Originally Posted by PencilGeek
You sure about this? I'm pretty sure there's only one catalytic converter on our cars, followed by a silenser, followed by a resonator. We looked very closely at all of the exhaust pieces that came off of my car, and there was only one set of catalytic converters. I'm open to the idea that I'm wrong here because we didn't actually cut all of the pieces apart and take a look inside. But based on weight alone (catalytic converters are heavy), there was only one section that appeared to be catalytic converters.
The first set is a pre-cat of sort. If you look under the car, where your cats were removed were the main cats which are further down stream.
Starting with the E46 M3, in the US market, BMW incorporated a pre-cat or primary cat, into the header. The reason for it, is it heats the fastest due to its location. It also saps power and the Euro E46 M3 did not have it due to different emissions regulations. This was part of the reason of the different HP ratings between the euro M3 and the US M3 (also due to different standards.)
I had the car on the lift the other day and was checking it out. Right where the header finishes there is a small bend and then you have the first set. This is the cat I would want to remove while leaving the "main" or secondary cat in.
Most X-pipes replace the secondary and leave the cat off the header in place.
Turner has realized the cheapest and best upgrade is removing this first cat and tuning for it. They did this for the M5/M6, and now the M3.