FPR Function on BMW 850

[dragon850]

Wanted to verify if my thoughts are correct on how the 3 BAR FPR functions on M70 cars.
So at manifold vacuum the FPR diaphragm gets pulled up and drops the rail pressure to 2.5 BAR or around 37 psi.
When manifold pressure changes to 0 the diaphragm locks down and rail pressure increases to 3 BAR or around 43 psi.
What happens when boost is applied to the vacuum line? Rail pressure remains the same or increases with boost?

[Hyper]

The intention is for the FPR to maintain a target pressure drop across the injector, from the fuel rail side to the manifold side so that fuel delivered is proportional to time, independent of changes in manifold pressure.

To accomplish this, the FPR references manifold pressure via its tube port. As manifold pressure changes, the fuel rail will track the change, so that the difference is always the FPR rating.

So if you were to attach a gauge to the fuel rail and boost the manifold, you would see gauge pressure increase, because gauges are by definition referenced to ambient atmosphere. An absolute sensor would also see an increase, because its referenced to a perfect vacuum. However the injectors would always see a static/steady pressure difference, rail side to manifold side, as per the FPR spec.

[dragon850]

Afterwords FPR will hold 3 BAR regardless of manifold pressure correct?

[Hyper]

Yes, constant 3 bar from the fuel side of the FPR to the reference port (which we assume is connected to the manifold), regardless of absolute manifold pressure. The injectors don't know they are in a boosted or NA system.

Several imperfections to consider...
- There are other specs in the world besides 3 bar, although that is fairly common. My mbz cozzy FPR targets 5.8 bar. Some FPR's can be adjusted (say to get a little more head room on the injectors).
- A static FPR could be out of spec and an adjustable one might be set inconsistent with the ECU's thinking.
- The pump may be delivering pressure below the regulation level. This might happen due to a failing pump, a line constriction such as a plugged filter, or during cranking when system voltage drops. Note that the pump typically develops head starting from atmospheric (unless two in series), so if a turbo adds 1 bar to the manifold then the pump needs to output at least 3 + 1 bar to stay in regulation. Not a problem for an ideal pump in an ideal system.

Because of these potential deviations from the ideal, my boxes measure absolute manifold pressure AND measure fuel pressure, then do the subtraction, then adjust injector time (when in open loop, multiply the nominal lookup table by the square root of the pressure ratio, actual vs anticipated), and report on any condition that can't be worked around.

Another imperfection is that referencing to manifold pressure is not quite the same as maintaining a fixed pressure drop across the injector, for the reason that the manifold plenum (where the reference is made) and the injector is typically at opposite ends of an intake runner. If there were no pressure difference between the two ends of the runner, there would be no air flow thru the runner.