Here is my orginal message..."I have a 99 mustang I hate the air filter on it, to me it's a small one. I had a 98 silverado and took the air filter off of it and it fit right on the mustang. My question is and I know it sounds crazy maybe even dumb but is there any way to put the air filter under the car like behind the bumper. The reason I ask I did not know if there was any way to force more air into the filter for better horse power. Thank's for any information any one can give me on doing this." I got some pretty good feed back from it but I would like to know if I do put my filter inside my fender will I gain anything from it. I dont want to put it inside the fender and waste money and not gain any power from it.

 

Putting the filter inside the fenderwell would get cooler air into the intake but behind the bumper would allow more dirt into the filter.
The Ram Air effect is negligible at street car speeds but if you locate the inlet to a high pressure area on the front of the car it can help somewhat.
I have a chart that was made for aircraft but I'm sure it holds true for cars as well and it says that at 100 mph the air pressure in a static ram air system is 0.1916 psi.
That being said,I once built a ram air filter box on my H-D Sportster and at top end it allowed the engine to pull an extra 100 rpm.Not a lot and it was ugly so I pulled it off.I gained 200 rpm just by getting down behind the fairing.