what would happen if i carbureted my 13B in my S4?? would there be a power increase/decrease? MPG increase/decrease??

 

You would kill gas milage and drivablility, and probably have no power increase. In fact, it would probably lower power for most of the rev range. The only reason to car an FC is if you have a monster port (bridge/peripheral) and are too cheap/lazy to get and standalone and possibly ITB setup.