"Much easier to just raise the bonnet up an inch or so at the back, this sucks air in directly above the carb. "
As you can see on the above pic, where there is red indicates high pressure areas, and theres one at the base of the screen as you see. If the rear edge of the bonnet was raised, without aerodynamic add-ons the high pressure would just keep the warm air in place under the bonnet. The other red areas though are good, the one at the front will be forcing air through rads/cooler etc and the one on the rear wing will keep the rear end planted.
the air doesn't flow cleanly over this area and carry heat away and over the roof, instead when it reaches the edge of the bonnet it basically swirls about in place and doesn't really disperse any heat. It does slowly filter out round the sides of the screen/car, but nowhere near quickly enough to be benificial in anyway whatsoever.
Although this is more relative to heat dissapation, it essential means that it could well raise engine temps and give the carb warm air instead of fresh cold air
