cfm of a weber
Old [Sch|F]ool
Joined: May 2001
Posts: 12,856
Likes: 568
From: Cleveland, Ohio, USA
The CFM is variable depending on the choke (venturi) size. Which, because Webers are wonderfully tunable carbs, is one of the things you can change.
Measuring the CFM wouldn't be of much value, since the carbs are 99% of the time used in individual runner configuration. Since each rotor/cylinder has its own carb barrel, and no other rotors/cylinders share it, the flow through the carb is not constant - it only flows about half the time, maybe more, maybe less, depending on the engine.
For example, a Ford 302 that might require a 650cfm Holley carb (plenum manifold so carb sees constant flow) mighr require 1450cfm worth of Weber carbs (individual runner, NOT constant flow).
That's why Webers and other carbs designed for individual runner use are not rated in CFM, you're just given a throttle size and then a venturi size. The sizing rule of thumb is that the throttle blade diameter should approximate the area of the intake port and the venturi should be 4/5 that size (so, a common carb is a 32mm venturi in a 40DCOE, or a 38mm venturi in a 48IDA) although the venturi size is one way you can tune your powerband - bigger venturi, less restriction at the high end but also less signal and poorer performance at the low end.
Measuring the CFM wouldn't be of much value, since the carbs are 99% of the time used in individual runner configuration. Since each rotor/cylinder has its own carb barrel, and no other rotors/cylinders share it, the flow through the carb is not constant - it only flows about half the time, maybe more, maybe less, depending on the engine.
For example, a Ford 302 that might require a 650cfm Holley carb (plenum manifold so carb sees constant flow) mighr require 1450cfm worth of Weber carbs (individual runner, NOT constant flow).
That's why Webers and other carbs designed for individual runner use are not rated in CFM, you're just given a throttle size and then a venturi size. The sizing rule of thumb is that the throttle blade diameter should approximate the area of the intake port and the venturi should be 4/5 that size (so, a common carb is a 32mm venturi in a 40DCOE, or a 38mm venturi in a 48IDA) although the venturi size is one way you can tune your powerband - bigger venturi, less restriction at the high end but also less signal and poorer performance at the low end.
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