would it give me any extra HP just with the tip or do i have to install the whole piping???
wat size piping would i need for a D16Y8?

 

Any muffler tip is purely aesthetic - it will not gain you any power whatsoever. In order to increase engine output you're going to need to either increase exhaust piping diameter or decrease the number of bends in the system. Since the latter is rather difficult while remaining legal and pleasant to drive, you're left with increasing the diameter.

On a Y8, I wouldn't go any larger than 2.25 inches or it'll start to sound like **** and you'll lose all of your low-end torque.