OK, I got a chance to review my Lyc 320 engine manual today.  Food for thought:

 

For a carbureted 320 Lyc asks that the max pressure to the carb be limited to 8 psi with the ideal being 3 psi and a minimum of at least at .5 psi.  I think a fuel head of about 18" would deliver the minimum pressure.  Hmmmmm, that's about the distance from the header tank to a bottom mounted carb.  What was the designer thinking?

 

For an IO 320-B, the max fuel pressure at the inlet to the engine pump should be no more than 55 psi with the mixture at idle-cutoff and 35 psi during normal operation.  They do not give a desired pressure but specify that the inlet to the engine fuel pump should see no less than -2 psi. 

 

In other words, the engine pump can draw fuel from below itself - like out of very low wings.  Hmmmmm... I wonder.  If the pump is actually sucking on the fuel line does that lower fluid pressure allow it to vaporize more easily?  Especially, can this occur if the path to the pump or the pump itself is heated somewhat?  Does this set up pump cavitation?  Certainly a boost pump could maintain a positive pressure at the engine pump inlet. 

 

Anyway, the inlet to the fuel injectors (after the metering device) is limited to a max pressure of 45 psi, no particular desired value is noted but a minimum of 12 psi is required.

 

In my airplane the pressure at the outlet of the engine pump is normally between 26-28 psi, with or without the boost pump engaged.