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  • Adjusting fuel pressure down to lean the car out?

    Good idea? Bad idea? (for a stockish NA KA24DE)

    I'm seeing my AFR dip down to like 10.2 or so when I try out the higher RPM ranges of 2nd 3rd and 4th. I think I'm kinda high right now, 50-60psi base pressure.

    Motor is the standard cams, intake, header, exhaust on a stock ECU. I also have a Bikirom I can run, but think getting my fuel pressure back to normal should come before playing with the maps.


    I think the main question I have is what should be the base fuel pressure at idle? Should I go back to that? And should I keep the adjustable FPR, or just go back to the stock FPR?

    Car has an aeromotive FPR and walbro 255 in tank from a previous turbo setup.
    '95 240sx

  • #2
    Originally posted by Sleepy_Steve View Post
    Good idea? Bad idea? (for a stockish NA KA24DE)

    I'm seeing my AFR dip down to like 10.2 or so when I try out the higher RPM ranges of 2nd 3rd and 4th. I think I'm kinda high right now, 50-60psi base pressure.

    Motor is the standard cams, intake, header, exhaust on a stock ECU. I also have a Bikirom I can run, but think getting my fuel pressure back to normal should come before playing with the maps.


    I think the main question I have is what should be the base fuel pressure at idle? Should I go back to that? And should I keep the adjustable FPR, or just go back to the stock FPR?

    Car has an aeromotive FPR and walbro 255 in tank from a previous turbo setup.

    Does it work? Yes. Perfectly? No.


    The percent change in fuel flow will be equal to the sqaure root of the percent change in the fuel pressure.


    Since your fuel pressure changes with manifold pressure, you will not see a linear change then in the fuel flow.

    Particularly, you will see a comparitively larger decrease in fuel delivery (or increase in "leanness") under lighter loads.


    The effect is small, and as long as you aren't planning running the car on the edge under load (keep it 11.5 or richer under heavy loads), then this probably would be an okay bandaid....

    50-60 psi base is really high....the OEM is 43.5 base fuel pressure....clearly, at idle, it's lower with vacuum pulling on the FPR.

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    • #3
      OK, so what should I see at idle / when the pump primes? 43.5 or something lower?
      '95 240sx

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      • #4
        Isn't it 36psi with vacuum? 43.5 without.

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        • #5
          I see... That would explain some things.
          '95 240sx

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          • #6
            the problem could be that the response of the FPR is rising-rate rather linear with manifold pressure, so it goes really rich up top. These FPRs are basically an aftermarket workaround for turbo'd cars with maxed out injectors and/or standard mapping that otherwise wouldn't be able to drop the AFR enough under heavy load, so I'd say first port of call would be a standard FPR and see if the problem goes away.

            - jspaeth I'm not sure what you mean with the flow/pressure relationship, as my understanding was that it is linear - can you explain further please?

            Jonty

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            • #7
              The stock 1:1 FPR adjusts the pressure to maintain a fixed pressure differential between the rail and the manifold; the base fuel pressure. This keeps the amount of fuel delivered independent of the manifold pressure. The injector opens and it always sees the same amount of pressure drop across it's nozzle. This nonsense about fuel added being non-linear isn't correct. If you pull out fuel pressure so that it decreases the AFR up top by 1pt, it will be 1pt lower everywhere. That percentage, as was said, is the square root of the new pressure over the old pressure.

              Just adjust the aeromotive back to 43.5psi. That should be measured with the vac reference temporarily unplugged. It'll be mid 30's at idle depending on what your manifold pressure is. When the engine is off and the system primes, it should be 43.5psi.
              She's built like a Steakhouse, but she handles like a Bistro.

              Comment


              • #8
                Originally posted by Epstein View Post
                The stock 1:1 FPR adjusts the pressure to maintain a fixed pressure differential between the rail and the manifold; the base fuel pressure. This keeps the amount of fuel delivered independent of the manifold pressure. The injector opens and it always sees the same amount of pressure drop across it's nozzle. This nonsense about fuel added being non-linear isn't correct. If you pull out fuel pressure so that it decreases the AFR up top by 1pt, it will be 1pt lower everywhere. That percentage, as was said, is the square root of the new pressure over the old pressure.
                Is that directed at my post or the other Epstein? Some aftermarket FPRs are non linear and are often advertised as such, but Steve could have ended up with one by accident. maybe I wasn't very clear as I wrote it quickly and tbh the issue had already been pin pointed as high base pressure so it is a bit irrelevent, but a quick google will show loads and loads of rising rate fuel pressure regulators (before you even finish typing rate!)

                Anyway, judging by how high the base pressure is I'd say your advice is spot on and will sort the issue... though personally I'd have stuck with the standard FPR to begin with, as unless it's faulty there's no need to replace/touch/look at it unless you're running massive power and it can't bypass enough fuel for the huge pump being used... which I doubt is the case with a stockish n/a KA.
                Last edited by Jonty; 10-12-2010, 08:13 AM.

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                • #9
                  Mine is a 1:1 regulator. http://aeromotiveinc.com/products-pa...efi-regulator/

                  The reason I have it, is because the regulator and fuel pump were left overs from a failed turbo project where the plan was to be able to push what I could get out of some 550cc injectors. The car is NA again, and runs stock injectors, but the regulator and fuel pump remain.

                  I would switch it back to the stock FPR, but those screws are so hard to get to when the fuel rail is still on the car.

                  Anyways, thanks for the tip on setting to 43.5psi with the vac reference removed.
                  '95 240sx

                  Comment


                  • #10
                    Originally posted by Epstein View Post
                    The stock 1:1 FPR adjusts the pressure to maintain a fixed pressure differential between the rail and the manifold; the base fuel pressure. This keeps the amount of fuel delivered independent of the manifold pressure. The injector opens and it always sees the same amount of pressure drop across it's nozzle. This nonsense about fuel added being non-linear isn't correct. If you pull out fuel pressure so that it decreases the AFR up top by 1pt, it will be 1pt lower everywhere. That percentage, as was said, is the square root of the new pressure over the old pressure.

                    Just adjust the aeromotive back to 43.5psi. That should be measured with the vac reference temporarily unplugged. It'll be mid 30's at idle depending on what your manifold pressure is. When the engine is off and the system primes, it should be 43.5psi.
                    Yes, my apologies for not saying this and actually, for forgetting it.

                    What I said about pressure drop versus flowrate WAS true, but I negelected the fact that pressure drop = Rail pressure - manifold pressure (pressure at the outlets of the nozzles).


                    So yes, like Chris said, if your fp in the rail rises 1:1 with manifold pressure, this gives a constant flowrate from the injectors.

                    Sorry again...

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