[cmichaelo]

Thanks for your replies.

If it's stainless, how do I tell the difference whether it 304 and 409?

Btw, I'm doing this mod predominantly because I like welding and because I've had success in the past putting an entire exhaust system together.

Obviously my goal is to improve sound and flow at the same time.

I have not cut the resonator out yet. But I suspect it does in fact restrict flow as follows. If you look closely at the input pipes to the resonator, you will see that the pipes have been crimped, or crushed, on the outer sides. It looks like a 3" x 1.5" long 1/2" recessed indentation. My guess is that this was done to cause turbulence of the gases in the resonator to make the resonator more effective. Turbulence, however, is not good for flow.

By removing this turbulence, the flow should improve. Secondly, by removing the resonator, the exhaust will become a little louder and probably, flow is improved a tiny bit more because it looks, from the outside, that the bend inside the resonator is tight.

Aftermarket crossover pipes are 2 1/4" all the way to the muffler. This, I believe, is too big for optimal flow at this portion of the exhaust. I think the optimal design is somewhere in the middle between the stock and the aftermarket crossover pipes. I feel that 2" piping, after the X-pipe section, is optimal for HP up to 400HP and that is effectively what the stock crossover pipe size is immediately after the X-pipe section.

See, gases are not that hot after the X-pipe section and therefore can benefit from a smaller pipe size to keep them moving fast and thus slightly aid the scavenging effect. After all, the secondary cat section is also made of 2 1/4" pipes, and here gases are much hotter. Thus, pipe size further down should shrink to make for a wholesome integrated exhaust system, and which BMW actually did. Even their mufflers are made of 2" piping. They just don't flow that well.

I also plan to wrap the entire crossover pipe in thermal wrap to keep the gases as hot and fast as possible.

-M