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Thread: How many people utilize their boost refrenced FPR

  1. #1
    Advanced Tuner angrygoat's Avatar
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    How many people utilize their boost refrenced FPR

    I was curious how many people actually have a vacuum lie hooked up to their regulators. When I got the car tuned, Nick pulled the line off and capped it. But I also was reading about how when you start making boost that it can affect/restrict the amount of fuel being sprayed and you can actually not receive the correct amount of fuel. Should I be running that vacuum line to the FPR? And will that help with me having to jack up my PE table because the MAF is maxing out? Thanks guys
    Joe
    2006 M6 GTO
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    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  2. #2
    the idea of a vac/boost referenced reg is to lower your fuel pressure at idle, then provide a linear fuel pressure increase as MAP increases. ie. every 1 psi of MAP increase should get you 1psi of fuel pressure increase, up to the max pressure (flow) your pumps can provide.

    The idea of lowering your fuel pressure at idle is to allow large injectors to use a higher pulse width so you don't get into erratic injector operation at low throttle.

    eg. on my latest setup i set my reg for 50psi at idle and this lifts to around 70psi at full boost. If i was to run 70psi at idle i would have a lot of problems getting the Motron 60's to work.
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  3. #3
    Advanced Tuner angrygoat's Avatar
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    Thats an interesting idea Chris. I take it this was on a LS- car that normally ran 58psi of fuel pressure? How does this affect you IFR table
    Joe
    2006 M6 GTO
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    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  4. #4
    if you run a vac/boost ref'd reg you set the IFR table to a flat line. ie. your injector flow rate should be constant with vacuum because the fuel pressure is changing with vacuum.

    i run a walbro in tank pump feeding large swirlpot then 2 x Bosch 040 pumps, a Holley reg and 1/2in supply and return lines so i don't have any fuel problems @ 16psi boost with the Motron 60's (Siemens Deka)
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  5. #5
    Senior Tuner eficalibrator's Avatar
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    Chris is right. There's two main benefits to running a 1:1 referenced regulator. The first is lower pressure at idle for better resolution on pulsewidth with large injectors. The second is to avoid effectively losing flow rate when manifold pressure climbs. If manifold pressure goes up without rail pressure, the delta pressure (change across the injector itself) drops which in turn reduces the flow rate just when you need it most.

    The factory went away from referenced regulators because the return lines were splashing heated fuel back into the tank, increasing vapor pressure and evaporative emissions. They just revised the flow rate variable which used to be a single number to become a table that varies with manifold pressure in order to model the effective change in flow rate vs MAP.

    If you run a 1:1 regulator, just set the IFR table to a constant flat line at the appropriate value for your nominal reference pressure. You should also have the same values across all pressures for the injector offset too. (Offset will now only change relative to voltage)

    None of this has anything to do with whether you're pegging the MAF. That's a separate discussion altogether.

  6. #6
    Advanced Tuner angrygoat's Avatar
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    Yeah, I realize that I def need a 2bar SD.

    Just so I understand, there is definitely a benefit to running a 1:1 with larger injectors. It helps improve low pulsewidths control on larger injectors. Now since my tune was orignally set up to not use the 1:1, all I need to do to take advantage of the 1:1 is recalculate the IFR for 60lbs injectors at 58 psi., make sure my pressure is set to 58 and enter the results into all the cells of the IFR table. Any changes to VE or anything? How do I know what to set the injector offset to?

    Also, if there is such an advantage to using a 1:1 with large injectors, why would someone disconnect the vacuum line to start with?

    Thanks a ton gents
    Last edited by angrygoat; 03-19-2009 at 12:36 PM.
    Joe
    2006 M6 GTO
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    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  7. #7
    Advanced Tuner angrygoat's Avatar
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    can anyone offer some insight to the above?
    Joe
    2006 M6 GTO
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    10.76@131

    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  8. #8
    in the end you will need to retune the VE table. Because you haven't been using the boost ref'd reg the VE table will probably be richer in boost than necessary and probably a little lean at part throttle depending what kind of slope the tuner put on the IFR table.
    I count sheep in hex...

  9. #9
    Good information here. I often thought you would want to flat line the IFR table when using a vac/boost referrenced regulator. Looks like I will be making some changes to my tunes.
    Regards,

    Brian Turner

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  10. #10
    Advanced Tuner angrygoat's Avatar
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    from what Chris is saying that is what you want to do. You would scale your IFR if you were running one fuel pressure all the time
    Joe
    2006 M6 GTO
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    10.76@131

    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  11. #11
    The flow thru the injector is mostly linear with the pressure drop across the injector ie. the difference between the fuel pressure and manifold pressure. This means that if you double the pressure drop you double the flow rate, if you halve the pressure drop you halve the flow rate etc.

    Lets say your injector flows 60lb/hr at 58 psi (above atmospheric). This means that with the fuel pressure at 58psi and manifold pressure at atmospheric (0 psi) you have a 58psi pressure difference across the injector.

