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Most single piped designed I've seen for 135/335's are 3.5. There are a few like Cobb that use a single 3". If I remember the measurements right a 3" should give about the same flow as the factory mids, slightly less than the aftermarket mids at 2.25 and the 3.5's flowing the most. But in the real world where the manifold is the real restriction and all the mids flow into the same 3" rear section I don't think any of the sizes really matter. And keep in mind most cat backs are only adding what around 5HP?
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2008 135i - MHD/JB4, MS DP's, AMS IC, UI CAI, CDV delete, M3 control arms/Links, RE Toe Link, ECS Trailing Arm, M3 rear SF & Trans bushings, SS brake lines, Michelin PSS.
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