44 IDF Weber main venturi
Genuine Weber calibrated part
The main venturi is the first part of the selection process when determining the appropriate jetting for your carburettor/engine combination. The smaller the venturi, the higher the air speed entering the engine. This equals more low end torque and response. Larger venturi will flow more volume and have the potential to create more top end power, however this is often at the expense and low-mid range driveability. As you can see, the venturi size needs to be determined for your application. ...and don't forget, a change in venturi size will most definitely mean a change in main jet, air corrector or idle jet.
As a very basic guide;
Cars used soley on the street where the useable rpm range is much lower will benefit from smaller venturi.
Dual purpose (street/club day) cars can go up a size or two
Race cars can go as big as the engine will support as the rpm is unlikely to drop below 3500-4000 on the track
There is lots of information out there to help you determine the correct venturi size for your engine. If you are having trouble, give us a call and we are more than happy to help.