Great info Mr. E!
Some of the terms in your post certainly bring back the memories (a.k.a. nightmares) of the predictability, or perhaps unpredictability of fluid flow. I was indeed curious as to what could be considered as the "effective" diameter with the louvers in place. Something else that I was just thinking about is how exhaust flow is actually a pulsating flow instead of a steady, constant velocity flow as is typically dealt with, and then the pulse frequency changes with RPM, which only further complicates things for exhaust designers.
