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Thread: Fuel Rail Pressure's relationship to fuel economy.

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    Default Fuel Rail Pressure's relationship to fuel economy.

    Is there a relationship between the Actual Fuel Rail pressure and the amount of fuel required to propel the truck down the road? I am debating this with myself as to whether I can prove that fuel rail pressure can be tuned up to run at a higher level and actually increase the fuel economy. I have sucessfully raised fuel rail pressure and decreased the required pulse width to idle esp. This in theory (kinda) would prove the be a sort of control for the experiment.

    My problem is proof. I have to have a computation on what happens at the injector level when FRP is increased 10% or even 20% at idle. 30-35 etc etc. If my injector pulse width is decreased the same 10% or 20% did I even gain anything in terms of a decrease in the amount of fuel used. I know that as you increase pressure, flow is also increased. If anyone can help me out on the injector fuel flow data at different pressures I would appreciate it.

    I also have to figure out how timing plays a part in the fuel rail pressure equation. I know it's the same as adding timing.. or so I was told. I have an older FRP-PW-Timing calc.

    Part 2 is if frp even matters, or if it can be tuned better with timing to control cylinder pressures for the same economy. At a point you can't get blood from a turnup.
    2007.5 RCLB 4x4 WT LMM 8594OS

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    In my tests so far i have found on my LBZ that the factory FRP is just about right on the money for MPG (in the cruising range) i did find also that raising FRP and advancing timing at idle did decrease fuel consumption quite considerably.
    '07 TBSS, LS2, T42, 3SS, Black on Black on Black, Pretty much stock at the moment. EFILive will fix that

    '06 LBZ/Allison 6speed, EC, LB, 1LT, Leather
    Current Mods: EFI Live/ PTO High Idle Mod/ Factory Exhaust Brake / BD Full Bore/ TransGo Jr./ TTS Twin Lift Pumps/ MBRP 4" turbo back/ Custom air box mod/ BullyDog Outlook Monitor

    465hp/1008tq---7/25/09---Dyno Day Fast Specialties Tuning by ME

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    Thanks for the reply. As pressure increases and as pulse width decreases, how do we know that the final fuel quantity also decreases?

    I also found that decreasing the idle speed to 600 also decreased fuel consumtion a lot more and also idles smoother and with a nicer deeper rumble than before.
    2007.5 RCLB 4x4 WT LMM 8594OS

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    Quote Originally Posted by duramaximizer View Post
    Thanks for the reply. As pressure increases and as pulse width decreases, how do we know that the final fuel quantity also decreases?

    I also found that decreasing the idle speed to 600 also decreased fuel consumtion a lot more and also idles smoother and with a nicer deeper rumble than before.
    Yes i concur. I have lowered mine to 630-640. 600 i thought was just a little too low. (oil pressure)

    and the answer to your question lies withing the equation

    fuel flow (v) is equal to injection pulse width (t) times the square root of pressure (p).


    V=t*sqrt(p)


    a simple calc_pid will yield a plethora of valuable information.
    '07 TBSS, LS2, T42, 3SS, Black on Black on Black, Pretty much stock at the moment. EFILive will fix that

    '06 LBZ/Allison 6speed, EC, LB, 1LT, Leather
    Current Mods: EFI Live/ PTO High Idle Mod/ Factory Exhaust Brake / BD Full Bore/ TransGo Jr./ TTS Twin Lift Pumps/ MBRP 4" turbo back/ Custom air box mod/ BullyDog Outlook Monitor

    465hp/1008tq---7/25/09---Dyno Day Fast Specialties Tuning by ME

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    Lifetime Member killerbee's Avatar
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    consider that there is a mechanical latency in the pulse width measurement. 800us is not double the fuel of 400us. The latency measurement appears to shorten with increasing pressure, as if this problem wasn't complicated enough.

    FWIW, I typically have not increased pressure, in fact, on some platforms I lower pressure at lower loads. So I use a wider range of pressure, spread across a range of loads. I do this with a timing adjustment that allows for greater economy, and lower noise.

