Fuel Pressure Dropping? ( FPR )

[krayton]

A wierd problem has risen up with my stock FPR. at idle with vaccum it sits at 3.5bars. now as i start going and start building boost, it starts to fall a bit. at about full boost (8psi) it falls down to 3.3 bars.

anyone have ideas? i thought it was suppose to rise with boost? did my fpr take a dump? im gonna double check my lines and fill up and see what happens.

*ps im using the Apexi gauge. so the brain is calculating pressure at the injector using the boost gauge. just incase any experts needed the lil details

thanks, let me know so we can turn up the boost and tune for 22psi

[themadscientist]

sounds like you pump has a problem. I would think a bad FPR is bad all the time and no matter what if it held any pressure it would stay the same or rise a little with boost. If you have an extra pump I would swap it in and see if it fixes it. If you an extra regulator try that first though as it's easier than the pump to pull but I would bet Fiz's asscherry it's a pump issue.

[krayton]

sounds like you pump has a problem. I would think a bad FPR is bad all the time and no matter what if it held any pressure it would stay the same or rise a little with boost. If you have an extra pump I would swap it in and see if it fixes it. If you an extra regulator try that first though as it's easier than the pump to pull but I would bet Fiz's asscherry it's a pump issue.

thats whats confusing me. cause at idle the pump is pumping at 3.5bar. so it can pump that much. but why would the pump start kicking out as i start to build up boost?

but yeah. i think ive needed a fpr anyways, so ill try changing that. dont really feel like digging into the gas tank just yet. *bad memories

[NISMO_RB25]

it drops because under boost you have your injectors open wide open, so the pump is trying to maintain the pressure while dumping fuel out the injectors. A strong pump can maintain it and a weak one can't.

Kind of like filling a bucket up with water that has a hole in it. You got to have more flow coming in than going out to keep the bucket full.

[Wulfgang]

*ps im using the Apexi gauge. so the brain is calculating pressure at the injector using the boost gauge. just incase any experts needed the lil details

I don't know what you mean by the "brain" or by calculating pressure using the boost gauge. Sounds like a backwards way to get a fuel pressure measurement. I bet if you put a fuel gauge in the line, you'd see the pressure rising with boost.

So what is probably happening, is what everyone is already saying. The Apexi gauge is giving you pressure differential -- not absolute pressure -- which will of course fall a bit as the flow increases. The fpr is there to keep the pressure differential roughly constant, but it is a simple, single-stage linear device, and cannot possibly compensate perfectly for changes in flow rate.

In control systems lingo, the fpr is a single-spring regulator and is thus a P (proportional) control, unlike the PID (proportional-integral-derivative) cruise controls in the better cars. Bottom line is that it will hold different setpoints (differential pressures) for different flow rates.

[themadscientist]

that hurt my brain

[krayton]

the full throttle thing makes since. i sure hope my walbro didnt take a dump.

anyways, the Apexi manual reads:

If the boost meter AND fuel Pressure Meter are connected to the Meter Control Unit at the same time, the Fuel Pressure Meter will display Fuel Pressure at the injector nozzle.

About Fuel Pressure ReadingsMost electronically fuel injected vehicles measure Fuel Pressure through a Fuel Pressure Regulator. The regulator ensures that the Fuel Pressure always corresponds to the Intake Manifold Pressure. In the diagram below, if the Fuel pressure is set to 300kpa, a Fuel Pressure Meter wthat displays pressure relative to atmospheric pressure will read 400kpa at 100kpa of boost. (this is Fuel Pressure at the FUel Rail, or Actual Fuel Pressure). Fuel Pressure Meters that display Fuel Pressure relative to boost pressure will display 300kpa. (This is Fuel Pressure at the Injector Nozzle)

*cant draw the fancy diagram

Wulfgang, I think you know what your talking about. if you could explain this to us, cause reading yours and reading apexi my brain hurts too

But I thought I remember my fuel pressure giving a slight rise rather then slightly falling off.

On the same note, whats a safe pressure? since it falls to about 3.3bar (thats about 47psi at full throttle)

am I still ok? these are 550cc injectors. not to sure what the duty cycle is up there.

thanks

[Wulfgang]

Ok, I misunderstood how your fuel meter works. According to what you just posted, you must have a fuel pressure sensor and a boost sensor.

absolute fuel pressure - boost = fuel differential pressure.

Fuel differential pressure should be kept relatively constant by the fpr.

The fuel pressure regulator is just a spring pushing on a diaphram, just like a wastegate. The diaphram also has fuel pressing against it on one side and air from the plenum on the other side (the same side as the spring). It uses the counteracting forces of the air pressure/spring pressure and fuel pressure to move the diaphram and thus regulate the fuel pressure in the rail.

Remember that the regulator is at the END of the rail, so it just dumps fuel back into the tank to regulate. So if you try to force more fuel through the regulator (i.e. low duty cycle on the injectors means more fuel will have to be returned to the tank), it will err on the high side, right? It isn't perfect... it's just a spring and a diaphram! If you push a lot of fuel through it, the spring will deflect a lot more. Since the spring is now producing more force, you'll end up with higher pressure in the rail.

By the same token, if you push less fuel through it (i.e. high injector duty cycle) it will tend to err on the low side. The spring will be deflecting less, producing less force, giving you less pressure in the rail.

In an ideal regulator, the spring does not deflect at all from its equilibrium position as the flow changes. Thus you get a perfectly constant pressure in the rail. But not in real life.

Anyway, I would not worry much about the drop off at high boost. As long as your fuel maps account for it, you'll be fine until you run out of duty cycle. Also, flow rate through the injectors is proportional to the square root of the pressure differential. So if it dropped to 3.3 bar, you'll still have sqroot(3.3/3.5) * 100 = 97% of flow. So 6% pressure loss corresponds to 3% flow loss.

[krayton]

good info and thank a lot. understanding. im not to worried about the small drop anymore. ill be watching my wideband and during the tune we can see whats going on.

i may just change to a adjustable fpr for my preference. ill keep an eye on it though. if it starts to get worse ill dive in the fuel tank again