It SHOULD be understood that ultimate CFM on an engine has many side issues, as to the carb sitting on top of the intake manifold.
The secondaries of the Holley 4bbls are "self-sized" by the spring in the secondary diaphram which opens the secondary throttle plates. A carb which CAN do 700cfm will not open all of the way on an engine that does not need that much air flow . . . according to the old Holley (when it was a part of Colt Industries) information. Can the springs in the diaphram be changed? Certainly, as to rate of opening and how much opening is allowed.
The secondaries of the AVS and TQuad carbs can be manipulated, too. The spring tension on the secondary air valve can be tightened and loosened as needed, from the basic "stock" tension. Too much and a bog happens when they open. Too little and little opening happens at WOT. THIS is why a 750cfm QuadraJet or 800cfm TQuad can work on 250cid motors, especially with their small 1.38" primary throttle bores (and smaller venturi clusters),
AFBs usually had a counter-weighted "air valve" above the secondaries, too. UNLESS it is like the one which was OEM on my '67 Newport which had NO such thing on the secondaries. Which means it is a full manual secondary-operation AFB! After I gently dressed the outside of the venturi cluster to remove casting flash, WOT from idle would slightly bog the 383 whereas it did not before I did that. I believe that OEM AFB was about 575cfm?
In an old Rochester carb book (by S-A Designs), it was noted that as the emissions era progressed, the tab on the QJet secondary air valve opening linkage was raised to limit how much that air valve opened, which reduced the ultimate total CFM to about 650cfm.
When I got into the "Holley upgrade mode" on my '77 Camaro 305, the first one was an OEM-spec 4160 direct factory replacement for a '76 Impala 350. I could not feel any secondary opening at WOT, which probably meant it was not opening all of the way. When I upgraded that to a 650cfm 4175 spreadbore, I had to put a weaker spring in the secondary diaphram in order to just hear the secondaries opening at WOT and 2500+rpm in low gear. About two notches weaker than the spring it came with (OEM Factory replacement for a 1979 L82 Corvette 350).
SO, rather than talk about ultimate CFM ratings, look at the primary throttle bore diameters (throttle bore and venturi diameter dimensions). A 318 usually came with something about 1.44" 2bbl throttle bores, so something in that area would work well in an aftermarket carb, NO MATTER what the secondary sizings might be! Used to be that an approx 600cfm carb would have 1.56" throttle bores on all four bores, no matter the maker. As the larger 1.69" throttle bores means an approx 780cfm 4bbl carb (as the Holley 3310 of the later 1960s).
In more recent times, it seems that Holley is using the 1.69" throttle bores on all cfms of 4bbls, using the venturi sizes to modulate the air flow capabilities? Unlike in earlier times when venturi diameters and throttle bore diameters were used together. Such a design orientation could allow for decreased part numbers of "main bodies" for a multitude of carb flow capacities, saving money for the "New Holley" organization.
Enjoy!
CBODY67
