I took the air dam off. It was too low, and was scraping, and looked horrible. I'm gonna cut it back to 3" from 4.5" and not extend the pieces in front of the tires. Instead i'm gonna have separate deflectors in front of the tires.
I got my oil change done, and that helped a bit. And i'm still considering the diffuser. If my reading is right, the diffuser is basically supposed to be an ideal nozzle in reverse, slowing down the air, and getting the pressure back up, and keeping the renolds number lower to maintain laminar flow.
.. Some of my fluid dynamics courses are coming back to me!
