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Thread: inj DC% for 1 or both tbi injectors

  1. #1
    Fuel Injected!
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    inj DC% for 1 or both tbi injectors

    Trying to figure this out to get the correct readings and adding templates to my .adx file.
    I have this for the inj DC% and have seen it in multiple adx files but from reading on the forum I have heard different results as it only reads 1 injector and you have to double it.
    <MATH equation="(X*4*Y)/1200">
    I have rad to possible add multiply by 2 needs to be added.
    I ask because I get max mid to high 50's for inj DC% and if this is the case and I need to multiply by 2 for 2 injectors then I am over 100%
    I get high 11's to low 12's with the wbo2 at WOT but the fuel psi is dropping about .5, trying to figure out if I need a bigger pump(have tpi pump with 65pph inj @ 24psi 350hp 355ci motor) or inj are just pushed too far and need more psi at the vfpr.
    I get high rpm very slight degraded performance like a miss or under powered for split sec.

    So if anyone has any info on the equation that would be great.

    thanks for your time

  2. #2
    Fuel Injected! PlayingWithTBI's Avatar
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    It seems to me it doesn't matter since these injectors are running synchronous so, 50% for one would be 50% for the other. Or am I looking at this wrong?

    Here's how it's calculated in the EBL Flash2, quoted from RBob when I asked about my injectors running 109% DC

    As for the DC%, it is correct. Here is the math:

    5200 RPM, divide by 60 to get revs-per-second:

    5200 / 60 = 86.667 revs-per-second

    As the TBI injectors are fired alternately on each spark plug firing (in sync mode). Double the above revs-per-second for how often a TBI injector is fired on an 8 cylinder:

    86.667 * 2 = 173.333 times per second

    Now invert by dividing the value of 1 by the above firings per second. This gives the time base (frequency):

    1 / 173.333 = 0.00577 seconds

    Multiply by 1000 to convert from seconds to milli-seconds (msec). This is the time between injector firings, anything higher and the injector is always open:

    0.00577 * 1000 = 5.77 msec

    Now divide the actual injector PW (6.3 msec) by that value:

    6.3 / 5.77 = 1.09

    Convert to percent by multiplying by 100:

    1.09 * 100 = 109 % duty cycle

    I was going to add that this is easy as pie, but pi didn't come into this... (bad joke).

    RBob.
    Last edited by PlayingWithTBI; 6 Days Ago at 08:45 AM.

  3. #3
    Fuel Injected!
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    I get your point. but is 1 injector = 50% then 2x = 100%
    but if 2x = 50% then 1 = 25% and if this is the case something is wrong.
    so if 1 injector is the equasion I posted and I can hit 59% @ 5200rpm then I am at 118% total. is each injector not suppose to go over 85% duty cycle or both together?
    I did order a vortec fuel pump acdelco EP-381 form summitracing just to make sure it is not the pump since it is 8yrs old and the tpi one and I got it from tbichips not knowing better back then so not sure if it is acdelco or aftermarket junk.

  4. #4
    Fuel Injected! PlayingWithTBI's Avatar
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    I get your point. but is 1 injector = 50% then 2x = 100%
    but if 2x = 50% then 1 = 25% and if this is the case something is wrong.
    No, 1 injector @ 50% DC is fueling 1/2 of the cylinders, 2 injectors are fueling ALL of the cylinders so (in sync mode), you're at 50% DC.

    I did order a vortec fuel pump acdelco EP-381
    That's what I have - when my engine was running 109% DC I had an 18lb spring in my regulator, I went to a 28lb spring out of a 94 7.4 TBI (and AFPR set to ~27-28psi). Now I'm at ~75% DC at 5200RPM WOT.

    Edit; I'm using 61Lb/Hr injectors.
    Last edited by PlayingWithTBI; 5 Days Ago at 10:46 AM.

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