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I've heard people say on here say that you need a restrictive exhaust because the turbo needs it.....right.....at least there is some actual testing by a trust worthy source to show the axle back IS restrictive. But like any car, the stock exhaust (headers on back) are very restrictive. That is why there are companies who make aftermarket exhaust systems to make the exhaust flow better and get the most out of the engine for power.
I'm sure with the N54 on the 135i the mid pipe and axle back need to be swapped out to get the same gains as the other 3 models did. But generally you only gain 5 to 10 HP at most from a tune, not noticeable on a car.
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