The stock exhaust is not too restrictive. Infact, if you remove the rear cat converters for those who and an N54, and replace the downpipes with catless ones, the stock exhaust is pretty good as is. Where other turbo cars may have a single 3 inch exhaust, the BMW has a double 2.-- midpipe that y-pipes into a 3 inch muffler. The catback can be upgraded, and also converted to all straight 3 inch, but unless you are running 5-- +HP there isnt any point to it IMHO.
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