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      11-06-2008, 04:42 AM   #83
Second Lieutenant
Drives: Z3 M Coupe(S54) and Z4 M Coupe
Join Date: Apr 2008
Location: UK

Posts: 218
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Originally Posted by ace996 View Post
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 may just see debris fly out of tht vent.

My question, which hasn't really been answered ...other than someone's "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.

I've previously referred to an article here: where the photograph below is found. In this photograph it shows the pressures on the bonnet of that vehicle recorded at 50mph (80km/hr).

As the photograph suggests, the hood vent on your M3 will be positioned in a region which might experience a +ve pressure at 50mph of around +0.4 to +0.5 inches of water.

I've previously posted the graph below showing the increase in pressure versus forwards velocity, and it's below.

As you can see the theoretical maximum attainable ram pressure at 50mph would be around +1.3 inches of water, therefore, it can be deduced that the pressure on the hood in the position of your vent Part 1, is at approximately 1 inch of water less pressure than the theoretical maximum ram pressure, BUT it is still at a pressure greater than ambient pressure.

I have previously posted the graph below, which shows the change in volumetric efficiency of my car that occurs related to vehicle speed.

This graph shows that with all my air-intake mods, my car now achieves 100% VE at WOT at around a speed of 60mph, when the theoretical maximum ram pressure will be at +2 inches of water pressure. Therefore, using all the above information and my deductions, it suggests to me, at least, that on my car, when the air intake system can provide a constant supply of air at +2 inches of ram pressure, then the pressure of air inside the air intake system, at the position of the throttle, will be at ambient pressure. As such, at WOT, the pressure inside my air intake system will ALWAYS be at a lower pressure than the ram pressure at the source of a front scoop (Part 3) which will be at around +2 inches of water pressure and ALWAYS be at a lower pressure than the pressure above the hood vent (Part 1) which will be at a pressure of more than + 0.5 inches of water. As such, assuming your M3 can achieve the same VE figures as my MC does, as I see it, your M3 air intake system is completely self-regulating: Part 1 acts as an air-intake under WOT and a vent at lower throttle openings, and therefore it doesn't need a flap at all to operate. The M3's air intake design is ELEGANT.
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