IIRC in Calum's book it was mentioned that they ran out of capacity in the original injection pump, and rather than design a new pump with bigger capacity they just added a 15th pump to inject fuel into the supercharger inlet.Some of the Fw 190s had system were they could inject (dump?) raw fuel into the air intakes and get around 140hp extra and use 8.8lbs of boost instead of 5.5lbs. (figures from Janes so they converted to British measure). Now why did they resort to a system like that? Maybe because the standard injection pump could not flow the required amount of fuel? Maybe because the fuel being sprayed into the inlet duct/s in front of the supercharger impeller cooled the intake charge somewhat?