it all depends. how much fp you are running, what size is the 5th injector, how much boost. everyhtign plays a role you have to just try it and tune it. stay stoich not under boost and make sure it stays rich under boost and doesnt go lean in order to stay away from detonation. try it with the fmu first and if it doesnt keep you rich enough wire up a 5th injector to kick in at WOT.