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Thread: Custom ...747 TBI issue. Help!

  1. #1
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    Custom ...747 TBI issue. Help!

    I’m looking for help with tuning a custom …747 TBI system. This arrangement runs very nicely except during deceleration when I “blip” the throttle to double clutch downshift it has a “flat spot” and the revs don’t want to come up.
    Some background first; this is originally a 1960 Rochester fuel injection plenum converted to TBI by custom machining two TBI injector housings and inserting them before the throttle in the Rochester inlet. The object was to disguise it to fool the judges at car shows and so far that has worked. The original 3” throttle plate was way too large in that “tip in” made it too sensitive to drive without laying rubber every time trying to take off smoothly so I reduced the throttle bore to 2.120” which is much more controllable. In normal cruise or hard acceleration the throttle response is instant when jumping on it.
    The only difference, that I can see, is this flat spot seems to happen only following very high manifold vacuum where using the engine for braking and dropping down gears. I thought this vacuum was cutting the mixture too much so I doubled the pulse time at 0 and 20 Kpa in the accel enrichment table but to no effect. Note; in early experimentation, using a stock twin bore throttle body, it did the same thing. Looking for ideas. Thanks.

    DSCF2584.jpgplumbing.jpg

  2. #2
    Super Moderator dave w's Avatar
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    Very clever TBI / EFI modification!

    Perhaps you can post a TunerPro RT data log (.xdl file) of the TBI system in normal driving conditions?

    dave w
    Attached Images Attached Images

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    I have been having trouble recording data on my laptop and seems to be a hit or miss issue. I'm wondering if it would be recorded anyway since the stumble is for half a second or less only during throttle punches while engine braking (high vacuum) double clutch. Punching the throttle while accelerating off cruise vacuum yields instant response.

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    Last edited by JF in MI; 05-29-2022 at 09:33 PM. Reason: addition

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    Super Moderator dave w's Avatar
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    I'm wondering what the VSS data is?

    dave w

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    Quote Originally Posted by dave w View Post
    I'm wondering what the VSS data is?

    dave w
    That's easy; Nothing. This system is on an antique Corvette so is very basic. Only sensors are TPS, coolant temp, MAP, RPM.

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    Super Moderator dave w's Avatar
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    VSS input can appreciatively help with Declaration Fuel Cutoff (DFCO) and Stall Saver Routines.

    Perhaps Dynamic EFI is an option for replacing the 1227747? Dynamic EFI is Plug-n-Play with the 1227747 harness.

    http://www.dynamicefi.com/EBL_Flash.php


    https://www.dynamicefi.com/EBL_Choice.php


    http://www.gearhead-efi.com/Fuel-Inj...21-Dynamic-EFI

    dave w

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    Quote Originally Posted by dave w View Post
    VSS input can appreciatively help with Declaration Fuel Cutoff (DFCO) and Stall Saver Routines.

    Perhaps Dynamic EFI is an option for replacing the 1227747? Dynamic EFI is Plug-n-Play with the 1227747 harness.

    http://www.dynamicefi.com/EBL_Flash.php


    https://www.dynamicefi.com/EBL_Choice.php


    http://www.gearhead-efi.com/Fuel-Inj...21-Dynamic-EFI

    dave w
    In my limited understanding DFCO won't function at all without an VSS. In any case I have the DFCO RPM parameters set so it shouldn't come into play. I'll be trying another program tomorrow. Will keep all posted.

  8. #8
    Super Moderator dave w's Avatar
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    Quote Originally Posted by JF in MI View Post
    That's easy; Nothing. This system is on an antique Corvette so is very basic. Only sensors are TPS, coolant temp, MAP, RPM.
    IAC?

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    Quote Originally Posted by dave w View Post
    IAC?
    IAC seems to be working as it should. Have a few more programs to try.

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    Super Moderator dave w's Avatar
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    Quote Originally Posted by JF in MI View Post
    IAC seems to be working as it should. Have a few more programs to try.
    Perhaps the information in the attached .pdf will be helpful?

    dave w
    Attached Files Attached Files

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    Quote Originally Posted by dave w View Post
    Perhaps the information in the attached .pdf will be helpful?

    dave w
    Let me explain what I have been doing so far. I’m trying to adjust the program that was meant for a normal dual port GM TBI. The custom throttle body I made up is basically uncharted territory. Through some crude flow testing I thought it would be similar but it turns out to be considerably more volumetric efficient hence needing much more fine tuning. One of the biggest changes I had to make just to get it to run smooth at cruise is jack up the BPW a lot. I’ve been messing around with the acceleration enrichment and power enrichment numbers (both at high vacuum levels) with no noticeable change whatsoever. It seems to be getting incrementally better but on today’s last test ride I noticed the minute “stumble” seemed to occur between 2400 RPM and lower (above this was fine). I just noticed on the Main Fuel table #2 that the numbers at 2400 and 2000 RPM were 10 points lower than other RPM ranges. I compared this to a stock factory 5.7L engine and they were all the same. I brought these numbers up to match the others and will try it tomorrow. It’s possible that the cam simply won’t allow it to gulp that much at that low RPM however during normal acceleration, mashing the gas at that RPM gets instant response so I’m not sure.

