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Congrats, when I look at my muffler it looks like opening the flap actually bypasses the muffler entirely on a 135i. The other cars aren't like that so that must be why they gain comparatively little but my car is quite loud, especially on cold starts. I love it, post a video of yours once you give it a few hundred miles for the muffler to 'break in' or whatever, they tend to get a little louder over time.
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