Panic you summed it all up in one sentence: "Adding CFM to a truck will only help at engine speeds you don't want to use." If one has a stock 235 engine in a stock AD truck, that is driven at speeds and manner that it was originally designed, the Carter YF (235 replacement model) is best.
Now, to satisfy the curious (like me) and the thrill seekers (who wish to go faster than stock): Again assuming a bone stock 235 engine. The 390 has a slightly smaller primary venturi than the smallest Carter AFB (9400S). The bores are the same size. Other than the venturi, what advantage does the 390 have? Are the secondaries even used?
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
