Okay, Im a newb tuner and I always read about psi. What exactly is psi? I mean my friend who drives an Evo VIII was talking about how he had 21 psi but I was confused as hell as to what he was talkin about....a little help here guys

 

psi stands for Pounds per square inch. Which is the amount of pressure that the turbo is creating when compressing the air. The higher the "boost" the more power that the turbo is adding. However high psi can do damage to the engine and to the turbo. A turbo basically takes the exhaust and compress's it, then pushes it back into the engine. Go here if you want to learn allitle more about the basics.
http://auto.howstuffworks.com/turbo.htm

 

yeah, agreed, a shatload of psi may be impressive but not exactly healthy for the car