the light bulb just came on.............
silly me ...lol
Yeah, it's cubic feet of air. Also the ratings can be a little deceptive because they're taken at some standard 'vacuum' state - 1.5 inches of mercury for 4bbl carbs (meaning that the "engine" is pulling on the carb hard enough to create some pretty good vacuum, so therefore by definition it's not able to get 100% of the air it wants.) That's why, up to a point, an engine's performance will improve with a carb whose cfm "rating" is larger than what the motor theoretically needs. For optimum performance you don't want the engine to have to "suck" at 1.5 inches of mercury to get air. In fact, the closer to 'zero', the better. (It can't ever be actually zero because then there'd be no air movement at all
Sooooo... if we go back to that theoretical 400 that needed 850+ cfm at 7360 rpm, at 100% efficiency --- let dial that back to say, 85% efficiency --- that puts us at around 720 cfm needed. To get 720 actual cfm through a carb without forcing the engine to pull 1.5 inches of mercury to get it might require a carb "rated" at 900 cfm or so.
So, why not just put the biggest honkin' carb on it you can and be done? Because for all the circuits to work properly a carb needs to "see" the air moving through it at certain minimum velocities for each circuit, so on a motor that's not "big" enough to move that much air it's just not going to work right.
Everything must be in balance, Grasshopper --- not too big, not too small