Be careful just blindly installing an x-pipe. I removed my center resonator and had a reputable local shop install a Magnaflow x-pipe with small Magnaflow resonators behind it to get a bit better flow. It was a disaster. I lost so much low end torque that I couldn't tow my empty trailer in 6th gear, let alone the loaded trailer like I could with the stock resonator.

Yanked it out, put straight pipes in and had the shop fab a basic crossover pipe between the straight pipes out of 2" pipe. What a difference! torque was great down low, pulled the trailer with ease, and a nice sound running just the stock rear mufflers.

Here's what I've discovered since then after chatting with a few folks who understand the American V8 hotrod market. The x-pipe is very sensitive to where it's placed in the exhaust stream. And it should be an equal distance from each exhaust manifold. On our X5's the distance is far from equal with the left side having to travel a lot further than the right, so the exhaust pulses going through the x can fight each other for space rather than "follow" each other through. A simple corssover (should be about 75% the size of the exhaust tubing) is much less sensitive to those distances.

I know others here have had luck with the Magnaflow x-pipe, but in my case I lost a good 50hp and a bunchy of torque. OK, it flowed really well at 5000 rpm and up, but that's not where any of us drive, we need the 2000-3000 torque on a daily basis and that's where my engine fell flat on it's face. And not being able to even maintain 65mph on flat highway in 6th gear with the empty 1100 lb trailer behind me was the killer since I can pull the loaded 4500 lb trailer on flats in 6th just fine.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R