I finally read the articles I linked above again (rather than skimming to find the link) the "Undertrays, Spoiler & Bonnet Vents, Part 3" one is the most relevant here's an image and quote:
Quote: This DaimlerChrysler graphic shows the typical frontal pressure distribution of a car. Looking just at the bonnet you can see that there is low pressure (blue) where the air wraps around the leading edge of the bonnet, grading to high pressure (green) as the air reaches the obstruction which is the windscreen.
So you wouldn’t site a bonnet outlet vent close to the windscreen – in fact that’s usually where the cabin ventilation inlet ducts are... they’re taking advantage of that high pressure! Looking at just the exterior pressures, what we want at the vent location is the very lowest external pressure.
That’s the theory – but what about on the road? By using the Magnehelic gauge (see the previous parts in this series for more on these gauges), it’s dead-easy to directly measure the pressures over the outside of the bonnet. Simple run a tube from the Magnehelic gauge to the area to be measured, making sure that the open end of the tube is at right-angles to the direction of airflow. Leave the other port of the gauge open, and swap the tube from port to port, depending on whether you find that you’re measuring a high or low pressure.
This is exactly what we did on the Maxima, using the same 80 km/h road speed for all testing. The results were astounding – not because they reflected the textbook example shown in the graphic above, but because the pressure variations were so great! As can be seen here, the front half of the bonnet surface was clearly a negative pressure zone, while the rear half was all in positive pressure. And the further forwards the measurements were taken, the lower the pressure; and the further backwards, the higher the pressures!
But as the author finds out earlier in the series it doesn't always follow the theory... so as GR said there's no alternative to doing the testing yourself, it seems the mini's requirements aren't the usual as we'd like to promote higher under bonnet pressures to promote airflow through the radiator and/or lower pressure in the guard. I can't remember the source but AFAIK the guards or wheelwells are generally a low pressure area. I figure the mini is different enough to upset a few of the theories.
Thinking of trying an undertray/sumpguard which unlike the author's (Part 2) would aim to seal off the under bonnet area and raise the under bonnet pressure.
Thanks 69k1100, gonna go read it now 
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