the 5th injector was an idea mainly used on the escort rs turbo (hisssss) it was an injecto mounted in the side of the top boost carrier and was conected to a boost switch+ a wot switch so when there was enuf boost at wot it would come on, the 8 injector setup was used on the 1st rs500's and rs200's (hissss) and worked in the same way but was more of an ecu controlled thing, altho if your car has the propper injectors, a decent management system and is mapped propperly there should be no need for a device such as a 5th injector