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Thread: Fuel Pressure questions

  1. #1
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    Fuel Pressure questions

    My e38 ECU no longer has the FPCM since I went to a return-style fuel pump setup. My FPR is boost indexed, and should go up to around 82 PSI (at 22 PSI of boost).

    1. Can a transducer be hooked up so that the ECU is again aware of the actual fuel pressure?

    2. Since the ECU doesn't know the fuel pressure, wouldn't this cause the EQ Ratio to be wrong? (Wouldn't it cause it to be overly rich, since the fuel pressure is causing higher flow than the ECU is estimating)?

  2. #2
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    under vacuum at the injector tip the injector can spray easier and under boost at the tip its harder to spray, all the referenced reg does is zero out the boost/vac at the injector tip so the injector pressure stays the same overall, so at 22psi of boost u have 22 psi pressing against the fuel coming out of the injector so it increases the fuel pressure by the same amount so it then will be injecting fuel like it had no pressure against it at all because its zeroed out and same for the vacuum side but reversed so it will have less pressure needed under a vacuume to spray the same fuel, also this is assuming its 1:1 referenced
    Last edited by 07GTS; 03-15-2015 at 03:22 AM.

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    Makes sense. Thanks for sharing.

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    just remember to change the other injector data to suit because now the reg is doing the delta pressure the ecu dosnt have to compensate for it

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    Senior Tuner mowton's Avatar
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    Quote Originally Posted by 07GTS View Post
    under vacuum at the injector tip the injector can spray easier and under boost at the tip its harder to spray, all the referenced reg does is zero out the boost/vac at the injector tip so the injector pressure stays the same overall, so at 22psi of boost u have 22 psi pressing against the fuel coming out of the injector so it increases the fuel pressure by the same amount so it then will be injecting fuel like it had no pressure against it at all because its zeroed out and same for the vacuum side but reversed so it will have less pressure needed under a vacuume to spray the same fuel, also this is assuming its 1:1 referenced
    Great answer...additionally that is why an IFR table with a 1:1 referenced reg is flatlined at one value, instead of decreasing (telling the PCM the injector is actually smaller than it is to incease the IPW) as the manifold pressure increases. A boostapump (BAP) is a bit different where it only applies additional voltage to the fuel pump to increase its pressure when the setup has entered boost. Some after market setups (MSD) are programmable vis software while others have a dial to tweak the voltage developed when activated.

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    Advanced Tuner jpb1972's Avatar
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    I'm new to tuning and currently waiting for my software to arrive. I will be up against this same issue with having installed a squash return system with the boost ref. reg. How do we compensate for the extra psi from the boost ref. within the tune? I should mention i'm also running an alky single nozzle methanol kit. Thanks in advance.

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    Senior Tuner DSteck's Avatar
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    Quote Originally Posted by jpb1972 View Post
    I'm new to tuning and currently waiting for my software to arrive. I will be up against this same issue with having installed a squash return system with the boost ref. reg. How do we compensate for the extra psi from the boost ref. within the tune? I should mention i'm also running an alky single nozzle methanol kit. Thanks in advance.
    The post above your's explained it.



    You can't hook a pressure sensor up when deleting the FPCM unfortunately because the factory pressure sensor is actually hooked into the FPCM and not the ECM. The ECM defaults to some value when deleting the FPCM. Flatline the IFR and the offset and move on.

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  8. #8
    Advanced Tuner jpb1972's Avatar
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    Quote Originally Posted by DSteck View Post
    The post above your's explained it.



    You can't hook a pressure sensor up when deleting the FPCM unfortunately because the factory pressure sensor is actually hooked into the FPCM and not the ECM. The ECM defaults to some value when deleting the FPCM. Flatline the IFR and the offset and move on.
    Thanks. I'm hoping once I get the software and dig into it that it will make more sense. Can you clarify how to access and modify the IFR and Offset? Sorry for my ignorance but like I said, tuning is new to me.

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    Quote Originally Posted by granatl View Post
    My e38 ECU no longer has the FPCM since I went to a return-style fuel pump setup. My FPR is boost indexed, and should go up to around 82 PSI (at 22 PSI of boost).

    1. Can a transducer be hooked up so that the ECU is again aware of the actual fuel pressure?

    2. Since the ECU doesn't know the fuel pressure, wouldn't this cause the EQ Ratio to be wrong? (Wouldn't it cause it to be overly rich, since the fuel pressure is causing higher flow than the ECU is estimating)?
    Your EQ Ratio or injector settings will also be off if you ignore the "Desired Fuel Pressure" in your tune. Even if you have "FPCM Fitted" set to "NO", it will still reference the desired fuel pressures and they will effect fueling. So if you have 80psi as your desired in the tune and you're only running 60psi with your return system, it will actually run leaner, not richer. The ECM DOES need to know what the desired fuel pressures are, even if it's not able to monitor them. Now that you don't have a FPCM or a fuel pressure input, try setting all the desired fuel pressures to 20psi and see how it runs.

    You can also use your stock fuel pressure sensor as an AUX input to your HPTuners, assuming you have the Pro version. It's a simple voltage setup. If you don't have the Pro version, you can still monitor fuel pressure off your stock sensor by using other ECM inputs/circuits for that input.

  10. #10
    Advanced Tuner jpb1972's Avatar
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    Quote Originally Posted by 07GTS View Post
    just remember to change the other injector data to suit because now the reg is doing the delta pressure the ecu dosnt have to compensate for it
    Can I ask exactly what you mean by this? From what I'm reading here, as the boost signal on the regulator increases pressure the injectors psi will even out due to boost, this I understand. However, if that's the case, wouldnt my tune go rich with using the original data for the injectors? This assuming the injector data is probably not accounting for boost pressure or a referenced fuel system. But others here are saying flatline, or change the data to compensate? Sorry, just trying to make sense of it all, or maybe I'm just overthinking it. Lol.
    Last edited by jpb1972; 03-26-2015 at 06:16 AM.

