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Thread: MV Patch Fuel Pressure vs Inj Delta Pressure

  1. #1
    Advanced Tuner
    Join Date
    Mar 2015
    Posts
    233

    MV Patch Fuel Pressure vs Inj Delta Pressure

    I'm working on a new fuel system, so been logging FP and inj delta pressure to see how everything performs. I was told that I need to apply the Manifold Vacuum Patch and enable "Supercharger Fitted".

    When I applied those changes and logged, I now see my inj delta pressure is calculating differently. But I dont understand which way is correct. Probably because I dont exactly understand inj delta pressure and how its calculated. Is actual fuel pressure supposed to match inj delta pressure under boost?


    I'll post two logs, one before the patch and one after. Let me know which looks right. Not sure if I need the MV patch.... I am running the Beta.

    Before Patch
    MicahBlepp7.hpl

    After Patch
    2pumps 78ths tank.hpl
    2011 Camaro SS
    Thompson Motorsports Dart 427
    ESS Tuning G4 Supercharger 1158whp/1017wtq
    DSX Tuning stuff

  2. #2
    Advanced Tuner
    Join Date
    Mar 2015
    Posts
    233
    MVPatch.PNG

    Why is my Inj Delta Pressure now not rising with boost? Isn't that exactly what this patch is supposed to FIX??? It's done the opposite. i was better off before...
    2011 Camaro SS
    Thompson Motorsports Dart 427
    ESS Tuning G4 Supercharger 1158whp/1017wtq
    DSX Tuning stuff

  3. #3
    Senior Tuner 10_SS's Avatar
    Join Date
    Mar 2012
    Location
    Detroit, MI
    Posts
    1,320
    that looks like it's working right then. Delta pressure = Fuel Pressure - Manifold Pressure.

    If you have constant 58psi fuel pressure at the fuel rail going into the injector, and 0psi boost (100kpa) you should be 58psi delta pressure (58-0). If you build to 10psi boost, then your delta should drop 10psi to 48psi. 58 fuel psi - 10 manifold psi = 48psi Delta.

    If you managed to build 58psi manifold boost then the fuel pressure would be the same as the manifold pressure so delta pressure would = 0. You wouldn't get any flow.

    This is what Vacuum or boost reference fuel regulators do, they adjust the fuel pressure depending on manifold pressure. They lower fuel pressure under high manifold vacuum so injector pulswidths can stay reasonable, usually required for very large injectors, and they are supposed to increase fuel pressure at the rail as manifold pressure increases at a 1:1 ratio, ideally. So if you have 58psi normal running pressure then build 10psi of boost, your Delta Pressure is 48psi, but if your regulator is working and you have enough fuel pump then the fuel rail pressure increases to 68psi to maintain a 58psi Delta Pressure.

    The problem with the 2010ish GM cars is they use returnless fuel system, so no external regulator and no return so the regulator is built into the pump, no way to get above 58psi on most LS3 non factory boosted type pumps. So my fuel pressure Delta comes down to around 48psi, though I have 55-58psi at the rail. Since the 48psi Delta is correct, it references the 48psi flow rate for injectors and my fueling is rock solid. I've had it down to 25-30psi Delta on my stock SS pump with no BAP and still had perfect AFR. You can force the fuel pump to slow down or turn off in HPT and watch delta pressure fall, and pulse width pick way up to compensate, and still have great fueling. Pretty neat.
    Last edited by 10_SS; 02-06-2020 at 09:52 PM.
    2010 Camaro LS3 (E38 ECU - Spark only). MS3X running complete RTT fuel control (wideband).
    Whipple 2.9L, 3.875" Pulley, kit injectors, supplied MSD Boost-A-Pump, stock pump
    LG Motorsports 1 7/8" Headers - No Cats, stock mid pipe with JBA Axle Back
    ZL1 Wheels/Tires