Not exactly sure. I'll take some exact readings on my way to the muffler shop next wkend and post on both rpm's and what the exhaust did for me.
Off topic but I love my IROK 37's. I was able to navigate 1 ft of snow at 30psi with ease, so no more airing down and back up as I traverse the snow line on my way to work. The only problem was that I needed goggles to run with the window down