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    Super Moderator dave w's Avatar
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    Quote Originally Posted by JF in MI View Post
    Let me explain what I have been doing so far. I’m trying to adjust the program that was meant for a normal dual port GM TBI. The custom throttle body I made up is basically uncharted territory. Through some crude flow testing I thought it would be similar but it turns out to be considerably more volumetric efficient hence needing much more fine tuning. One of the biggest changes I had to make just to get it to run smooth at cruise is jack up the BPW a lot. I’ve been messing around with the acceleration enrichment and power enrichment numbers (both at high vacuum levels) with no noticeable change whatsoever. It seems to be getting incrementally better but on today’s last test ride I noticed the minute “stumble” seemed to occur between 2400 RPM and lower (above this was fine). I just noticed on the Main Fuel table #2 that the numbers at 2400 and 2000 RPM were 10 points lower than other RPM ranges. I compared this to a stock factory 5.7L engine and they were all the same. I brought these numbers up to match the others and will try it tomorrow. It’s possible that the cam simply won’t allow it to gulp that much at that low RPM however during normal acceleration, mashing the gas at that RPM gets instant response so I’m not sure.
    I previously mentioned Dynamic EFI, the AFR Tuning screen shots below are from a Dynamic EFI system. The '7747 is a good ECM, and works well for engines that " are factory". For engines that "are not factory" Dynamic EFI is a very good option. AFR Tuning is strongly recommended for engines that "are not factory". The '7747 is not easily adaptable to AFR Tuning. Likely the Dynamic EFI computer processor will provide better performance

    dave w

    Lower AFR.jpg


    Upper AFR.jpg

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    In addition to the volumetric efficiency, you also have a much larger plenum that is not heated in the same way as the original TBI intake manifold. Generally when moving from original TBI to single plane carburetor style intake folks notice that the values simulating accelerator pump function need to be changed, sometimes quite a bit. The 7747 is not highly flexible in this regard. I would look at pump shot duration vs MAP and pump shot duration vs TPS to assist with this. The situation occurs when there is a large change in manifold vacuum so I would expect the vs MAP table to be very useful.

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    Quote Originally Posted by 1project2many View Post
    In addition to the volumetric efficiency, you also have a much larger plenum that is not heated in the same way as the original TBI intake manifold. Generally when moving from original TBI to single plane carburetor style intake folks notice that the values simulating accelerator pump function need to be changed, sometimes quite a bit. The 7747 is not highly flexible in this regard. I would look at pump shot duration vs MAP and pump shot duration vs TPS to assist with this. The situation occurs when there is a large change in manifold vacuum so I would expect the vs MAP table to be very useful.
    Yes, I agree with you on this 100%. I have increased the (accel enrichment) pulse duration in the 0-20 Kpa range 40% with no effect (thinking of trying 100%). Herein lies the part that puzzles me; At normal idle manifold vacuum (about 15 inches of mercury) when I punch the throttle it responds instantly (along with any off idle RPM) with no trace of a flat spot. Cruising, under any load and below 15" it also responds instantly. Only under deceleration, using the engine for braking where the manifold vacuum is 20" or higher, when I jab the throttle during a double clutch does it stumble with a flat spot. All other driving conditions are perfect. Only looks to be effected by transitioning out of high manifold vacuum.

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    If the calibration is going very lean during deceleration then it's likely the manifold has no fuel left by the time you press the throttle. This is a tough condition to detect and to tune around with 7747 / $42. The challenge is covering the difference from very low kPa MAP to high MAP without causing flooding during transitions from idle range kPa to high MAP. What is the range of MAP transition logged by the scantool when the problem occurs? I'm thinking above 60 kPa change? You may also want to adjust the lean decel so it is not quite as lean. In a carburetor where changing the pump shot is not as easy we might richen the lean stop, change a spring under a primary rod, or change taper on primary rods if those options are available.

    You may also want to think about IAC operation. IIRC $42 does not have a way to differentiate between idle and deceleration when VSS is disconnected. Closing throttle to decelerate when VSS is not present signals to ecm that engine should be at idle. ECM may consider idle speed to be above threshold and will close IAC to attempt to bring engine speed into line. Stepping on throttle (Again, IIRC, study Rob Rauscher's IAC info) trips ecm to open IAC for "throttle cracker." There are time delay values that could be manipulated to prevent or increase IAC response time in order to reduce airflow changes while throttle is changing. Are you using a manual transmission calibration? Comparison between manual and auto trans cals may suggest which direction to go.

    7427 is truly much more capable. 7427 is better able to handle a wider range of conditions imo. 7427 code is built with the knowledge learned while calibrating earlier ecm's.
    Last edited by 1project2many; 06-02-2022 at 09:20 PM. Reason: changed RPM to IAC (in italics) for clarity

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