Agreed with both of the above although these days with returnless liquid fuel systems they no longer try to keep a constant pressure differential across the injectors. They just measure the fuel rail and manifold pressure and let the ECU do the math to determine the necessary pulse width. 30 years ago the ECU's didn't have enough compute power to handle that so the pressure drop was kept constant by a pressure regulator mounted on the fuel rail that had a tap off the manifold for it's reference pressure. The pressure regulator bleed excess back to the tank in a return line. That very nicely accounted for the range of manifold pressures from decel vacuum to WOT turbo boost. Without a return line to the tank that doesn't work anymore. The injector static flow rate has a nearly perfect (delta P)^.5 relationship that's easily calculated and they can map in the change in injector response time as a function of pressure differential. Not sure exactly when they transitioned to returnless, I'm sure it took quite a few years, but I know my 2003 350Z was the first returnless system I've owned.
Also, with GDI systems the rail pressure is so high the cylinder pressure is insignificant. They ramp rail pressure up and down all over the place depending on engine speed and load.
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