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  1. #1
    LTX Master


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    Default 24x Tuning with Fixed Fuel Pressure and Retrun Fuel System

    I'm building a 396 LT1, it has modified fuel rails with an external FPR set to static 60 PSI with no vacuum, but it is still a return fuel system. It is a 24x system with 411 PCM. I'm using HP Tuners. I have set the injector flow rate to match. Does anything else need to be changed in my tune? I'm just a little confused since the LT1 is referenced with vacuum and the LS1 is not, but the LT1 is a return system and the LS1 is not. I maybe over thinking this, someone bring me back down to earth.
    engine.jpg20170715_193250.jpg
    1995 TA: Converted Nose...381 LT1...4L80E...Moser 12 bolt...Edge TC...
    Best 1/4 10.71 @126, 1.46 60ft, 3450lbs
    http://www.youtube.com/user/MysteryBird1?feature=watch

  2. #2
    LTx Guru


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    Default

    I've always wondered about that. My system using the MoTeC ECU runs constant 58 PSI, with no vacuum compensation. The compensation would have to be done internal to the program, adjusting the programmed injector flow for MAP. I just never asked the tuner.

    With a fixed rail pressure, flow through the injectors is going to vary with MAP..... low MAP increases flow across the injector orifice, high MAP reduces flow. Mathematically, it’s a simple calculation to make. It would seem something internal to the program has to calculate rail pressure minus MAP (in PSIg), divide by rail pressure (is there any place where you enter the set rail pressure in the program?), take the square root, and multiply it times the programmed injector flow constant, and adjust the pulse width accordingly.

    No answer, just speculation. Will be interesting to see what the answer is.
    Last edited by Injuneer; 02-23-2018 at 08:23 AM.

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  3. #3
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    Default

    So my numbers in the PCM for IFR look like this for 42# injectors. I'm just not sure if this is correct for this situation, and/or does something else need to be changed.

    ifr.JPG
    1995 TA: Converted Nose...381 LT1...4L80E...Moser 12 bolt...Edge TC...
    Best 1/4 10.71 @126, 1.46 60ft, 3450lbs
    http://www.youtube.com/user/MysteryBird1?feature=watch

  4. #4
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    Default

    Assuming:
    - injector rated exactly 42.0 #/HR rated at 43.5 PSIg
    - rail pressure at exactly 60.0 PSIg
    - 100 kPa = 14.5 PSI

    Then subtracting negative vacuum from rail pressure = rail - (-manifold vacuum) = rail + manifold vacuum = injector DIFFERENTIAL pressure, I built an Excel spreadsheet. My numbers for flow vs. manifold vacuum are virtually identical to the numbers in your chart. Deviations are a fraction of one percent, and appear to be due to rounding to a single decimal point in your table.

    Looks good to me.

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

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  6. #5
    LTX Master


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    After much research I found my injector data from the manufacturer. From that data I was able to change the injector offsets vs. volts, minimum pulse, flow rate multiplier, and short pulse adder. I couldn't figure what to do with the short pulse adder numbers so I subtracted 10%. Feeling pretty confident now with these numbers.
    1995 TA: Converted Nose...381 LT1...4L80E...Moser 12 bolt...Edge TC...
    Best 1/4 10.71 @126, 1.46 60ft, 3450lbs
    http://www.youtube.com/user/MysteryBird1?feature=watch

  7. #6
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    Default

    Did you make your own base tune for your motor? About to start putting a tune together for my 24x swap and making sure what all needs to be changed from the ls tune to the lt tune.

  8. #7
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    Mike Storm "Stormy"
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    May I say that's one nicely put together engine bay.

    It's not how fast you mow. It's how you mow fast.



    1995 Trans Am - CAI - TB coolant bypass - Performance Muffler - Poly mounts - stock LT1/A4 - 13.3 1/4 - 1.9 60 foot , driven daily. .

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  10. #8
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    Quote Originally Posted by Jcvititow View Post
    Did you make your own base tune for your motor? About to start putting a tune together for my 24x swap and making sure what all needs to be changed from the ls tune to the lt tune.
    I had a mail order tune but it was so far off the car would not run and threw about 20 codes. I ended up buying HP tuners and an LM-2, then started researching all I can about 24x tuning. I'm not new to tuning but this is the first time doing a conversion.
    1995 TA: Converted Nose...381 LT1...4L80E...Moser 12 bolt...Edge TC...
    Best 1/4 10.71 @126, 1.46 60ft, 3450lbs
    http://www.youtube.com/user/MysteryBird1?feature=watch

  11. #9
    LTX Master


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    Quote Originally Posted by Stormyweather View Post
    May I say that's one nicely put together engine bay.

    It's not how fast you mow. It's how you mow fast.
    Thanks, I wanted to carry the black and red theme throughout the whole car.

    black thunder.jpg
    1995 TA: Converted Nose...381 LT1...4L80E...Moser 12 bolt...Edge TC...
    Best 1/4 10.71 @126, 1.46 60ft, 3450lbs
    http://www.youtube.com/user/MysteryBird1?feature=watch

  12. #10
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    Default

    Yea I'm in the same boat I have tuned ls cars and trucks but this will be my first lt engine.

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