6paks came from the factory with a 8-9lb pump, BUT they had a .060" return orifice in the vapor separator that helped control fuel pressure. i just went thru a high pressure fuel thing with one of my street cars. anything over 7lbs can cause problems and seems that 6-6.5lbs is a sweet spot. i use the 6903 pumps and do some mods to them but run a return of some sort. i also think large needle and seats can be a problem with higher pressures. the floats just can't control the flow, especially at WOT. i don't know your power level but stock 6pak fuel lines with the separator can handle 450hp. if you want to keep the braided hose then try to rig up a return line. i prefer hard lines myself, but braided may be better for someone pulling a lot of maintenance.
The vapor separate comment is misleading, because Mopar fuel pressure spec was measured at inlet to carb and included the separator as part of the system. Hemi pump was 7 to 8.5psi, the 440 HP was 6 to 7.5psi. Mechanical pumps are constant volume pumps. If the diaphram or the airdome age and harden the pressure at outlet rise because less give/expansion of the rubber to buffer the pressure pulse.
have you ever had your hands on this stuff and worked with it before?
Yes in great detail documenting fuel pumps and rebuilding them. I have several dozen disassembled and bagged, what do you want to know. Your comment about large and small needles should have been obvious. The same as MC bore. You apply pressure to a surface area. Multiply the area by the pressure and that tells you the force applied to the area. A set pressure against a larger needle means a larger force on the needle. Hence a smaller needle and seat sees a smaller force for a set pressure and therefore less force opening, which means the float can handle keeping the needle shut. High school physics.
The same applies inside the fuel pump with the different size chambers, size of the washer plates, and thickness of the air dome gasket.
Original carter carbs were designed for a higher pressure, and Chrysler made the move to higher pressure in the 50s. Higher line pressure keeps the fuel in a liquid state when engine compartment gets hot. Less vapor lock. Plenty of good info in the Carter manuals and old Army tech manual.
