i'm driving a Downing/Atlanta s/c'd Z3 1.9. the OBC used to say about 26-29 mpg before the install, now it says 23-26 ish. by actual math the OBC was a little high before, so i assume it still is? most of the time the engine is operating in a vacuum condition due to the Downing/Atlanta systems' bypass. it allows economical non boost operation at light throttle, but the boost is right there at the tips of your toes. yes, big difference! i can pass without downshifting and waiting for the rubber band to wind up. it just starts pulling like you would think it should. i just installed a vacuum/boost gauge on the windshield frame pillar. i was kinda surprised at how much of the time it's in vacuum, but like i said, the boost is right there when you want it. the gauge says almost 9 psi when i'm into it hard, then back to 20" of vacuum to cruise. about the only con is that i won't be filling up at the goat piss pump anymore. premium is recommended from here on. you can check out the unit that Downing/Atlanta uses at Eaton's website, i think it's the 62 size.