hi there,

fifth injector kit is only needed if you are going to run high boost (anything over 15psi) because the KE fuel management cant put enough fuel in to match the air, so this is when the fifth injector comes in.

Once you reach a programed boost level the fifth injector starts to fire fuel into the air charge pipe (the one that crosses the rocker cover) and your safe to run higher boost levels. But if your only going to run about 14psi its not worth fitting as it will cost anything from £200 to £350.

hope this help