This is only useful when you're stationary, the way the air travels over the bonnet when moving doesn't let any air out. This is called the Venturi effect.
No it's not?! There is no fluid pressures involved for it to be classed as a venturi, the technical term is "**** airflow."
base of your windscreen/top of bonnet is going to be area of high pressure when moving. Compared to lower pressure under the bonnet. Therefore its not going escape. Its debatable that cold air might go in thoughIt must have some effect whilst moving, hot air under the bonnet will have a higher pressure than the air flowing over the bonnet, the high presure air will want to go to the low pressure air, the gap at the back should act as a heat extractor. If you fit bonnet vents they dont force cold air into the engine they draw hot air out, some cars have vents in the wings, like the escort rs cosworth or clio 197/200 which work in the same manor. Although some cars do have a scoop like scoobys and GT4s that force air into the engine or intercooler on these cars.
If only you could mount the filter right up under the scuttle for that high pressure. Bit like the Gruppe M intakes for EP3`s having the scoop on the scuttle.
Just had to have a quick google on the Gruppe M kit for our 8L S3's.....
To say their having a laugh with the price is a huge understatement!
Buy Audi S3 8L 4WD 1999-03 Gruppe M Air Induction Kits | PPCGB
You couldn't be more wrong.
Base of windscreen is a high pressure area, not low, air will be drawn IN, not out by raising the back of the bonnet.
Don't do it. No gains.