I posted earlier about a double filter on my US M coupe. There is an Orange filter and a grey filter. I have come to the conclusion that the grey filter is a charcoal filter and is only on US models. I decided to do a test. I got my friends G-tech and we did several runs and consistantly got 188 hp going one direction (210hp the other way, but not consistant) where we were doing the test. Then we pulled off the road, took out the charcoal filter, and did more runs in the same direction and peaked @ 189hp. TO THE COMPUTER BATMAN. so we downloaded the info and found that the peak gain was only 1hp, but over the mid range the gain was max 5 hp. So, if you want the extra 5 hp mid range, remove the grey filter from the airbox.