^^ Ya Guys, get with it
I believe that on MOST 4 Cylinder Cars, you can run upto 6 PSI WITHOUT an Intercooler.
OR, you can go upto 10 with an Intercooler.
I takes the HOT air that is coming in by the Exhaust Manifold, and essentially cools it down to increase performance and reduce the risk of Detonation (motor goes KAPOW!)
This is on a STOCK motor, BTW.