I came on a little bit different one today. Perkins 4 cyl, DB pump which gets both its supply and return via hard lines to a Stanadyne filter head with the "fuel manager" type filter that is held on by a ring. The other three ports are the fuel supply from lift pump, fuel to the Perkins flame intake heater, and a very tiny return orifice. My question is this. I have heard about the DB pump having variable timing based on housing pressure. This one is running all the return right back to the filter head, so the return port is seeing the same fuel pressure as the inlet, which is being fed by the lift pump. The return to tank is from the filter head, so it is tiny and will not affect fuel pressure in any way. The owner has installed an additional electric lift pump before the whole mess, that is surely increasing system pressure by a few PSI. The DB pump has its own electric advance mechanism/solenoid on the bottom of the pump as well, connected via a temp switch and 12VDC. I'm looking for insight as to the effect of this back pressure on the pump return, or if I should care. Perkins built it this way with Stanadyne parts but the electric pump was installed as a crutch for other problems, which we think are fixed now.
Tagging in. I’ve wondered about this configuration myself. Doesn’t make sense when Stanadyne says no back pressure on the return.
I think that the pump operates with an internal pressure higher enough above the external transfer pump pressure that it doesn't matter too much. The added electric lift pump won't add to the pressure of the cam mounted diaphragm pump, it will only pump to a higher pressure on it's own if it is capable of a higher pressure, the diaphragm pump is self limiting in pressure. If it pumps 5 PSI, it will do that whether it is sucking a 3PSI vacuum, or has 3 PSI pressure in it's supply.