Honestly, I don't know that there is a reason to spend more than $150 for the headers and then about that for new, good quality cats. I arrived at that conclusion by reading a ton of e46fanatics.

There is not a ton of power left on the table on the exhaust side on these motors where X5s can use it - the M54 and S54 motors were designed to make torque up higher in the RPM band than A)is useable by a 5500lb truck, or B) it turns out is feasible for the M54 because the harmonics over 6k rpm are bad and the oil pump nut might spin off, so you don't want to be up there anyway.

Go look at dyno charts of M54 to confirm that.

The major point I see on the X5 exhaust manifold is that the bends right off the head are tight and the cat is mounted way up high to get it to light off faster so it could make the higher tier of emissions (PLEV...some stupid acronym).

So I don't know that other than having better bends and having hi-flo cats there is much to do on an M54. I have a set of ebay headers I am about 25% of the way through getting on. There are about 4 different kinds of ebay headers that fit, but they are all more or less the same 2 designs. And then there are the 1500 ones.

EDIT: You have a Z3 - I actually looked at using factory Z3 manifolds, which are more like a header, but IIRC there was some emission thing they had/didn't have that I did not want to bother with. The high level Z3 and 330 OEM steel header type manifolds look pretty good. Could be an improvement over X5 OEM.

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2001 E53 3.0 5pd