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Thread: Some simple GM TBI Questions

  1. #1
    Carb and Points!
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    Some simple GM TBI Questions

    While being a very experienced tech and trainer, I have never dipped my toes into ECM programming before, but the time has come. I've been wanting to do this for years, and now I am in a position where I want to dive in.

    Project at hand is an old V-12 Flathead that I want to TBI Fuel-Only Inject
    Initially plan on using a 4.3/5.0L TBI Single and then possibly duel 2.8L TBI's later.

    First question is while I have found other solutions that would allow me to TBI this application, can the GM 1227747 for example, be run in Fuel-Only mode?

    If so how is that done? is it turned off in programming?
    Is the ignition output just not used?

    As this engine utilizes basically 2 6 cyl ignition systems I'm assuming a single coil input can be used for the rpm signal.

    I have spent countless hours digging through site after site and thread after thread and have not found anything to confirm these questions so I decided, at the risk of being flamed to just go ahead and create this post asking for help.

    My plan is to fully document my progress in a step-by-step fashion to share with others who may wish to finally ditch the fuel toilet(s) which on Flatheads, due to the very low fuel pressure and the fuel pump mounting design are very prone to vapor lock issues.

    Thanks for giving me a shot and I appreciate all the knowledge here.
    Regards,
    GT

  2. #2
    Super Moderator dave w's Avatar
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    The 1227747 can be fuel only. The 1227747 needs an RPM reference pulse from a GM Ignition Module or a suitable square wave tachometer input to trigger the injectors on / off.

    TBI triggers the injectors "Once Every Distributor Reference Pulse (DRP)" The Injector Base Pulse Width (BPW) would optimally be 135 like the factory setting, attached spreadsheet is helpful with BPW.

    GM TBI bodies are generally 535 CFM (4.3 / 5.0 / 5.7) and 670 CFM (7.4). Using one throttle body will simplify the V-12 installation, because the 1227747 has TWO Injector Drivers. Likely the 1227747 will require electronic modifications to run FOUR TBI injectors.

    Dynamic EFI has good information about Injector Drivers: https://www.dynamicefi.com/InjectorDriving.php Copy / Paste Information from Dynamic EFI Below.

    The stock TBI ECM will drive:
    2 TBI injectors, 1 on each driver
    4 port saturated injectors, 2 on each driver (in parallel)
    4 port peak-and-hold injectors, 2 on each driver (in parallel)
    6 port saturated injectors, 3 on each driver (in parallel)
    8 port saturated injectors, 4 on each driver* (only if the injector resistance is greater then 14 ohms) (in parallel)
    * saturated port injectors range from 12 to 17 ohms. FMS/Bosch are typically in the 12 ohm range. AC Delco/Delphi are typically in the 16 ohm range.
    *A simple modification to the TBI ECM will allow any 8 port saturated injectors, 4 on each driver (in parallel). This mod is required for saturated injectors in the 12-14 ohm range.

    With the 4-injector upgrade (a Dynamic EFI exclusive) the TBI ECM will drive:
    4 TBI injectors, 2 on each driver (in parallel)
    6 port saturated injectors, 3 on each driver (in parallel)
    6 port peak-and-hold injectors, 3 on each driver (in parallel). Not exactly correct as-is, this can be changed to suit if a user is running a 6 cylinder with PnH port injectors.
    8 port saturated injectors, 4 on each driver (in parallel)
    8 port peak-and-hold injectors, 4 on each driver (in parallel)
    There are other combinations that will work. However, they are in the "this is not the best the way to do it" book.

    Link to Dynamic EFI 4 Injector Upgrade: http://www.dynamicefi.com/4-INJUpgrade.php ***Note that this is only an upgrade to the EBL Flash ECM hardware ***

    dave w
    Attached Files Attached Files

  3. #3
    Carb and Points!
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    Great info Dave. As I gather more info in order to make an informed decision, your response will most certainly help in that regard. Is there a signal conditioner I can use to enable simply tapping into the ground side of one set of points in order to drive the injectors?
    Regards,
    GT

  4. #4
    Super Moderator Six_Shooter's Avatar
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    Honestly with a 12 cylinder engine, I wouldn't even look at a GM ECM. AFAIK, there is no setting and CALPAK/MEMCAL for any OBD1 GM ECM, especially a TBI ECM that will have settings to properly divide the signal of a 12 cylinder engine.

    Yes you can have a 6 or 8 cylinder ECM setting run a 12 cylinder, the RPM will be off, 33% to 100% higher than actual as far as the ECM sees. This doesn't sound bad at first, but with a lot of settings being based in RPM, and some of these have max values they can use, this CAN be an issue. VE tables typically only go to about 4000 RPM, which could be at 2000 to 3000 actual crank RPM, so some drivability issues could be seen, cranking RPM, DFCO settings, among others.

    Also, the '7747 can run 4 injectors, you just need to run pairs in series.

    I would look at an aftermarket ECU in this case, which can allow for timing control as well and much easier than trying to fudge a GM ECM settings.

    A Megasquirt is what I would look at, the MS2 (Megasquirt 2) variant can run TBI injectors (It has Peak and Hold injector drivers built in), it can be set for 12 cylinders, and with board mods can be used for 12 cylinder DIS wasted spark ignition, or drive a single coil with proper timing control for a 12 cyl, e fan control, generic outputs, along with plenty of other features. an MS3 has even more features.

    I love the GM engine control, but sometimes it's just not the right choice. I also avoid the '7747 as much as possible, mainly due to the agonizingly slow datalogging (160 baud) that means you can miss transients because it was between packets.
    The man who says something is impossible, is usually interrupted by the man doing it.

  5. #5
    Super Moderator dave w's Avatar
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    Quote Originally Posted by ProfessorGT View Post
    Great info Dave. As I gather more info in order to make an informed decision, your response will most certainly help in that regard. Is there a signal conditioner I can use to enable simply tapping into the ground side of one set of points in order to drive the injectors?
    Points Ignition with the MSD Ignition Box TACH output will trigger a 1227747: https://documents.holley.com/6425.pdf

    dave w

    1227747 MSD Trigger.jpg
    Last edited by dave w; 11-19-2021 at 09:07 PM.

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