    Your post is a good discussion starter
    Michael, Systems Engineer 04.5 D-max LLY, Phoenix, Arizona Email
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    Quote Originally Posted by bballer182 View Post
    Yes i concur. I have lowered mine to 630-640. 600 i thought was just a little too low. (oil pressure)

    and the answer to your question lies withing the equation

    fuel flow (v) is equal to injection pulse width (t) times the square root of pressure (p).


    V=t*sqrt(p)


    a simple calc_pid will yield a plethora of valuable information.
    Is there a way to make that calc_pid? I would like to be able to monitor that. I have been curious on this very topic as well.

    In Michael's post.... would lowering pressure not decrease power and economy? Would you have to couple that change with an adjustment in pulse for proper atomization?

    Also, when increasing pressure but not decreasing pulse, I have found that the truck appears to override my pressure adjustments for the most part and utilize values close to stock, am I seeing that right? So in order to actually get a noticeable change in pressure when cruising I have no choice but to shorten pulse time? I'm still experimenting but hopefully this will speed up my learning curve. Thanks
    Peace Through Torque

    Punisher Performance DSP5 Tuning, PPE Dual CP3's, 68mm Fleece Cheetah, Punisher Built trans w/ Suncoast Parts, Airdog 165, SoCal 100% Overs, ARP Studs

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    Lifetime Member killerbee's Avatar
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    Quote Originally Posted by LBZoom View Post
    Would you have to couple that change with an adjustment in pulse for proper atomization?
    You are asking all the right questions and the answers are not so intuitive.

    Remember firstly and lastly, for a given set of conditions, and total drag, the dmax is going to find the power to exactly offset the drag for steady state conditions. If you lower pressure, and do nothing else, then the truck will have to have more pulse. Or you will have to command more pressure with pedal. Keep digging into these concepts. It is a great (albeit painful) learning curve.
    Michael, Systems Engineer 04.5 D-max LLY, Phoenix, Arizona Email
    Custom Tuning Services
    DURAMAX/CUMMINS ECM's TCM's for sale, all years, 20% wholesale discount available
    EFILive's first VGT "Software Wastegate Tune"

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    It might be such that you have to have a pilot and a post for idling and no main. Get the smallest shot possible to light a charge and then a post to give it enough downward momentium to keep to running smooth.
    2007.5 RCLB 4x4 WT LMM 8594OS

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    Lifetime Member LBZoom's Avatar
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    Quote Originally Posted by killerbee View Post
    You are asking all the right questions and the answers are not so intuitive.

    Remember firstly and lastly, for a given set of conditions, and total drag, the dmax is going to find the power to exactly offset the drag for steady state conditions. If you lower pressure, and do nothing else, then the truck will have to have more pulse. Or you will have to command more pressure with pedal. Keep digging into these concepts. It is a great (albeit painful) learning curve.
    Thanks, I'm never finished, only finished for a few days at a time haha.
    Peace Through Torque

    Punisher Performance DSP5 Tuning, PPE Dual CP3's, 68mm Fleece Cheetah, Punisher Built trans w/ Suncoast Parts, Airdog 165, SoCal 100% Overs, ARP Studs

    2013 NHRDA Sportsman World Champion

    www.PunisherPerformance.com

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    Senior Member DmaxHawk's Avatar
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    Quote Originally Posted by killerbee View Post
    consider that there is a mechanical latency in the pulse width measurement. 800us is not double the fuel of 400us. The latency measurement appears to shorten with increasing pressure, as if this problem wasn't complicated enough.

    FWIW, I typically have not increased pressure, in fact, on some platforms I lower pressure at lower loads. So I use a wider range of pressure, spread across a range of loads. I do this with a timing adjustment that allows for greater economy, and lower noise.

    Your post is a good discussion starter
    Micheal, I was under the impression that increasing pressure would better atomize fuel and therefore create a better burn. Or is it not much of a difference?
    07 El Bee Zee
    Little faster than stock...

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