Shawndude did the testing years ago, and I believe the data was lost in one of the various site hacks.
Basicly here is the info, from what I remember.
First, I'm assuming everyone knows that air always flows from high to low pressure. No exceptions. And that your car's cooling system is designed to use that reality to flow air over your coolers, and to create areas with minimal lift, or low pressure areas, on the body of the car. (IE: Like a plane uses low pressure areas over the wing to create lift, you don't want to make your car fly off the road if you can avoid that.)
The stock undertray is important as it creates a better low pressure area behind the radiator, so air naturally wants to flow through your FMIC, Condensor, Oil Cooler and Radiator while your moving down the road.
Without it, the air pressure is not maintained, and air will take the easy path, and just dump under the car v/s go through your coolers.
Now, on the top of the car, over the hood for example, there are areas of higher and lower pressure.
The highest pressure is right at the base of the windshield to hood location. Right where 99.9% of all OEM air intakes are for your HVAC system. This high pressure area is used by "Cowl Induction" type hoods to force air back under the hood/air filter. (Think Chevy Chevelle, or the older Corvettes with this type of hood design, and filters over the carb.)
The lowest air pressure readings from Shawn's data was on the hood, just about as far back as where the radiator to engine gap is located. (Perfect for what we can do.) It was slightly ahead of that, but close enough.
The pressure goes up about 1/2 way back on the hood, and then starts to increase as you get back closer to the windshield base.
It slips off on the sides, where air can spill over around the fenders, along the edge of the hood to fender gaps.
So, the guys who shim up the rear of the hood, in an attempt to allow air to escape? They really are just ramming high pressure air back under the hood at speed, and that screws up the low pressure area you want just behind the radiator.. so your cooling is reduced by shimming up the back edge of the stock hood.
The readings from the inside of the engine bay indicated that the lowest pressure area was just around the oil pan area of the bay. (With the stock lower tray installed.) This indicates that as the air flows through the radiator, and then engine bay, it then flows out under the car at the engine oil pan area and transmission tunnel. This is a good thing. But I think it can be enhanced.
The goals should be to lower front end lift, and to enhance cooling air flow while smoothing out air under the car to increase fuel economy and performance.
To lower front end lift, you want to limit as much air as possible from going under the car, and allow as much air as possible to exit the hood into that low pressure area formed by the curve of the hood, much like an airplane wing.
The bonus is that by "venting" the hood, just aft of the radiator, and ending that vent just about as it reaches the engine in the middle, and strut towers on the sides, you will allow quite a bit of air to be "sucked" out by the low pressure area formed as the car passes through the air.
This should help to keep the front end from generating lift, and improve cooling as a bonus.
A well sealed up undertray/splitter would further make the cooling and downforce benefits even more effective. The splitter would limit air from choosing paths other than the coolers, and the smooth underside is a good start for the air to start it's journey under the car.
Strakes that start very short, and then triangle up to about 3" tall at the wheels, angled like a point under the car, with the outer edges of the strakes blowing the air off them at the brakes/wheels would further create the low pressure area you want at the oil pan, sucking the hot engine air out from under the hood. (And as much of that air would be vented through the hood, this would enhance downforce under the car now as well.)
Flat panels from the rockers to the tunnel would enchance this effect further, and as I've seen on some new cars, a "wing" or upside down 80's Camaro tail type flip of the panel disconnects the air over the rear suspension, and then smooth panels at the gas tank and exhaust area to the rear cover would allow it to re-attach, and then be drawn out with a simple diffuser design in the rear of the car. (You want to smooth out the air, and draw it up into the low pressure area right beind the car as it's moving.. Strakes can be used here to create a super low pressure area, around the rear differential, so air will flow over it to keep it cooler too.. Again, that "V" would be pointed directly at the diff, and then go wider as it curves up to the rear cover. Vertical side strakes can be added to keep air from flowing in around the rear wheels, sort of closing the "Box" on the sides. (Look at some race cars, and you will see what I'm taking about.)
It would be quite a bit of work to make it all happen right, but the under panels could be formed up from plastic or aluminum, the front diffuser/undertray could be modeled on the stock setup, just extended in the front to replace the stock short air dam lip. The rear diffuser in alluminum would be best, as it's going to be subjected to heat from the muffler.
Thing is, other than the hood vents, the front diffuser end hanging out, and the rear diffuser being there to see when your following the car.. Not much of this is very "performance" to the average person. (IE: no fancy bumper kit, no side panels, no rear bumper skirts... No cool looking intake scoop on the hood.)
Just panels cut out of the stock hood, some mesh perhaps istalled to keep fingers away from the engine, and panels of plastic or metal to control and smooth the wind under the car.
The only addition that might look "modified" is a wing that extends the rear hatch.. a trick to not only lower drag, but also create even more low pressure behind the car, that the rear diffuser can then direct air into, enhancing downforce and lowering drag.
A wing positioned like this would actually do that. (Hope the owner does not mind, but I saw this years ago at SILV, and liked it so much, I photographed it, and might someday copy it with some slight changes...)
Here you can see how the wing hangs back, over the end of the hatch.
