i was stuck in traffic last night behind a bmw 328i with my lights shining right up its exhaust. it had two tips on a single muffler (like ours) on the driver's side of the car.
i've seen faux dual exhast tips, you know, just two tips originating from a single outlet like a lot of domestic cars. of couse the bimmer had functional tips and i could see that they were of different lengths inside the muffler, again, similar to ours.
what was interesting was that the function of the two outlets was reversed from ours. i could actually watch one of the openings open and close via (i think it's called) a butterfly valve. at speeds above ~10mph one would close and when we slowed to a stop, it opened. so from a standstill, the engine worked with two open outlets and once it got going it operated with only one open.
i thought ours was backward, one closed at a standstill/low speed and opened at wot? the rationale behind this being that the increased backpressure at low speeds would help give some additional torque and of course we could still breathe at wot when the exhaust opened up fully. this seems intuitively correct so i never questioned it before.
now having seen the bimmer do this in reverse makes me wonder what their reasoning is behind it?? any ideas??

 

Perhaps it is to give the freest breathing at idle seeing as power is not an issue.

 

What?

 

I'm just guessing maybe for an emmision reason or corrosion reason they have both sides open at idle. Must be some reason.

 

i too saw something interesting on a bmw a while ago.....clear/euro taillights! i was amazed that a company actually made those. im sorry, but i had to post this....it just amazed me that much, and the thread title jogged my memory...btw, the lights looked nasty.

 

Hi,
I own a 2002 325ci and had a 97 328i and this car had the exhaust system that you just described.
The flap is the vaccum flap which is controoled by the ECU in terms of the engine rpm.
At rpm's above 2800 the flap opens and at idle it remains open.
This is because the ECU determines at idles speed the maximum outing of it's exhuast since no work is done at speed of 0 mph.
The Vanos system (which was single Vanos upto 99 model years, closes the flap upto 2800 rpm to boost low-end torque whereas at rpm's above 2800 it opens to boost mid-range torque.