If you have a straight river flowing south and you have a stream flowing into it from the east, then if the stream joins the river at an angle of less than 90 degrees, then the flow of water going south in the river will pull
water from the stream into the river. Think about it, if the stream joins the river at angle of, say, 60 degrees, then there is absolutely no chance of the river back-flowing into the stream because the pressure of water upstream in the river, will force all the water in the river down the river and not into the tributary streams along its route. In the case of your air-intake, think of the river as being the air coming from the scoop in the bumper (Part 3 in the diagram of your car) and the stream being the air coming from your vent in the hood. Does this example help?
-well, yes, but in our case the rivers are sealed and they lead to a lake...they don't "t-off" with one another which would have a scavenging/venturi effect...I understand your point I just don't think that exact situation is happening in the box.
Either way, I do appreciate you putting in the time...your posts contribute a great deal and bring another perspective on this issue.
My S54 engine has a 3246cc engine and at the rev limit of 7600rpm, and at a Volumetric efficiency of 100% it consumes 12.33 cubic metres of air per minute. Your E90 M3 with a 3999cc engine and at the rev limit of 8400rpm and at a VE of 100% consumes 16.8 cubic metres per minute which is 36.25% more air than my S54 engine can consume. Now my air intake has just the scoop in the bumper and a secondary scoop behind the headlamp, which I see is equivalent to Parts 2 and 3 in the diagram of your air-intake.
I have done a total of seven individual mods and tweaks to my air intake and exhaust systems, all of which have individually, and collectively, helped me improve the Volumetric Efficiency at increasingly lower speeds. Now if you had to rely solely on Parts 2 and 3 to provide 16.8 cubic metres of air per minute, then I would suggest that you will not see a VE of anything near 100% until very high speeds. However, with your additional third intake, Part 1, your engine is able to obtain the necessary air to achieve 100%VE at realistic useful road speeds. -yes... 'at what speed' does it happen is the answer to sealing the hood vent for good. If it's100mph, then that's not really worth doing...
I am sure that if I were to cut a hole in my MC's hood and provide an additional intake to my air-filter box, to make my MC's air-intake system function similar to yours, that I would see a considerable improvement in VE at lower speeds in addition to those I've already achieved with my present 7 tweaks/mods.yes, it effectively makes it an 'open' airbox but with only cold-air feeding it. LIke those cone filters people change their OEM airboxes forthat sit in the engine bay without a box...except in this example...you'd have that heat-shielding box.
Take a look at the photo below, it shows the rain marks on the bonnet of my car after I'd made a high-speed motorway journey (Germany). These marks clearly demonstrate the flow of air over the hood of my car. Notice how the lines seem to come closer together and concentrate in the region on the hood where your M3's hood vent is located. This concentration occurs on both left and right sides of the hood. In view of the fact that the pressure of air towards the rear of the hood is at positive pressure on a moving car, then it seems obvious to me, that your bonnet vent is perfectly positioned to provide the additional air required to supplement the air-intakes from Parts 2 and 3. I'm afraid that I just can't bring myself to cut a hole in my bonnet to allow my engine to benefit from this additional source of air, purely for aesthetic reasons.
if the pressure zone at that point is less than what is present inside the airbox(the pressure fed from the front two inlets), then air will come out of that vent. I notice this with rain/water/leaves whilst cruising.
I do not think that zone/vent is allowing any positive pressure to be added to the airbox, on the contrary, I think it acts as a true vent...allowing pressure to be released out the box. Why else would rain fly out of the box or leaves poke out through the mest on the vent when drivng?
If your car is parked outside in a thunderstorm, where does the rain water go that falls into the hood vent?
It is either the fog-light inlet or small holes in the bottom of the box..like other cars. I'll have to take the top of the box off to be certain.