Back on my quest to build my own header, because that's about the only choice we z5 owners have. And the crack in my manifold may have something to do with it...
So I had previously purchased these:

I plan on port matching the primaries to the factory exhaust manifold gasket(and then the head to the same gasket).

So my trip to the junkyard today discovered the oft elusive and pesky pre cat:

This little bastard almost got away...the junkyard told me they won't sell catalytic convertor related equipment. I asked them to wait a minute, went around the corner and hammered out the internals. It's just fat downpipe now. The guy found that humorous and sold it to me for $15.


I was originally going to make my own pipe to replace the precat and just wanted it for external dimensions and fitment. But now, I'm thinking this is the sneakiest way to get what I want without tipping off the smog folks...


I'll use a 2.5" pipe insert flared at one end and welded in at the bottom...or even a velocity stack might work. I'll pack the surrounding empty space with exhaust material(steel wool or fiberglass). I'll put some small holes in the pipe so that any exhaust gas passing through the sound deadening material can still pass into the exhaust piping.

And I'm going to order one of the secondary o2 sensors replacement resistors...we'll see how this works. Hopefully this all be done next weekend.

 

So...I'm finished. If I get another cracked manifold, so help me, I'm selling this car.
This is the second time I've replaced the exhaust manifold.
The results of my modified header? I started the car up, no MIL, pull out and drive real easy to warm up. No obvious drawbacks yet.

Get out on the main street and wail on it as it goes into second.
2k...nothing different
3k...nothing
4k...nothing, oh, wait is that,
5k...hey that is a mild improvement
6k..things feel...almost peppy

I shift into 3rd...

same results again...upper rpm improvement.

I'm digging it. I don't know how it would compare to a real header.
If anything, this whole thing was worth it for the MIL eliminator for that second pesky, expensive, stupid O2 sensor.

We don't have smog testing here. Yet. It's starting mid to late 2005.
When we do have to start smogging, I don't anticipate the absence of precat material to cause any problems.
1. It's an invisible mod...looks stock as it did last week. Nothing to tip off the smog tech.
2. The precat being so close to the primaries is designed to get hot quick. So the precat lights off and starts cleaning sooner after startup. I'll bet this was done to satisfy EPA emissions qualifications. But by the time I drive down to the emissions station, the main cat is nice and hot and hopefully doing it's job.

 

Don't portmatch the exhaust to the gasket, just leave the port a little bit smaller. Helps to keep reversion down. Something you don't want.