Im not sure how to start explaining this; so here goes nothing...
On my 94 325iS; and every other 325 and 328 since 92-98;
the exhasut comes from the two split manifold and down into twin 2" intermediat pipe. The pipe then goes from the twin 2" setup and gets funneled into a signle 2" pipe for about 12" to where there is a single oxygen sensor and the it splits back off into it's previous 2x2" design and then continues into the twin cats and of to the dual in dual out muffler.
Now.....i work at an exhaust shop and i know a thing or two about flow and they way exhaust systems function. I'm thinking that if there wasn't that section that squeezes the two pipes into one you would have better flow and therefore gain somewhat in the horsepower department. I have done before and can make up a section that would replace the single setup with twin pipes with a section contecting the two in order to equalize backpressure and utilize the single oxygen sensor. Also known as an "H" pipe.
The only thing thats stoping me from doing this is that fact that BMW does know how to build a car and why would they put this single section in what would be a "true" dual exhaust set-up. There must be a reason for it; whether it be to use only one Oxygen sensor.....to equalize backpressures between the first 3 and last 3 cylinders.....or in order to compress the exhaust gases into a smaller pipe therefore making the concentration higher and allowing the Oxygen sensor to get a clear and proper reading.
The first two things don't cocern me that much cause the "H" pipe design still does those two things.....BUT the last on is what could mean the difference between the car runing better or worse than before....
SOO....what I'd like to know is; has anybody had something like this done with their exhasut system and if so how did it turn out.
I'm not overly concerned because the worst thing that could happend is the car won't run as well as before and I can just weld the original piece back into place