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Thread: TBI Injector DC%

  1. #1
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    TBI Injector DC%

    TunerPro shows my W.O.T. injector DC% as being over 50%, I'm assuming that's for one of the two injectors?
    With the stock 55 lb/hr LO5 injector should I look at increasing the fuel pressure and redoing my VE table to compensate?

    In the attached screen capture my injector base pulse width looks like it's decreasing as RPM rises, is that correct?

  2. #2
    Fuel Injected! jim_in_dorris's Avatar
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    The duty cycle is the same for both injectors, it is one value. You should not worry about DC until it is over 90%. you are fine with the DC.
    Square body stepsides forever!!!

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    Quote Originally Posted by Kitch View Post
    TunerPro shows my W.O.T. injector DC% as being over 50%, I'm assuming that's for one of the two injectors?
    With the stock 55 lb/hr LO5 injector should I look at increasing the fuel pressure and redoing my VE table to compensate?

    In the attached screen capture my injector base pulse width looks like it's decreasing as RPM rises, is that correct?
    Your duty cycle calculation is wrong!!! At 7 msec at 4,000 rom you are very close to 100% duty cycle. With two injections per crank rotation at 4,000 rpm you have 133.33 injections per second. 1000 ms per second gives you 7.5 ms between injections. It looks like they are not doubling the value for two injections per rotation. 7.06/7.5100 = 94% DC
    Last edited by Fast355; 08-23-2015 at 07:31 AM.

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    Hi Fast,

    From what I had previously read it sounded like I should expect the stock TBI LO5 injectors to be getting maxed out.
    I was noticing a trend while using the spreadsheet to correct my VE table, the corrections were working well except at the 90-100kpa end of the table where it kept staying lean. The fuel pressure gauge would read nice and constant and if tweaking the VE table was making no difference then I was wondering if that was all the injectors had to offer!
    From the two attached images what variable do yo think needs to change, something in the DC% equation or is there a problem in the actual base pulse width calculation?

    Incorrectly displayed DC% aside, from a tuning perspective is increasing the fuel pressure the next step?

  5. #5
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    Quote Originally Posted by Kitch View Post
    Hi Fast,

    From what I had previously read it sounded like I should expect the stock TBI LO5 injectors to be getting maxed out.
    I was noticing a trend while using the spreadsheet to correct my VE table, the corrections were working well except at the 90-100kpa end of the table where it kept staying lean. The fuel pressure gauge would read nice and constant and if tweaking the VE table was making no difference then I was wondering if that was all the injectors had to offer!
    From the two attached images what variable do yo think needs to change, something in the DC% equation or is there a problem in the actual base pulse width calculation?

    Incorrectly displayed DC% aside, from a tuning perspective is increasing the fuel pressure the next step?
    Duty cycle equation is wrong. For an easy fix multiply by 200 rather than 100. That equation is assuming one injection per rotation and TBI has two meaning half the time available or two times the duty cycle.

    At close to stock fuel pressure I would expect a TBI engine especially with headers to be running out of pulsewidth at higher RPMs. Next step is to increase pressure or injector size. I prefer increasing pressure for mild changes.

    Your stock injector is 61# @ 12 psi. At .5 bsfc and 85% duty cycle they are good for ~210 hp. The 55# in the 305 are good for about 185 hp worth of fuel. The 46# 4.3 injectors are good for 165 hp. GM closely sized the injectors and fuel pressure for the HP of the stock engine and did not leave much room for added airflow and fuel requirements at the stock fuel pressure or injector size. I have had a L03 F-car that headers and a catback required me to toss 350 injectors into it to keep it from running lean and static as early as 3,500 rpm.
    Last edited by Fast355; 08-23-2015 at 10:15 PM.

  6. #6
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    That was quick!

    I'd previously tried the multiplier change to 200 and it it worked pushing my DC% off the graph above 100% but I thought that was too easy and changed it back

    I take it that upping the fuel pressure means a VE table redo?

    Do you think my base pulse width mV's falling off as RPM rose was because the injectors were working so hard or would there be another reason?

  7. #7
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    Quote Originally Posted by Kitch View Post
    That was quick!

    I'd previously tried the multiplier change to 200 and it it worked pushing my DC% off the graph above 100% but I thought that was too easy and changed it back

    I take it that upping the fuel pressure means a VE table redo?

    Do you think my base pulse width mV's falling off as RPM rose was because the injectors were working so hard or would there be another reason?
    Base Pulse Msec follow the VE table. If you have 99% @ 3,000 and drop to 85% at 5,000 your base pulse will drop but the duty cycle can still increase because RPM has increased and time between firings has decreased.

    If you are talking O2 mv pushing the injectors over 85-90% DC will make them go static and really do some weird things to your air/fuel ratio.

  8. #8
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    Raised fuel pressure to 15 PSI and sure enough it ran rich and felt sluggish on the throttle, logged the run, ran the VE table through the spreadsheet, uploaded to PCM and way better! I'll filter the VE table a few more times and see what difference a couple of PSI can make.

    Is reworking the VE table to compensate for the change in fuel press the best way to deal with it? While it no doubt works, I couldn't help but wonder if there was some variable related to injector flow somewhere that I could change to represent the increase in fuel pressure and the PCM could automatically compensate?

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    There is an injector constant which represents injector flow and engine displacement. Typically changing this constant alone is not enough to address all performance changes that occur because of pressure changes.

  10. #10
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    Quote Originally Posted by 1project2many View Post
    There is an injector constant which represents injector flow and engine displacement. Typically changing this constant alone is not enough to address all performance changes that occur because of pressure changes.
    TBI PCMs like the 7427 have an actual injector flow value in the constants. Like you said he will have to make some changes to straight pulsewidth tables like cranking fuel and AE to name a few. Might also have to adjust the injector compensation table to keep the VE table from belling and proportional gains to keep the engine from surging.

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    I didn't know what ecm/pcm he's using. The C3 TBI's (and the 7749) use the combined constant. It was such a blessing when we got the 7427 and had a "real" injector flow value. AE is time based and isn't corrected with the injector constant.

  12. #12
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    Quote Originally Posted by 1project2many View Post
    I didn't know what ecm/pcm he's using. The C3 TBI's (and the 7749) use the combined constant. It was such a blessing when we got the 7427 and had a "real" injector flow value. AE is time based and isn't corrected with the injector constant.
    I just remember it being a mid 90s G-van.

  13. #13
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    It's a 8625 or basically the same as a 7427 running a $OD MASK ID.

    I had a look and found the injector flow rate (TBI lb/hr) as per attachment. :-)
    A quick search for a formula and it looks like changing from 13.0 PSI up to 15.2 PSI should increase my stock injector flow rate from 61lb/hr to 66lb/hr (assuming the stock injector gets it's rating @ 13PSI?). Can I just change the "Injector flow rate" to 66lb/hr and then start to rework the VE table again?
    Sounds too easy, what am I missing here?

  14. #14
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    I'll be interested to hear the answer. The way I thought it would work is if you upped the FP you would be rich and the ve tables would need to be lowered. If you entered the new flow in the parameters things should be close on ve with very little ve work required. The base pulse would have mathematically changed but the ve's should remain close. I will be interested to see if I am on the right track or don't have a clue.
    Last edited by myburb; 08-26-2015 at 02:32 PM.
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  15. #15
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    I think this will work as you've just outlined. Even though just tweaking the VE table seems to work it felt like a very round about way of doing it.
    Changing the fuel pessure and then changing a constant related to increased fuel flow sounds better than faking the efficiency of the engine to compensate but I too am on the steep part of the leaning curve.

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