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      11-05-2008, 05:15 PM   #76
Second Lieutenant

Drives: Z3 M Coupe(S54) and Z4 M Coupe
Join Date: Apr 2008
Location: UK

iTrader: (0)

Originally Posted by swamp2 View Post

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).
You will find that the pressure differentials along the length of the hood will always be relatively the same: i.e -ve pressure at the front and slightly +ve towards the windshield. As the speed increases, the pressure at the front of the hood will become more -ve and the pressure towards the windshield will become more +ve. Motor manufacturers have known this for ages and that's the reason why the air-intakes for the cabin venting are located at the end of the bonnet/base of windshield.

Originally Posted by swamp2 View Post
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.
As I've described in my posting to Ace996 above, I bet this doesn't happen under WOT at any speed.