The chicken or the Egg (fuel flow vs. presuure and volume)

[turboglenn]

So, in my late night thoughts that often keep me from sleeping i had a rather weird question pop in my head.

Okay, say you're running 43.5 psi fuel pressure but your pump flows more volume at 35 psi. When your fuel pump starts to run low at the 43 psi setting because volume is falling off, along with this pressure drop shouldn't the fuel pump start flowing more volume like it' supposed to since the pressure got lower on it's own?

That's one of the reasons i find it hard to believe that you can make much more benefit by lowering base fuel pressure. Because pressure is going to drop anyway when it starts running out..and according to the charts, lower pressure= more flow (i know this is redundant, but seriously... lets here some thoughts on this) Why will the pump not flow more when the pressure falls off on it's own since it doesn't know wether the pressure is falling off because you set it lower or that the car is consuming more than it can flow at the given pressure....

My assumption is that the pump can maybe push more volume agaisnt' less pressure aslong as the volume isn't being consumed, but all in all it's only going to flow so much either way and when you start running out , that's it you're running out.

 

[PieEyedPiper]

If you have some exact numbers it might be easy to see what is and what isn't just through the math.
Interesting thoughts though.

 

[turboglenn]

Yea, i'm no math expert so i'll leave that to some one else. But does that theory not make sense? Pressure drops when there's not enough volume to maintane pressure so then flow should pick up according to what we've all been told about volume at a certain pressure. It must be something in the way they get the numbers, or just assuming you aren't using all of the fuel being pushed to the rail..i don't know honestly this subject confuses me and with all teh ish i've been through trying to maintane flow on E85, i'm starting to want to call BS on some of these "facts" we've all been filled with.

 

[turbo addict]

The problem is the boost. as you are increasing the fuel you are increasing boost on our cars. so base line is 43psi, ok now you are in boost, so lets say that you are running 30psi of boost. so your fuel pressure (pumping resistance is 73psi) but your injector flow is still only 43psi because you have 30psi of boost pushing against the fuel (back side of the injector). So if your fuel pressure (pumping pressure or rail pressure) drops 20psi to 53psi lets say, and you are still at 30psi of boost your injector is only flowing 23psi through it. there is the problem.

you try to increase your duty cycle to keep up with the pump running out and your pressure keeps going down.

stuff like that keeps me up too. that is why I answered at 5am. damn it.

 

[turbo addict]

ps we should meet up some time, I kind of got the mirage running again.

 

[crimsondragon]

Pumps do not create pressure. Restrictions and regulators create pressure. Regardless of fuel being low, pressure will stay the same and flow will not be affected...till you run out of fuel then the pump is pumping nothing.

 

[99gst_racer]

Pumps do not create pressure. Restrictions and regulators create pressure.

Mostly true. It was to my understanding that the actual outlet of the pump is restriction enough to create some pressure. It would be more accurate to say that it's not the duty of the pump to regulate and/or maintain pressure.

 

[Boosted98gsx]

Solution: 4 Bosch 044's. Series AND Parallel; Y'd into -10an. Problem solved.

 

[TurboTiger]

I believe the difference on walbro pumps is just the pressure relief valve is higher on the HP pumps than the LP pumps. Modifying the pressure relief valve will allow you to run even higher fuel pressure than the HP pumps.

 

[turbo addict]

Mostly true. It was to my understanding that the actual outlet of the pump is restriction enough to create some pressure. It would be more accurate to say that it's not the duty of the pump to regulate and/or maintain pressure.

true but it pushes the fuel to the rail etc. and different fuel pumps respond differently to different pressures. the overall result that we are all trying to accomplish is fuel flow into the engine. pressure kind of works the same with anything, it is a measure of resistance. not exactly the same but the engine determines the boost from a turbo as the resistance of the boost geting into the engine and what the engine can flow.

 

[T is for TURBO]

Don't forget pressure can also affect the spray. to little and you get poor atomization of the fuel...

 

[JWoodley]

Also most injectors are rated at XXX cc/min based off of the stock fuel pressure. I could be making this up, but I'm pretty sure I read it somewhere (FIC website, IIRC).

 

[tkelly27]

i either know this one, or I'm a bottle of wine off... You're right, as the pressure drops off the flow capability should increase, in fact, if you're asking for more flow than it can deliver thats why the pressure will drop. However tuning for that would be impossible. It's like trying to tune out the pressure being too high if you don't use the proper regulator.

http://treperformance.com/FuelPumps/FuelPumpGraph.gif

If you turn that on it's side, it makes more sense. Instead of saying at XXpsi you have a XXX flow limit, you can say at XXX flow you have a XX pressure limit.

Your injectors are what will be affected and will flow less at the lower pressure. If you have bigger injectors and they run well at the lower pressure, you can squeeze some more out of the pump.

... 4 fuel pumps? : D they do make really big fuel pumps for big hp applications.