Thanks for the reply! Maybe I wasn't clear enough in my first post. I'm strictly talking the intake air temperature. I was comparing a cold air intake (e.g. drawing air from the front or base of the windshield) vs. drawing air from the engine compartment. Obviously the temperature difference decreases with driving speed as the heated air is replaced faster (if the engine compartment is properly vented).
Let's say you are driving @60mph - what's the temperature difference between engine bay and outside air?
Actually it all boils down to the pressure difference (colder air being denser) - so drawing cold air from a high pressure zone would be the best but is worth the hassle to make all the ducting etc.?
I might buy a differential pressure gauge and do some investigating..