What I like about AFBs is the secondary air door. It's sorta like an "on-demand" limiter. When you're running on the primaries, you only have a maximum air flow of half (or a little less) than the rated CFM capacity. Then when WOT is reached and the secondaries kick in, the air door limits the amount of air to that which is demanded by the motor.
It's a little different from a mechanical secondary setup on say, a Holly double pumper. There the secondary butterflies open and you got the full capacity of the carb wide open. But the AFB (and the Quadrajet) limit the amount of airflow, so that it doesn't overwhelm the motor. That's why some factory Q-jet motors (L69 305 for example) have a carb that's rated at a maximum cfm of 695...which would normally be WAY too much for a 305. But by limiting the secondaries, that carb works fine (well, as fine as a q-jet can) with a small motor.
I'll letcha know how the 625 Carter works out on my 292. We're gettin closer and closer. I just got the headers painted with high-temp stuff and as soon as I can get my El Camino functional, I'll dig into the truck.
