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A cat by-pass is just a straight pipe. A pre-muffler has a chamber with a perforated "liner." The pre-muffler is quieter, and it's suppose to give 8-12 HP over the cat. I'm sure there is some gain with the by-pass pipe, but no one has quantified it...that I know of. It's likely to be very close behind the pre-muffler.
I made my own pre-muffler recently that is even quieter than the Danks or Fabspeed pre-muffler. It has a very throaty sound at idle and filters out the tinny sound of a 911 across the RPM range. I have to wait for warmer weather for performance testing.
Last edited by MotoSook; 01-04-2005 at 12:23 PM..
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