I installed a K&N filter along with a 4" exhaust with straight through muffler, new bellowed up pipes and deleted the back pressure valve. Didn't change anything else. My fuel economy dropped by about 2 MPG. Anybody have any ideas? Did I do something I shouldn't have? It seems that these mods should improve economy and performance but neither is better. The only thing I've noticed is a different exhaust tone and I can hear the turbo whistling a bit more.