[tetsuo111]

1) The archives are full of passionate opinions about exhaust design. There are also enough dynos posted to draw some informed conclusions. In any case, exhaust design triggers a lot of heated discussions.

- consider what you want to accomplish: lighter weight (the OEM system is very heavy) / louder sound / dual or quad tips / optimized system for HP or TQ? Is the car going to be used mostly on road or on track? Different goals need different designs.

- my interest is optimizing for power. The Cliff Notes for this is to understand a 5-stroke ICE cycle. The 5th stroke is scavenging burnt gasses from the piston cylinder by improving the velocity / literally suction of the exhaust gasses out of the cylinder to maximize the volume of air/fuel on the intake stroke. the design can only be optimized for specific operating points at specific engine speeds / RPMs.

- Optimizing for power usually also means LOUD. LOUD usually upsets neighbors and spouses.

- stepping up the tube size post-headers is simply the result of using SuperSprint section 1 test pipes. I forget the numbers off the top of my head, but I think the SS pipes are 1/8" larger than stock. Too large and velocity drops, even if the air mass is similar.

- merge position has a significant impact on the performance. in general, placing the merge further forward, closer to the headers optimizes scavenging at higher RPMs. this may be more desirable for track applications, while trading some lower-end TQ for a few more HP on top. The stock merge is in the aft half of the system. This position generally optimizes TQ at lower RPMs in exchange for a loss of some HP on top.

- my personal opinion is that the correct way to design the exhaust is to begin with an informed plan, fabricate the system and measure on a dyno, then iterate until satisfied with the results. I think this rarely happens because the process is time-consuming, impractical, and expensive. ALL of the commercially packaged N52 exhausts I've seen retain the merge near the stock location. This doesn't mean it's "The Best" position. It means it's the best for maximizing TQ in the lower RPM range most-often used on a daily driver.

2) another great question! IME the AA and BPC tunes result in a bit more power. I have dynos posted of results on my old car. I think the old car made around 238 HP and 212 ft-lbs TQ, but I'm reaching back. The OEM 330i tune is cool, at least for me, because of some implementation issues it resolves. If power is your concern, I think there are many many satisfied members running both AA and BPC DME sw. The marginal power takes the Camry-like 328i and gives it just enough push to feel athletic. The 330i sw is noticeably less zippy compared with AA and BPC, at least IME, but functions with a level of product maturity that's superior. So choose your goal and spend accordingly.

Originally Posted by justrokkit

Two things I wanted to ask about:

1) In regards to the exhaust setup, is the midstream-merge single exhaust a better design than a full dual exhaust for this engine? Also, can you explain the theory behind stepping up the diameter after your headers? I want to learn more about the thought process behind finalizing this design for the N52 because I've been thinking about a full-custom exhaust of my own.

2) Is the 330 DME a better platform for adding a tune rather than just going off of a single tune from a tuner like BPC? If so, would you mind explaining why?