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Thread: 7427 MPFI tuning

  1. #46
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    Quote Originally Posted by PJG1173 View Post
    from what I've read the flow rate shouldn't change much. with the vac ref FP since at idle manifold vac helps draw fuel out of the injectors so less pressure is needed to provide the same amount of flow as higer pressure with lower vac (higher Kpa) signals.
    Quote Originally Posted by EagleMark View Post
    But don't think theres any coding in MPFI ECM for this as it's handled by the regulater. The vacuum referenced FPR is lower at idle and high vacuum as vauum sucks fuel out, then when vacuum is not present or low like WOT it adds pressure to compensate. Keeping same deleivery of fuel across board.
    correct and correct. i actually forgot about the returnless style duel systems that use a FPR that isn't vacuum referenced. i know some LSx cars are like that and for applications i'm used to, the 3500 and 3900 V6 units are actually similar, they have an in-tank pump/regulator combo with no vacuum reference. pretty sure that they don't use a PWM fuel pump either....

    and it's pretty easy to allow the ECM to know that you're removing the vacuum reference to lower fuel pressure, just modify the VE table at the various MAP points.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  2. #47
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    Another thing I forgot to mention that I noticed when I unhooked the regulator. My WB AFR stayed closer to 14.7 while driving/crusing like it did with my old tbi. As I have been running the mpfi with the vac refrenced fpr, my WB AFR wanted to stay at around 13.5 at both idle and most all driving conditions, and my blm's stayed in the high 130's to low 140's. When I unhooked the vac line, my blm's dropped to the mid to high 120's and the WB AFR jumped back up to around 14.7.
    So I dont know why is running way better with the vac line off, because according to yall, it shouldnt be?


    Heres one thing I just thought about. Dont know why I didnt think of it before, but the more I think of it, the more it makes sense. When I entered the injector flow rate into the pcm, I entered the flow with them at the max (no vac) setting, (what they flowed at 45 psi). Then when you run the engine and you get vac, pressure and therefore flow is less. I'm thinking this is screwing with things.
    SO, what if I run the engine till warm, hook up my pressure gauge, see what the pressure is at idle with the vac line hooked up, then calculate out what the injectors flow at that pressure, and then input that number into the injector flow rate?? Seems bpw calculation may be a lot better and closer to what they should be at idle and high vac/low load conditions, then when vac drops and you need more fuel, the pressure and flow go up and you get the fuel you need.
    Make sense to anyone?
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  3. #48
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    Quote Originally Posted by JeepsAndGuns View Post
    When I entered the injector flow rate into the pcm, I entered the flow with them at the max (no vac) setting, (what they flowed at 45 psi). Then when you run the engine and you get vac, pressure and therefore flow is less.
    while this is true, it's only if you go by the gauge.... you need to go by absolute values.

    the vacuum referenced FPR's job is to lower the fuel pressure seen by the injector to account for manifold vacuum. while this will appear as lower pressure, it's actually compensating to maintain the same amount of absolute pressure at all vacuum levels.

    take an injector that's being pressurized with 45PSI of fuel, shooting into a manifold that's at barometric pressure(~100kPa for the example). you have 45 gauge PSI on the injector and 0 gauge PSI in the manifold.

    now, take the same 45PSI of fuel pressure and shoot into a manifold with 15" of vacuum in it. now your injector is flowing a LOT of more fuel since the pressure differential between the pressurized side of the injector and the intake side are now even greater(greater differential = greater flow).

    it's a PITA to think about, but that's why none of the vacuum referenced MPFI masks use any kind of injector flow compensation based on MAP. it's also why the early MAF setups that use no MAP sensor at all work relatively well.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  4. #49
    Fuel Injected! JeepsAndGuns's Avatar
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    I understand what your saying about pressure diffrence and the need for the vac fpr, on paper it makes sense. I remember reading about it when I was in the planning and parts gathering stage. It was explained to me back then, thats the main reason I went ahead and got the vac refrenced fpr, and hooked up the vac line.
    But what I'm really having trouble figureing out, is why unhooking the vac line from the fpr and running a static pressure made so much of a diffrence, in both the way my engine ran, my blm's and my WB AFR, all got better.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  5. #50
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    wouldn't know....

    and honestly, if the engine likes it, by all means, do it. that's what tuning's entire purpose is.

    the only real issue i could think of by not running a vacuum references unit is in near-idle conditions if you have large injectors, since the PWs can get into the range of the injector being inconsistent.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  6. #51
    Fuel Injected! JeepsAndGuns's Avatar
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    I havent decided yet to run without it. My new/bigger injectors should be here this week, so once I put them in, I will probably hook it back up and do some tuneing and playing around with it and see how it goes.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  7. #52
    Fuel Injected! JeepsAndGuns's Avatar
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    Ok, got my LS6 injectors during the week and had some time this afternoon to put them in and do some tuning. I went ahead and hooked the vac line back to the fpr. I also left the fuel pressure at 45psi, mainly cause I was too lazy to set it back down to 43.5. Adjusted my bin to the new injector flow rate, and lowered the high map areas of my fuel map down to mid 90's.
    Fired it off and took it for a spin. Surprisingly, the blm's were not to far off. From about map 40 down to 20 I had to lower a bit, and the map 90-100 I had to raise back up to about 96-97. I did those changes and then did another log, blm's all seem good, lowest was 120, highest was 129. I may just call that good.
    I will say this, with these bigger injectors, the engine seems to run smoother during accel, and has noticeably more get up and go. Guess the old ones were just not quite big enough to give me the fuel it wanted.

    And oh yea, my clutch slips
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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