Originally Posted by e39_touring

I highly recommend doing headers even if you integrate cats as the stock manifold/cat combo is VERY restrictive. Maybe some of the aftermarket manifolds like pacesetter or magnaflow are better. I'm easily getting 10% better MPG's now and quite a bit more power in the mid to upper RPMS.

If I were to integrate cats, I'd use the same headers I got as they fit well and seem very well built. I'd cut the center resonator out and merge the pair of pipes into something like a single 3" cat and then Y it back out to the pair of pipes that goes on to attach to the rear mufflers. It's a straight shot, and you could probably even do it without welding if you don't mind a few extra clamps down there.

But, to answer your question on removing the manifolds, it's not that hard but is tedious. The sway bar and reinforcement plate need to be removed. My car doesn't appear to have ever seen salt, and all nuts/bolts were pretty clean so YMMV.

I don't know if it's required, but I did remove all O2 sensors first. Getting at all the nuts on the head was the tedious part. Some came off in a minute or two while others took 1/2 hour each. Have lots of extensions, wobble extensions, universals, short and deep sockets, etc., handy. They're not that tight and 1/4 ratchet worked for me on most of them even if I had to break it with a box end wrench. I'd say 1/4 to 1/3 of the studs came out with the nut. It's no big deal once the stud's out, just spin two regular nuts (M7) on and back off the bronze lock nut. Then do the same to spin the stud back into the head. I did about a third of them from above and the rest from below. The front manifold will come out first and then the rear. Both have to go out the bottom.

That's all I can think of - good luck!