Who's got a N75j or other variant?

Hello all... in the quest for power iv read a lot on these valves.

Are they a direct swap and can they affect my car in a bad way?
What do they actually do? Which ones are best?
Where can i get one from?

Thanx all!

 

Tried that and cudnt find much! /ubbthreads/images/graemlins/frown.gif