In terms of numbers to which I clearly don't have much experience with, how much cfm(?) will a flowmaster muffler output? I was told by an exhaust dude that my 4.0 sohc will take in @ 500 but the flowmaster only puts out 250-260. Is this salesman talk or do different mufflers have different outputs to consider? By the way he recommended the Mac catback but gave no numbers on that setup. Thanks in advance.