I think its 735 cfm - unless '68 was different.
Yep! I got called on this by another member as well. Here is what I know:
I should have said 735 CFM for the C9AF-9510-U. The same basic carb was sold as the 0-3310 and in the day it was rated at 780 CFM although it is now rated at 750 CFM. Oddly enough Mustang Tech has the C9AF-9510-U rated at just 700 CFM, but the same carb without the U suffix is rated at 735. I have a C8OF-9510-AB 68 Cobra Jet carb and it looks exactly the same venturi wise as the C9AF-9510-U. Then in 1970 the same basic carb becomes the D0OF-9510-N and it is rated at 780 CFM. What makes this all so puzzling to me is that the venturi sizes are all identical at 1 3/8" and 1 7/16". I have looked at a vintage 3310 and I can’t really see anything that would change the CFM rating. I have a D0OF and it too looks identical. Maybe I am missing something?
I think that some of the numbers came from marketing. For example, an engine of about 426 to 429 cubic inches, at 6000 RPM with 100% volumetric efficiency will flow 780 CFM. So rating the carb at that would be good from a marketing perspective as it would make the carb “big enough”. On the other hand carbs are supposed to be rated based on how many CFM they flow under a specific level of vacuum (15" of Hg I think). I think this number would be lower.
Another possibility is that the annular boosters might be a factor, but I don’t have any good information on them to say for sure.
Royce, can you speak to any of this?
Bottom line is that the CJ carb runs great on 390 GT, 427 and 428 applications for me.
Great info, Bill - thanks! Looking forward to Royce’s comments if he chimes in.
For the 1970 Ford equipped Holleys, the 428CJ/SCJ and Boss 429 engines were equipped with a 735 cfm. The Boss 302 and 429CJ/SCJ were equipped with a 780 cfm. The difference is in the secondary venturi diameters.
On a 735 CFM (D0ZF - AA, AB, AC AD, and S) they are 1.340.
On a 780 CFM (D0ZF - N, R and Z) they are 1.440.
I would imagine the 68 and 69 4150-C’s are the same.
Dave where are you getting these numbers?
Every bit of Holley information I can find shows the same primary side venturi of 1 3/8’ 1.375, and the secondary side 1 7/16" 1.4375 for all of these carbs.
When I see the drop to 1 1/4" 1.25 primary and 1 5/16" 1.3125 secondary it is on a 600 CFM carb. Nothing at 1.34.
Numbers are from the 1970 Ford Shop Manuals. Found in volume two 23-06-10 and also in the 1970 Car Service Specification (7202C-70) page 23-06-01 picture attached,
What does the book show for the 68 and 69 carbs?
Bill,
I only have 1970 and 1971 versions. Someone with a '68 and a '69 shop manual will have to look them up. I would think they should be in the 5 volume shop manuals, Engine volume 2 group index Fuel system (23 in 1970, 24 in 1971). The Boss 302 may not be in the '69 volume due to its late release (April 1, 1969) and the 429SCJ did not exist until the 1970 model year.
The 1971 shop manual (volume 2 24-23-11) only shows the 780 CFM as found on the “planned Boss 302 (D1ZF-VA)” and the 429SCJ (D1FZ-YA, XA) with Venturi Diameter of Primary 1.38 and secondary of 1.44.
The C9AF-U carb was a service replacement carb. They are plentiful. They are much cheaper than the original C9AF-M or C9AF-N assembly line correct Cobra Jet carbeurators. Many people try to hype up the U carb as original, but it is not correct for assembly line concours application. The U carb works great as discussed in this thread, just don’t think they are rare or spend more than $250 for a core. They made lots of them, we can find one. 
The Chevrolet 396 CI Corvette carburetor 0-3310-1 had straight venturi boosters so it flowed 780 CFM in its original form. The Cobra Jet carburetors have more responsive down leg boosters that impeded air flow a bit yielding 735 CFM. Throttle plate size is not the only variable affecting flow.
These are nominal numbers. A 750 CFM carb on a stock 302 will likely never approach its full flow number. It has been proven that a 0-1850-1 carburetor rated at 600 CFM can flow close to 700 CFM on a healthy 454 Chevy big block. These flow ratings are based on standard industry test procedures, not actual as installed flow.
Thanks Royce.
Royce,
Did the GT-E carb have a dashpot originally?
From what I can tell, no. There is a Holley dash pot that fits but I think it was for a GM application originally.