BMW ALPINA B3s exhaust flap does work.

B3Sのエキゾーストフラップ、動画撮影に成功しました。 詳細は、「みんカラ」で!

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BMW E36 328i exhaust silencer flap operation

M3 & M4 Exhaust Valve Mod How To - F80 F82 F83 - Free Mods To Stock Valved Exhaust
In this instructional video, I show you how to perform the Exhaust mod (a free mod) on the F80 M3 (also applicable to F82 M4 and F83 M4 convertible). This valved Exhaust mod allows you to run your 2015 M3 with the Exhaust valves open at all times by simply disconnecting the wiring harnesses but you must have the car in sport plus with valves open before turning it off and unhooking the harnesses. Make sure to protect the harnesses with tape etc... to prevent road debris and water from getting in them. The Exhaust sound is somewhat louder with this mod at idle and driving at partial throttle. It is like having sport plus on at all times. You can do this and drive in efficient and still get the rev matching feature in the 6 speed manual transmission.

e90, e92, e93 BMW Exhaust Flap Rattle Fix
This happens to a large amount of these cars. If you drive a e90, e92, e93 or maybe other ones, this will definitely help you at one point. This mod also makes your Exhaust sound very nice (a tiny bit louder)

Capristo RS5 exhaust flaps