Hmmmm.. very interesting comments all around. Some are true.. some are not so correct and some are commendable logic.
Again.. speaking like a old fart, the whole flow of fuel from fuel rail into injector and then manifold is depending on the pressure differential between the components. Let this be the fundamentals. This is material and law of dynamics. the higher the pressure difference, the more flow. Fluid flow from High Pressure to low pressure. Ok.. onto next step.
Does higher pressure always deliver more fuel?? Yes.. but only if your fuel pump is strong enough. As Volume is a function inversion of pressure. m=p/v.
So, what does Fuel Regulator Do?? it relieves excessive fuel pressure in the fuel rail to maintain a constant pressure, therefore you get a constant flow. it doesnt give you anymore fuel that what the pump can do by itself. Now.. i know this is getting a little more confusing and still do not answer your question of what is the correct fuel pressure to set.
Point one.... idling.. NA or Turbo.... same old same old... unplug the vacuum hose... i would say normally we want the pressure about 1.5 bar atmospheric (or 2 bar absolute). Note that on all engines, at idling, you will have -0.5 bar atmosperic. Some more.. some less. So what we are creating here is a 2 bar absolute between fuel rail and mainfold.
When you throttle and rpm goes up, the engine consume more fuel, thus less fuel being let off by the regulator back to tank.. in some cases the drop is significant that the pressure drop.. this is not the fault of the regulator.. the pump is working its heart out but still cannot fulfill the consumption..
In a good fuel regulator and we assume pump capacity is well enough, it will maintain this 2 bar pressure differential accross the rpm and power band. Meaning at -0.1 bar manifold pressure, the fuel rail pressure will be +1.9bar (approx) and at 0.8 bar manifold pressure.. fuel rail goes up proportionately to 2.0+0.8bar= 2.8 bar ... most cases of the power addict here..boosting 1.8 bar... then it should read 3.8 bar... Faham tak ooiii!!
The vacuum hose function is to maintain base pressure.. and then whatever manifold pressure add on to it to create a constant different between the two side of pressure differential..
well.. if still cannot understand.. but me a teh tarik.. some afternoon in sunway.. i am more than happy to share.. as long as i am in town..
ciao..