Re: Two separate outlet pipes....but
J. Kidd said:
they share a resonator and muffler.
All 6 cylinders dump into two header banks - 3 cylinders per. The 3 tubes squeeze down into one and continue the length of the car (times 2 - one for each header bank). These pipes share what seems to be some sort of resonator and then finally they share the muffler. This is a very efficient exhaust. A performance mod available for other cars with dual exhaust is an "x-pipe" where, on a V8, each cylinder bank goes down the pipe, meets at an "x" and then splits again. The theory is the x does not restrict exhaust flow but actually balances out exhaust pulses aiding in performance. Sharing the resonator the way our cars do seems to me like it would accomplish the same thing. Also having the dual pipes share the same muffler saves in the weight of an additional muffler and still flows very well (as long as there are just as many outlets as their are inlets - which our mufflers have).
Hope that answers your question.
great answer but I'm not sure it's the answer to this question....
yes, muffler shops have been adding an "H pipe" (a crossover pipe) between the left and right pipes in a dual exhaust configuration for years. in fact, I wouldn't call it a mod in and of itself. any well designed dual exhaust for a V8 will have this feature built in. the diameter of the pipe as well as its location (relative to distance from the header) are to the best of my knowledgecalculated. forthermore, there is only one crossover point. as I recall, the way that it "aides performance" is that it may induce some pulse scavenging. I'm not 100% sure, though. they taught us this in high school autoshop but it's been a while. I also seem to recall that there was an easy way to calculate which diameter to use for the H pipe based on the diameter of the main pipes and where to place the H pipe in relation to the length of the system.
I just remembered.... my BMW motorcycle (boxer twin) had a crossover pipe inthe stock exhaust as well as in the supertrapp system that I later bolted onto it.
anyway our bimmers are quite different since there *several* points at which the dual exhaust streams come together.
in terms of a 325/330: the manifolds are sepeareate and have their own cat. converters. the secondary converter (maybe it's one of two resonators and not a converter) is shared as well as the resonator and muffler. so there are three points of "contact" here. while there may be some of the same effect here, I don't think it's exactly the same thing and don't think that it's the intended purpose of the shared components in this case. I think the shared components are there to keep noise and cost down more than anything else.