Originally Posted by exdos
I think this may have answered a big question many of us have had and still seems to be overlooked (in that no one replied to this yet)
. The intake on the front bumper cover being at positive pressure is obvious. The fact that the pressure changes signs ALONG the hood going from - to + is the big surprise I think none of us realized. From my perspective I always guessed the hood intake was at a negative pressure from a simplisitic Bernouillis equation argument (high speed = low pressure). I would be very curious to know if this is a function of speed (perhaps + at some speeds and - at others). The next question is not just the absolute pressure on the hood, but the pressure differential between hood locations and the inside of the airbox. This is what really governs flow into or flow out of the box. If the M3s hood intake is indeed always under positive relative pressure this answers many questions. Unfortunately, some who have observed the situation report that there is an outward flow from this location (don't recall which speeds, but I think many). They noticed this from either debris or using a thread. This seems to be contradictory.