Now I know the BMW engineers have done a good job with their exhaust design, particularly with the M5. However, I have seen true dual exhaust systems (such as domestic muscle cars etc) that have benefited from a cross pipe about halfway down the stream. Would putting a link between the pipes, either like an H or an X benefit our systems? Has anyone tried/thought of this? I wouldn't expect a huge power increase or anything, but perhaps a slightly thicker torque curve. Any thoughts?
-Lloyd

The E34 M5 exhaust is a "true dual" system. Two exhaust manifolds, two pipes all the way to the rear muffler.

And last time I looked under mine, there is a siamesed section that is effectively the same thing as any X- or H-pipe design.

From discussions on the Yahoogroups E34M5 list, nobody has really found any worthwhile power by changing the M5 exhaust. You can change the sound and you can save some weight, but no real gains.

On the US 3.6l cars, however, you may find some gains by using the larger 3.8l Euro exhaust manifold, basically a tubular header system. Combine this with the matched Euro exhaust, and there should be some gains.

One particular reason why the E34 M5 exhaust design may be so efficient has to do with the timeframe in which the cars were built. The early '90s Motronic was very good (for a factory engine management system), and all the OBD-II nonsense did not exist. Additionally, exhaust noise regulations were not yet as strict. Combine the two, and realize that BMW had been tuning M30-series engines since the early '70s, and M88/S38 variations since the early '80s, and it will become apparent that while improvements in power do exist, you have to start looking at hard parts such as cams, valve size, and even aftermarket engine management. The intake and exhaust systems are really things that should be left alone on these cars unless you are specifically tuning in response to other modifications.

Hmmm, that's interesting. :ponder: The B36 has tubular headers too, or at least mine do. If you really want to imnprove the lower/mid range torque, then get cam gears. No seat off the pants, but complement the chip. They aren't too expensive if you get the B38 cam gears.

Cross pipe is virtually only for sound. The new viper has it so that the side exhaust play all 10 cylinders, instead of just 5.

Originally posted by ///MDriver
Cross pipe is virtually only for sound. The new viper has it so that the side exhaust play all 10 cylinders, instead of just 5.
While yes, the Viper did have the cross pipe put in to make the sound quality better, there is also a performance gain to be had. I saw a full exhaust test (I believe it was a Camaro ...) where they went from a 2-1 single exhaust, to a true dual, and finally to a dual with an X-pipe in place. The X-pipe produced a much thicker torque curve and higher overall power than the other two systems.
Anyway, I think I'm going to have a "track" pipe made to go from the headers back to the rear muffler, taking out the resonators and cats. I'll have them siamese the pipes somewhere in there and see what happens.