ls3 returnless fuel system offset select vacuum or MAP?

[danall]

Gen 4 ls3 with a returnless fuel system. Of the tunes i have looked at they all select the vacuum offset (not MAP) and the "offset vs volts vs MAP" table is zeroed out. Flow rate multiplier is constant at 1.000. Is that correct for a returnless system?

[07GTS]

it dosnt matter if its return or retrunless its only if your fuel rail pressure is vacuum/boost referenced or not ? if its still steady 58psi then keep as stock or if it varys with manifold pressure then there are some tables u need to adjust

[danall]

I reckon i'm a bit confused, i was under the impression that returnless systems were by default not vacuum boost referenced? Having recently taken Evans Performance Academy modules on Gen 4 tuning, when addressing these tables the distinction was returnless vs return system (or i missed something). Anyway i do not have any external vacuum fed fuel pressure regulator. Just the standard returnless system. So i take it mine is NOT vacuum boost referenced? if so do i use the MAP offset and populate the MAP vs volts table? What's confusing is the ls3 tunes i look at have Vacuum selected for the offset and do not have the MAP vs volts table populated would those be all return style systems then?. And then to make it more confusing i ran across this from Injector Dynamics:
"Some returnless systems will actually vary the pump output to emulate a referenced system, or to offer more fuel pressure at higher demands and less fuel pressure at lower demands. Ford fuel systems modulate the pump in an effort to maintain effective fuel pressure at 3 bar. The Corvette ZR1 runs fuel pressure in the 30s until an increased demand is on the system, at which point it will ramp the fuel pressure up to 88psi in the rail. Systems like these use sensors that record the fuel pressure, and when combining that pressure with the manifold pressure, the PCM knows what the effective pressure is and will determine a pulse width for the injector accordingly. Systems like these offer the best of both worlds."

[07GTS]

if ur all setup same as factory steady 58psi then all good and just use the vacuum table as its against pressure delta so it will be fine for all uses, the ecu dosnt know how the fuel is returning as it dosnt make any difference all that matters is if its factory steady 58psi or if its referenced to manifold pressure or FPCM,
if its steady then the injector data has to compensate for the manifold absolute pressure thats where u see the as pressure rises so does the offsets as more pressure on the injector tip the harder it is to inject fuel, for example if u run 40psi boost with 40psi fuel rail pressure u wouldnt be able to inject any fuel as the pressure is equal on both sides of the injector so the injector data has to increase the PW to suit,
but if u have a manifold referenced regulator for fuel rail then when the MAP pressure rises/falls so does the fuel pressure so the injector delta stays the same mechanically so if u have 40psi boost with 40psi fuel rail the regulator will increase it to 80psi fuel pressure so the injector can work as normal and the injector data can be adjusted to suit as it then dosnt have to adjust for it anymore, some use the FPCM so as that changes the fuel pressure its also knowing what pressure it is so the tune is constantly adjusting for it within itself in the injector data