Filtration is 10 microns.
In both Eric’s and Steve’s post on the feedback from the Bosch Engineer, the figure given for the minimum pressure (translated from bars) was –2psi. I would take that to mean the minimum pressure before cavitation begins to eat the pump and/or the minimum before the high side pump begins to starve. In the “pumps, lines & whatnot” thread, Mopar-muscle consistently measured 30ghp flow back to the tank at WOT at anywhere from 6psi to 0 psi. It would have been interesting to cut the flow on the intake side and see what happened at –2psi, -4psi, etc. If I were a betting man, I would say that the injection high side could keep up with the demand, at least initially, at –2psi. So, at WOT there is an excess of 30gph that is being pumped by the low pressure pump inside the VP-44 that isn’t being used by the high side part of the VP-44 and that is being returned to the fuel tank. .
So, any positive pressure maintained on the inlet side of the VP44 guarantees an adequate supply of fuel at our power levels. I don’t see a need to pressurize the low side beyond what the pressure relief valve is going to vent back to the tank. And don’t confuse the relief valve with the return on the VP.
Here’s a thought. If we force the relief valve to open because we are pushing an excess of 14psi, does any less fuel go to the lubrication of the VP-44?
Bill Kondolay wrote in that thread mentioned…” I talked to Lawrence at DD saturday night, since he is the only guy i know with R&D data on the vp44 pump , he said too much pressure at the inlet of the vp44 pump can actually cause more wear to the vp44 pump. ”
I missed that or forgot about that on the first go around. So there has been a previous indication that we can put too much pressure on the inlet side of the VP.
Pressure vs. flow has been discussed ad nausea, so if you read the rest of this, prepare for that. For some, I belive the following may clear up some confusion.
We are not simply measuring the static pressure of fluid restricted through an orifice, we are measuring the differential pressure between two pumps, the Lift Pump (LP) and the VP-44 (VP). If the LP operates at a maximum of 15 psi @100gph, then it can supply fuel in any amount up to that limit, which is the head pressure. The VP’s ability to pull or push fluid is proportional to the speed at which it runs, i. e. the faster it spins, the more fluid it will pump. If the demand (suction) of the VP is greater than the supply (head) of the LP, negative pressure; vice a versa, positive pressure. In other words, the LP provides a head of pressure to the VP. When the VP takes fuel from that head, the pressure will drop. The faster the VP operates, the more fuel it draws and the flow is increased both through the LP and VP but the pressure differential is REDUCED between the two pumps. That is why flow is increasing with a reduction of perceived pressure. It’s not a static orifice and it’s not static pressure. It is simply a pressure differential between two pumps. It would be hard to fathom if you were trying visualizing static pressure. We are not pushing fuel through a carburator, the same logic doesn’t apply.
I imagine, because I don’t know, that fuel passes into a reservoir chamber that is shared by both the inlet of the hi-pressure pump and the relief valve inside the VP-44. The valve is designed to open at between 12-14 psi. The relief valve acts as an orifice, not able to accept the entire volume of fuel that the LP is trying to deliver. Then, the excess pressure (0ver 12-14psi) developed inside the reservoir would only be reduced by the fuel demands of the hi-pressure pump in the VP. The pressure has to be pushing, at the very least, on a seal. It would also be nice to know if the lubrication for the VP is supplied by the low or hi pressure pump inside the VP-44.
Attention: TDR Forum Junkies 
} many reported VP-44 failures are a matter of poor/no diagnosis by D. C. techs, that find it easier to R&R than test. Again just my somewhat educated opinion, I hope to hear from some real flow bench experts. Jim G. 
