[kenntona]

I've read that besides downpipes, many BMW owners in the US either (1) decat, and/or (2) install H-pipe or X-pipe in place of the secondary cats. The theory of the latter mods stemmed on creating a crossover connecting the two sides of a dual exhaust system to balance the two banks of the engine. The H-pipe is good at balancing sound pulses between the two halves, but does little to promote "scavenging" because the exhaust gases tend to follow the path of least resistance, which is straight through each pipe rather than taking the 90-degree turn through the H-pipe into the other half of the system. In an X-pipe system, however, where the two sides of the system intersect, the gasses have no choice but to intermingle as they pass through the junction. This promotes improved "scavenging effects" by smoothing out uneven exhaust pulses from the engine’s firing order. It also helps quiet down the exhaust, resulting in a mellower, less raspy tone.

Can I assume that H-pipes and X-pipes are actually redundant for the 135s/335s, since a straight through exhaust system with the least resistance is the optimal setup for FI cars?