

• 2.0 liter displacement:

  • 30mm for Solex cams; peak HP @ 6200 RPM

  • 32mm for S cams; peak HP @ 6600 RPM

  • 34mm for GE60 cams; peak HP @ 7000 RPM

  • 36mm for GE80 cams; peak HP @ 7800 RPM

• 2.2 liter displacement:

  • 32mm for Solex cams; peak HP @ 6200 RPM

  • 34mm for S cams; peak HP @ 6600 RPM

  • 36mm for GE60 cams; peak HP @ 7000 RPM

  • 38mm for GE80 cams; peak HP @ 7800 RPM

• 2.4 liter displacement:

  • 33mm for Solex cams; peak HP @ 6200 RPM

  • 35mm for S cams; peak HP @ 6600 RPM

  • 37mm for GE60 cams; peak HP @ 7000 RPM

  • 40mm for GE80 cams; peak HP @ 7800 RPM

• 2.6 liter displacement:

  • 35mm for Solex cams; peak HP @ 6200 RPM

  • 37mm for S cams; peak HP @ 6600 RPM

  • 39mm for GE60 cams; peak HP @ 7000 RPM

  • 41mm for GE80 cams; peak HP @ 7800 RPM

• 2.8 liter displacement:

  • 36mm for Solex cams; peak HP @ 6200 RPM

  • 38mm for S cams; peak HP @ 6600 RPM

  • 40mm for GE60 cams; peak HP @ 7000 RPM

  • 43mm for GE80 cams; peak HP @ 7800 RPM

• 3.0 liter displacement:

  • 37mm for Solex cams; peak HP @ 6200 RPM

  • 40mm for S cams; peak HP @ 6600 RPM

  • 42mm for GE60 cams; peak HP @ 7000 RPM

  • 45mm for GE80 cams; peak HP @ 7800 RPM

• 3.2 liter displacement:

  • 38mm for Solex cams; peak HP @ 6200 RPM

  • 41mm for S cams; peak HP @ 6600 RPM

  • 43mm for GE60 cams; peak HP @ 7000 RPM

  • 46mm for GE80 cams; peak HP @ 7800 RPM

Guide to OEM Weber throttle body sizes Vs. Venturi diameters

The Weber throttle bodies are available in two bore diameters: 40mm and 46mm. The range of recommended venturi sizes for each of these bodies is:

• For 40mm bores:

  • 27mm for stock, 2.0 liter 911T engines of 1969 and 914/6 engines

  • 30mm through 36mm for 2.0 through 3.0 liter engines depending upon engine performance specifics

  • Larger venturis than 36mm in 40mm throttle bores are not recommended due to the 40mm bore with blockage from the throttle shaft is equivalent to the flow potential through a 36mm venturi.

• For 46mm bores:

  • 38mm is the best size for most all applications

  • 40mm is used for 3.2 or larger engines with big cams

  • 42mm was the venturi size for all Porsche race engines from 2.0 liter through the 2.25 liter 911ST. They have an almost non-existent venturi action and as such do not provide much drive-ability in normal driving situations

• Venturi selections larger than 42mm require throttle bores larger than 46mm. Performance Oriented recommends 50mm PMO throttle bodies be selected for 40mm and larger venturis to help provide a sufficient venturi action for normal driving situations.

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.