High impedance injector issues with 1227747

Discussion in 'The Injection Section' started by bp71k5, Mar 13, 2019.

  1. bp71k5

    bp71k5 3/4 ton status Premium Member

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    I'm using the Affordable Fuel Injection TBI injector tree which is using 2 Deka 840cc@43.5 psi injectors. (80#)

    I'm using a BPW spreadsheet to calculate what my BPW should be and it gives me ~134 which is close to the BPW I see in most of the stock bins for a relatively stock GM 7.4L. I'm running a moates emulator to start the engine and I can only get the engine to idle with 165 BPW, 65 psi, and the fuel table in the 50% range. This doesn't quite seem right given that many of these engines seem to do fine with 80lb injectors running with much less fuel pressure. It just seems to want much more fuel than expected and make me think something is not right.

    I wonder if there are some injector settings in the BIN that I might be overlooking given that these injectors are quite different than the originals? I see posts about injector voltage bias, and PW, but am not familiar enough with those to know if that's what I need to adjust. I can get the truck to run ok, it just seems like I’ve got some of these base settings off and I may just be OCD about getting things right.

    Appreciate any pointers.
    Brian
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Hopefully you get more qualified answers than mine lol.

    I know on the later 454 TBI setups they were using the 80PSI injectors, but I want to say that GM used a non '7747 with those applications? Is the injector impedance a change?

    Not sure why, but interwebs aren't working for me, was trying to see if the ~13PSI and 80PSI injectors were different impedance.
     
  3. bp71k5

    bp71k5 3/4 ton status Premium Member

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    I’ve been searching for a bit to know what the difference is, but nothing definitive. There must be something software related since 13psi + 80lb injectors work on the stock tune from GM. No reason I should have to increase fuel pressure 500% as well as increase the BPC to crazy amounts.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I cannot find ANYTHING with some decent searching on those 80PSI injectors. They are relatively rare, but I would think some info would be available!
     
  5. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Here’s what they sent with the original 60lb injectors. Supposed to be the same just flow a bit more.

    55324911-2268-438B-8C88-1B7288D70BEC.jpeg
     
  6. bigblock72

    bigblock72 Elmer's glue taste tester Premium Member GMOTM Winner

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    Try these numbers:
    Offset: 0.065ms
    Turn on time @ 14VDC: 1.3ms, Multiply the whole turn on time table by .9912 and it should get everything close.
    Turn off time: 0.7ms @ 600KPa
     
  7. bp71k5

    bp71k5 3/4 ton status Premium Member

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    You know what the settings are named? I’m using tuner pro so I wonder if the names are different? I owe you a huge thanks if it’s that easy.
     
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  8. bigblock72

    bigblock72 Elmer's glue taste tester Premium Member GMOTM Winner

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    I thought you had programmed what was shown in the table 5 already. :blush:
    Let me look and see what I can find.
     
  9. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Ah, no but I see people referring to those tables but haven’t been able to figure out what they’re referring to in the bin. Thanks!
     
  10. bigblock72

    bigblock72 Elmer's glue taste tester Premium Member GMOTM Winner

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    I just looked at the xdf and now I remember it's all handled by the BPW number. In this case the latency, opening and closing times are all quite a bit different than the TBI style injectors. You will just need to set the BPW to where your INT and BLM numbers are centered and tune from there. Don't be surprised if you have to change your "acceleration enrichment vs delta map and delta tps" settings as well. Having a small chip limits the number of available tables and is a handicap of these old computers. The nice thing is they are perfect to learn on since they aren't ridiculously complicated.
    This is not the computer to have if you can't keep your OCD in check. Adjust settings until it runs right and the BLMs are where they need to be and run it. If things aren't what you want then you will need to upgrade to a newer style TBI computer like a 16197427, or a megasquirt.
     
  11. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Ah, that’s helps a ton. At least it makes sense why the BPW needs to be so high.
     
  12. bp71k5

    bp71k5 3/4 ton status Premium Member

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    A bit of more info. I found a table called “injector correction multiplier vs voltage” and by increasing the values a little bit, I was able to richen up the mixture at idle enough to let me lower my fuel pressure to 50psi and the BPW to 175. Also gets my idle fuel table value at 40% which I’m fairly happy with. Still playing with it to see if there are any negative side effects but looks promising.
     
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  13. bp71k5

    bp71k5 3/4 ton status Premium Member

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    After some more fooling around with it, I just started over with a stock 7.4L bin and adjusted the BPW, injector multiplier, injector bias, and bumped the fuel pressure up to 60psi. The throttle body seemed to just be dumping gas into the engine by only using the injector multiplier at idle. So I figured adjusting a little bit of all those settings would be more likely to work out. The good news is that the injectors look to be spraying a lot less fuel, or it’s just atomizing better so maybe my fuel economy will improve from horrible to just pretty bad.

    Also adjusted the valves to make sure I wasn’t losing any extra vacuum. Seems to stay more consistently at 20in now, rather than 19/20.

    I did want to write this down for future reference since I spent 3 hours trying to figure out why the idle wouldn’t go below 1000 rpm. Tried tweaking every setting until I saw the “C/L idle RPM in ALDL” was set to 1000 rpm. Burned up so much gas forgetting about that dumb setting. :doah:
     

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