Originally Posted by exdos
On my vacuum cleaner, there is a little sliding vent on the solid connector to which the flexible pipe and the solid pipe for the end fittings attach, which can be opened to reduce the suction from the end of the pipe; I have yet to see anything blow out of this vent when opened, because the flow of air through the solid pipe from the suction tip towards the vacuum cleaner produces a "venturi effect" which always sucks air into the vacuum cleaner. I bet you NEVER see rain or leaves fly out of the hood vent under WOT.
That's what's undetermined. Actually, one poster said that with strings he saw the hood vent reverse flow at WOT...I believe this was on a track...so, yes, our (the forum members) experiences seem to lead to that fact.
To addressyour vacuum cleaner example, you would never get debris/dust to fly out of that vent because you have nothing forcing the air "in" the tube. It is being sucked. Now, an extreme example, if you took a leaf-blower and put it in front of your vacuum's hose...you may just see debris fly out of tht vent.
My question, which hasn't really been answered ...other than someone's string...is "do the intakes in front push enough air into the airbox, at WOT, to still allow a positive, ram-air effect with the hood-vent closed"?
I need to place a pressure sensor inside that tube and go WOT at various speeds and see if there is significant drop-off of pressure.
Originally Posted by swamp2
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.
Without testingequipment, I can only surmise that the pressure gradient is not enough at our vents to cause meaningful positive pressure. Back at the windshield...yes, that's why "cowl-induction" was so popular with the muscle-cars of past...but I believe, and the flying rain drops agree, that we do not have sufficient pressure on the outside of our hoodto allow any meaningful amount of ram-pressure to be contained in the airbox.
But, as we have all been saying, we don't have the proper data to support either position.