    Now in a standard fixed pressure fuel system like many of GM's the fuel pressure is constant @58psi. The only thing that affects the pressure difference across the injector is the manifold pressure or vacuum. Now, if your manifold vacuum is -5psi (keeping things in psi to make it easy) now the pressure drop across the injector is 63psi. This has a simple relationship to flow and the flow rate will increase to 60*(63/58)=65lb/hr. Its the same with boost if you have 5psi boost the flow rate will decrease to 60*(53/58)=55lb/hr. The important thing here is that the PCM needs to know about this changing flowrate, hence the IFR table.

    Ok so now you might think thats fine i can just keep raising my fixed fuel pressure to suit the maximum flow i need for my mad crazy FI setup, and sure you can but it has problems. The biggest being that at very high fuel pressures (and hence flowrates) it becomes increasingly difficult to get the injector pulse width down low enough for idle, light cruising and especially coastdown. Also at such high fuel pressure and low flows a lot of stress is being placed on fuel pumps and the whole returnless fuel system. So you might start to think hey wouldn't it be great if i could have a lower fuel pressure at low flow rates and a higher fuel pressure at higher flow rates.

    This is exactly what a 1:1 boost/vac referenced fuel reg does.

    Ok so with a FPR, the idea is to vary the fuel pressure by the same amount as the manifold pressure, this is the critical point to understand.

    This means that if the manifold pressure is 5psi (boost), then the FPR will increase the fuel pressure by 5psi. Looking at the numbers again our new pressure difference would be 58psi - the 5psi in the manifold + the extra 5psi from the FPR or 60*(58-5+5)/58=60 ie. the pressure drop across the injector remains the same and hence the injector flowrate remains the same regardless of the manifold pressure.

    So you can see that when the manifold is under vacuum the fuel pressure will drop and if it is under boost then the fuel pressure will increase. You will also notice this is exactly the situation you wanted to help with the idle problems the big injectors are creating.

    Many old skool engine builders will tell you that for FI, every psi of boost you add is the same as losing a psi of fuel pressure, this is not exactly the case. It would be more correct to say for every psi of boost you add the injector (fuel) flow rate is reduced by the same amount as losing 1psi of fuel pressure.

    Hope that helps!

    Chris...
    Last edited by Chris@HPTuners; 03-24-2009 at 05:36 PM.
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  12. #12
    Advanced Tuner angrygoat's Avatar
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    wow...great explanation Chris. Thanks

    Question, could not increasing the fuel pressure in relation to boost be part of the reason that I have to command a lower PE AFR than what I want to be at?

    Why would a tuner remove the vacuum line from a FPR if its so beneficial?
    Last edited by angrygoat; 03-24-2009 at 05:56 PM.
    Joe
    2006 M6 GTO
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    "The goal of tuning is for the tune to run well enough you dont need any corrective mechanisms"

  13. #13
    Quote Originally Posted by angrygoat View Post
    wow...great explanation Chris. Thanks

    Question, could not increasing the fuel pressure in relation to boost be part of the reason that I have to command a lower PE AFR than what I want to be at?

    Why would a tuner remove the vacuum line from a FPR if its so beneficial?
    For the first part yes you either need to command a lower AFR or more commonly you will find you are jacking up the airflow (MAF or VE) numbers to keep the fuel where you want it.

    I don't know why someone would fit a 1:1 FPR and then not use it on a forced induction application. I often ask my kids the same question about why they always put the TV remote beside the TV
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  14. #14
    Advanced Tuner angrygoat's Avatar
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    LOL...I understand. Great info and insight. Thanks again
    Joe
    2006 M6 GTO
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  15. #15
    Senior Tuner eficalibrator's Avatar
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    Quote Originally Posted by Chris@HPTuners View Post
    The flow thru the injector is mostly linear with the pressure drop across the injector ie. the difference between the fuel pressure and manifold pressure. This means that if you double the pressure drop you double the flow rate, if you halve the pressure drop you halve the flow rate etc.
    Close, but not exact from the physical side... They actually follow the Bernoulli Equation for flow through a fixed orifice where the flow rate is proportional to the square root of the pressure ratio.

    Final flow rate = (Original Flow Rate) * SQRT [(new pressure drop)/(old pressure drop)]

    Where "pressure drop" = (actual rail pressure) - (manifold pressure)

    The result is really a gentle curve that can be approximated as flat across very small changes, but it gets fairly nonlinear if you start doubling.

  16. #16
    yeah yeah i know i just try to avoid the word square root when trying to explain something in simple terms
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  17. #17
    it reminds me of a university math lecturer i had once who commented on the first day of complex number theory, "Some of you will leave here today amazed at how the square root of -1 created a whole new world of mathematics, some of you will leave here asking why wasn't the guy who asked 'What is the square root of -1' burned at the stake".
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  18. #18
    Advanced Tuner angrygoat's Avatar
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    mmmmmm..... imaginary numbers...LOL
    Joe
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  19. #19
    Advanced Tuner passingpower's Avatar
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    This thread should be immortalized as a sticky.

  20. #20
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    would I be correct in saying that I could get more flow out of my injectors using boost reference FPR.. (getting away with smaller injector when compared to larger one on constant fuel pressure)?
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