The only things that comes to mind with 174 and 175 as regarding Zenith:
174 - (nothing)
175 - type 175CD (sidedraft English Zenith) carbs
As far as the flange sizes are concerned:
The S.A.E. flange sizes in question are the size 2 (2 11/16 inches center to center) and size 3 (2 15/16 inches center to center). Size 2 being used by Chevrolet on 216's, size 3 on 235's and 261's.
In Zenith terminology, size 2 is a 10, size 3 is a 12.
So think of the 11 as a 10 that wants to be a 12!!!
More to the point, throttle body bore size:
10 (size 2 with 1 7/16 inch throttle bore)
11 (size 2 with 1 9/16 inch throttle bore)
12 (size 3 with 1 11/16 inch throttle bore)
This just explains PHYSICAL dimensions. For air flow, one must consider the internal venturi size which Zenith expresses in millimeters.
For a given venturi size, a Zenith 12 will flow MARGINALLY more than a Zenith 11.
It should now be as clear as mud on a foggy night!
Jon
