[cooljess76]

9hp, not a chance. In fact, if you remove your cat, you'll not only gain a raspy drone, but you'll probably lose a lot of low end torque. The best catback on the market might get you 1-5hp max(not even enough to notice) and yes, people here install catbacks for sound. The only real benefit besides sound is possibly a little weight reduction. The M42/M44 engines came with a really nice header from the factory. The factory muffler is much lighter than the ones used on the e36 coupes sedans and verts. I know it's hard to believe, but BMW actually got it right with these little engines. They already squeeze out a good amount of horsepower while still maintaining efficiency. Without a bore/stroke or FI setup, there's very little if any power to be gained. Sure, you could get a custom tune. However, whatever you gain on the high end, you'll likely lose on the low end. And whatever you gain in power, you'll lose on economy. If you want the car to feel sporty, get a lightweight flywheel, sport tuned suspension and some sticky tires. If you want power, suck it up and do a 6cyl swap or go FI. It's laughable that you want to keep the economical engine, but you want to remove the catalytic converter.

FWIW, we had a Stromung groupbuy a few years ago. Most people got 2 1/4" piping to match the factory cat and a few members with DASC's ordered the 2 1/2" piping for better flow. All of the DASC'd guys reported a loss in low end torque and none of the NA guys reported any performance gains that I can recall. It's a placebo effect. Just like the guys who install CAI's, they drive harder to hear the suction and then claim their car is faster. It's not really faster, they're just driving more aggressively to hear the sound.

Some people just don't want to believe that BMW designed a peppy little engine. It is what it is, no matter what you bolt onto it, it's always going to be a 1.9L.