west

You clearly haven't done your research. The RPI the OP is referring to is an axle-back. Where does your comment regarding catbacks being 'disappointing every time' come from anyways?

Crckrjck

Yeah, I have a catback RPI on my 06' 530i and it definitely made the car quicker off the line. Not to mention that it sounds f'ng awesome!

LessIsMore

You clearly don't see that if it's an axle-back, my point is more poignant. it will not increase horsepower in a meaningful amount.

The comment comes from the fact that modern NA engines are, as time goes on, more and more optimized for power (and sound, mpg, etc.) from the factory. They have to be to stay competitive and regulation-compliant. Yes the factory exhaust system compromises all these aspects to some degree in the final form, but across platforms - GM, Ford, BMW, Nissan, whomever - people just don't pick up much power from a catback (and much less so from just an axle-back) anymore, on normally-aspirated vehicles. Vehicles with forced-induction engines are excluded for obvious reasons.

I'm talking about on the dyno, not manufacturer's claims. You might get a few, of course, not doubting that - but not enough to justify the cost of the part+install if your goal is horsepower. if your goals is only changing the sound, obviously that's a different topic.

LessIsMore

How do you KNOW? Have a dyno chart of before/after runs taken in similar weather conditions? Or even data logs, or quarter-or-eighth mile ETs, again in similar weather, to show a quantitative and measurable difference?

We all know the butt-dyno is notoriously unreliable. Even if you did gain 5 horsepower, that's not enough to actually feel that in your SOTP, despite how much you might think you do. Your car's horsepower probably fluctuates more than that on any given day, even in stock form, with just temperature changes from day to night.