  11. #11
    Senior Tuner DSteck's Avatar
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    Stock tunes are set up to vary fuel flow rate based on injector delta pressure. Most GM stuff assumes 58psi in the fuel rail and use manifold pressure to calculate the delta pressure. 2010 Camaros, ZR1s, CTS-Vs, and other vehicles with a FPCM actually measure rail pressure and use that in combination with MAP to get a true delta pressure reading.

    When you use a boost referenced regulator, the delta pressure becomes constant because as MAP goes up and down, so does fuel rail pressure. However, the ECM doesn't know that this is what's happening, so it is going to try to constantly recalculate delta pressure which means it is going to grab a fuel flow rate from more than one cell of the IFR table. Since IFR becomes constant on a 1:1 setup, you have to set the IFR table to one value across the board. You also have to set the voltage offsets to the same across the board for pressure (it should still vary with voltage).

    On vehicles that use a FPCM, when you delete and go boost references, it doesn't matter where the default pressure is set because proper setup of the injector tables makes it not matter.

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  12. #12
    Advanced Tuner jpb1972's Avatar
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    Great post! I appreciate you taking the time to explain this. I will defer on any further questions until I actually start working on this. I have a feeling it will all make better sense once I do. Lol.

    Thanks again

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    Quote Originally Posted by DSteck View Post

    On vehicles that use a FPCM, when you delete and go boost references, it doesn't matter where the default pressure is set because proper setup of the injector tables makes it not matter.
    I don't really care if you believe this or not Dave, but for others reading this and trying to gain accurate information, what DSteck said here is NOT true. The "Desired Pressure" settings DO matter.

  14. #14
    Senior Tuner DSteck's Avatar
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    Quote Originally Posted by gmtech16450yz View Post
    I don't really care if you believe this or not Dave, but for others reading this and trying to gain accurate information, what DSteck said here is NOT true. The "Desired Pressure" settings DO matter.
    Not when you're using a boost referenced fuel system and delete the FPCM. I have done dozens of these, and it's the same story every time... Fuel pressure goes to the default value when the FPCM is no longer active.

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    Quote Originally Posted by DSteck View Post
    Not when you're using a boost referenced fuel system and delete the FPCM. I have done dozens of these, and it's the same story every time... Fuel pressure goes to the default value when the FPCM is no longer active.

    Ok so how about you put 80psi in all of those desired pressure settings in your car, go out and do a WOT pull and tell me how it goes. lol.

    Like I said Dave, I don't care if you think you know everything and are incapable of learning. I tried to make clear to the others that might read this that THE DESIRED FUEL PRESSURE SETTINGS DO EFFECT FUELING EVEN IF "FPCM NOT FITTED" IS SELECTED. You can think or do whatever you want in your tunes. Others might actually want to learn new things. What happens when some previous tuner changed those settings and you're telling everybody they don't matter? They matter.

  16. #16
    Senior Tuner DSteck's Avatar
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    We aren't talking about "FPCM Not Fitted".

    We are talking about the FPCM physically removed. You aren't reading.

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    Quote Originally Posted by DSteck View Post
    We aren't talking about "FPCM Not Fitted".

    We are talking about the FPCM physically removed. You aren't reading.
    Yeah, if you remove the FPCM and leave it on "Fitted" in the tune, what I'm saying ABSOLUTELY makes a difference. My point was that it even makes a difference if you select "not fitted" in the tune when you don't actually have a FPCM. So yeah, of course I know we're talking about physically REMOVING the FPCM. I don't have a problem with reading comprehension dude.

    I honestly don't know why I try to help you learn. You clearly know everything already and have the attitude to go along with that. I'm sorry, I'll go away. If anyone else reading this is having issues when deleting the FPCM, hopefully they'll understand what my point was, even if you don't.

  18. #18
    Sitting in a 2012 Ctsv with fpcm deleted with a boost referenced standard regulator system. When you delete the fpcm , the computer assumes what your default fuel pressure setting is, hence the name "default"... I just did a little test, I set the default pressure to 75 psi... Now look at what the inj delta pressure , request pressure, and desired fuel pressure in the scanner.. Sorry messed up iPhone pics...
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  19. #19
    once again sorry for the bad iphone pics, my camera is messed up, and this computer doesnt seem to want to screen shot anything.
    notice the actual pressure is 0 even though all others are 75psi....
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  20. #20
    Senior Tuner DSteck's Avatar
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    Quote Originally Posted by gmtech16450yz View Post
    Yeah, if you remove the FPCM and leave it on "Fitted" in the tune, what I'm saying ABSOLUTELY makes a difference. My point was that it even makes a difference if you select "not fitted" in the tune when you don't actually have a FPCM. So yeah, of course I know we're talking about physically REMOVING the FPCM. I don't have a problem with reading comprehension dude.

    I honestly don't know why I try to help you learn. You clearly know everything already and have the attitude to go along with that. I'm sorry, I'll go away. If anyone else reading this is having issues when deleting the FPCM, hopefully they'll understand what my point was, even if you don't.
    You do have a problem. You chimed in about something not even discussed.

    When the injector data is set right for a boost referenced system, none of those settings matter. It just goes to default pressure. End of story. Period. I don't get what your problem is... You can't accept when you are wrong even when flaring evidence is put forth like what Ryne posted.

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