When looking for high pressure zones and low pressure zones, the best bet is to find a car that is a similar shape and copy where they put their vents. Our cars have a low pressure zone close to the front of the bonnet (good for extraction of hot air), so you would have to mount the vents towards the front of the bonnet. The high pressure zone is where the factory have placed the windscreen wiper plenum and hence the vents. This means high pressure air can find its way into the cabin/engine bay where necessary.
Don't forget the high pressure zone where the front grille is. This will force high pressure air into the engine bay (and the frontal area is quite high) and pressurise the engine bay if the air can't get out.
Air flow is achieved by pressure differential.
They are kind of.. BRICKish
Makes the aerodynamics of it a little... unorthodox. What I am hoping is that, with the placement of the turbocharger, there will be a low pressure area behind that. Given there is going to be a significant portion of the heat being created around that area, it'd be awful handy if that placement would work as it would offer the biggest benefit. What I am thinking though is that there wouldn't be a great deal of high velocity air moving over that area of the bonnet...
I will have to do some image searching I think... See what I can find.
Thanks for the reply mate.