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It does have one very interesting safety feature built in.
What you are looking at is a small subcompartment within the airbox itself on the firewall side of the box. The outer shield meets right up against the filters foam and basically blocks 10% of its area during normal driving.
From the outside all you see is a circle within a circle.
As you saw in the first picture there is a spring which keeps the inner circle pressed against the airbox itself sealing the hole.
If the vacuum inside the box is too great that spring will compress and let air into the box from the rear as a vacuum release.
Why?
Hydro-lock. BMW must have felt that this setup was a little more dangerous then other airboxes so they built in this safety measure.
Normally the only vacuum that would occur below the airfilter at WOT would be from any restriction in the inlet of the box itself. But if the inlet was blocked (or submerged in water) the vacuum in the box would increase greatly and the vent would open to release it and prevent the engine from sucking in a lot of water and killing it.
Kinda cool.
Shawn
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