Thought about this quite a bit.
Seen a few of these done in person, and every single one of them looked hacked up to say the least, and some were down right shitty.
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To solve the problem of more air flow through the stock sized hole, you have a few options.
1) Make the hole bigger, but destroy the looks of the car by going wider, or lower, or just cutting out the bumper clear through to make the opening larger.
2) Be smart, and keep in mind that air is like any other fluid, and will react to what you design in ways you would not normally think.
a) Look at the nose of a corvette, or F-body GM. They are bottem breathers, and the designs are very effective at drawing air over the radiators. So much so that most vette's have a "trash dump" of leaves and pet hair packed up on the upper part of the radiator/condensor.
b) We can get much of the same effect on the Supra with mods that you can't see unless you lay down and look up UNDER the bumper.
c) Cut the bumper impact bar from a square tube to a triangle. A simple shroud then directs the air over the FMIC and also mounts the stock grille in place so when your standing in front of the car, you see a stock grille. When you lay down, you see the grille opening is really about 2x the size it used to be.
Fluid dynamics and the fact that air always flows from high to low pressure works for us here. A reverse vented hood would further improve the flow, but that's another post.
So, here's what happens to the air that reaches your bumper/grille. Becasue the hole is now larger surface area wise, it would create a low pressure area and draw that air up behind the bumper and direct it into the face of the intercooler. The lower pressure behind your radiator due to the fan, and the design of the car then draws that air through the FMIC, Condensor, oil coolers and your radiator of course. Higher speeds actually make it work better, especially if you have effective shrouds that don't let the air escape off to the sides, or back under the car. (The lower tray pan for example.)
You also need to design vertical panels that keep the air flowing over the FMIC and don't let it go around the sides. (Air is like water, it will take the path of least resistance, and if you leave larger holes on the sides, it will go there instead of going through your intercooler where you want it to go.)
Ok, end of my thoughts. One of these days I'll build this setup, and find out if it really works.