Is it safe to assume then that the fuel return line isn’t able to return all the fuel that isn’t being used?  Maybe there are restrictions in the return line that prevents fuel from retuning at a flow rate that would prevent it from also bypassing the mechanical pump?

Or does the return line start after the bypass value for the mechanical pump?

 There is a variable orifice as well as the bypass valve involved in the fuel return system that will affect fuel rate beyond the return line size.  To get fuel injection to work somewhat automatically with a pure mechanical system is very difficult.  Adding a turbo further complicates the effort.  Having a mixture knob controlled by the pilot is a big deal.  A good deal if properly controlled.  My advise is to be very careful when trying to modify the way the mechanical fuel injection system meters fuel.  VERY simply put, the pressure at the injector controls the fuel rate.  Increasing the inlet fuel pressure at the mechanical engine driven pump will increase the fuel rate.  Let me give you some food for thought.  The increased fuel rate caused by turning on the high boost pump may be acceptable to the engine during the winter months in Minnesota but will the same fuel rate increase be acceptable to the engine today in Arizona (110 def F).  In my opinion high boost on the TSIO550 should be used for aid in starting and as a back-up in case of a mechanical engine driven fuel pump failure only.

Craig Berland