Thread: inj DC% for 1 or both tbi injectors

1. 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  Reply With Quote

2. 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.  Reply With Quote

3. 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.  Reply With Quote

4. 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.  Reply With Quote Posting Permissions

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