Per 2002 Dinan Catalog the cold air intake system added 13 bhp at 5,700 rpms and 12 lb/ft. of torque at 5,700 rpms. It doesn't go away so you either are accustomed to the gain or something else is happening. There is no adaption that will compromise the tune though the ECU is always adapting the transmission to driving style. You could be driving less aggressively than when you first installed the tune.

Dinan tunes are historically on the conservative side though they are designed to match to Dinan Mods. I suggest adding the Dinan transmission software rather than Dinan Stage 2 tune since you already have a tune. The transmission software makes a big difference and is well worth the cost. It changes the shift points so you benefit more from what you got out of the cold air addition, it speeds up the shifts and makes them firmer. All benefits are especially noticeably in either manual mode. If you are in the full manual mode where you have to shift the transmission it will stay in the gear you select until you shift.

More air means more air out. There will be benefit from lower back pressure mufflers and a better sound comes with them.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas