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On a 135i, the PE will mount up directly to the stock midpipe. Typically the PE comes with a Y-pipe. The Y-pipe replaces the resonator on the stock mid pipe, but is not necessary to attach the PE. In order to install the Y-pipe you have to cut the midpipe and attach the Y-pipe with clamps, or have it welded together. The PE sounds pretty good without the Y-pipe (better than stock) but not as loud as with the resonator replaced. I prefer the PE together with the Y-pipe, but to each their own.
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