Quote:
Originally Posted by BMWforLife
The left pipe has a flap that closes off the outlet upon start up, and once the engine is warmed up it will open and both pipes are flowing exhaust.
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This is completely opposite of how our N55 equipped motors and exhaust work.
Ours work exactly as the OP described and is normal.
On cold startup the flap opens until motor reaches a certain temp. I think our flap closes pretty quickly, like within a minute. Then it remains closed in normal MILD driving situations. Under hard throttle or anytime above 4k rpms the flap opens to reduce back pressure which ultimately increases performance and sound.
Totally normal to have one dirty pipe and one 'cleaner' pipe. I do agree on the flap mod though and it is something I've done on all our N55 vehicles, in one way or another. For 1 I like to have my pipes get dirty in symmetry and second, I like the sound of the exhaust when the flap is open all the time.
I haven't confirmed with a 335i (ours is due in a couple weeks) but I suspect this methodology is consistent across all N55 equipped vehicles.