Quote:
Originally Posted by elp_jc
If you know anything about engines, there's no free lunch man. A higher flowing exhaust increases a few HP at the higher (and sometimes mid) end of the rev range, while reducing torque a bit down low.
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This is a very common misconception.
Engines do NOT need backpressure. Increasing the scavenging pulls in more air, no matter what.
However, there is a reason that BMW does not quote a power difference... there isn't one with a catback. The factory catback flows well enough to where there is zero difference.
Even with the cat removal, there is no power loss anywhere practical to dyno. (there may be some 2000 rpm and below, but it's not enough to matter, and BMW limits throttle there anyway)