Steve - looks like you're doing a great job of providing technical input on the cat topic. I'm not an expert, but I know that for years (maybe still), many US catalytic systems were "3-way"... ie, they controled CO, HC, and NOx, all of which are of concern in the US. Problem is that if you have too much O2 in the exhaust, the cat system is less efficient in removing one of these pollutants (don't remember which one), and if there is too little O2 in the exhaust, the cat is less efficient on a different one. So there's an optimum O2 level in the exhaust that the combustion system has to control, in order to optimize efficiency of the cats. Not sure, may not be as much of an issue today, but I always thought that the exhaust tubes were all for the O2 sensors for this system, except for temporary exhaust taps that may be there for testing. Is there any other use for O2 sensors that you're aware of Steve?
Anyway, at risk of making this a cat thread, I'm still interested in 328 exhaust options. Has anyone installed an aftermarket exhaust on their 328, besides a Tubi? PS Steve - I'm leaning toward a 328, but still want to take a look at a TR before deciding for sure. You're right that a TR is bigger (specs show TR is almost 10" wider than BOTH the 328 and Boxer), and 9" longer than 328, as well as being heavier.