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Thread: Seasonal Idle Fine Tuning

  1. #41


    Have you played with the IAC Offset at all?

  2. #42


    Yes, that's the plan, just waiting for the problem to occur again. It's working really well right now.

  3. #43


    It got hotter today and engine started to overshoot on return to idle. And the overshoot gets more aggressive as MAT gets hotter. More airflow fixes the problem. (None of these problems happen on a cooler 70°F day with more air mass). I think I've got two issues: I need more IAC opening when MAT goes up. And I need to change my IAC Ramp Down strategy so that I can depend on it.

    Zwede suggested in a different thread that I'm using IAC Ramp Down the wrong way and that's why I'm getting inconsistent results. The more I look at my datalogs the more I'm starting to believe him. But I don't think that change alone will fix it because I clearly have an airflow dependency on MAT too. I think I need to blend in the idea from allan5oh to make a custom table of IAC offsets.

    So I think what I'm going to try next is:
    1. Change my IAC settings to begin ramp down around 1500 RPM. Hopefully that will make IAC Ramp Down function more consistent on a day by day basis by keeping it out of normal cruise range.

    2. Make an Advanced 2D Table of IAC Offset based on RPM and MAT. This will be my airflow compensation for MAT. And I'm hoping I can also temper the engine descent down to 1500 RPM where Holley's IAC Ramp Down feature will take over and finish the controlled glide to idle.

    2b. Another idea instead of #2 above is to reset my throttle blade base airflow as the weather gets hotter and hotter until I've got a minimum airflow sufficient for the hottest of days. (And then hope the engine doesn't hang in cold weather like it's done in the past.)

    Now I do want to address something else: It's pretty common to read statements that you don't need IAC to control decent to idle because carbs don't have IAC. That's not true. Carbs have idle circuits that are always active and the carburetor is naturally over-damped which causes the engine speed to fall slowly. IAC is faster so it has to be controlled properly or you'll get these oscillations.
    Last edited by Qwktrip; 06-03-2020 at 10:52 PM.

  4. #44


    Try making the Advanced 2D Table CTS & MAT. Add more Offset with colder CTS and hotter MAT. Then your IAC Ramp Down matters less all the time. What really matters is the transition from Ramp Down to idling. It's closing too much or too fast. If you Offset it with an Advanced 2D Table, that becomes the new minimum, and your IAC Ramps Down to that number.

  5. #45


    I began doing that at first and switched to the MAT and RPM so I could control engine glide path at 0% throttle (if needed). That table has to be tuned at full operating temp. I figured if I wanted to fiddle with IAC Offsets during the engine warm-up cycle then I'd make another Advanced 1D Table of IAC Offset vs. CTS when the opportunity presents itself next winter. What are your thoughts on that?

  6. #46


    Very controlled descent to idle today. This might be the major breakthrough I've needed.

    Basically what I did was adapt the concept of an OEM style "Throttle Cracker" to Holley EFI. I made a custom 2D table of IAC Offset as a function of RPM and MAT. Think of it as a dynamic IAC Hold Position. I used datalogs to establish proper IAC values up to about 130F. Above that I don't know what it's going to need and hope I never have to find out.

    With the Throttle Cracker table in place I was able to place the RPM to Enable IAC Ramp Down at a low level below normal driving. This keeps the IAC from ramping down at the wrong times, which has been a big problem for me. The Throttle Cracker table sets up the proper IAC Position to start from for controlled ramp down, and then the built-in Holley IAC strategy finishes the job to settle the engine at idle.

    Also, my Learn Table is super tight now (under 2%) with the new Baro and Intake Air Temp compensation strategy. I'm hoping I'm on a good track with things getting really dialed in. It's been a long and frustrating process and I'm ready for a rest from it.
    Last edited by Qwktrip; 06-05-2020 at 04:36 PM.

  7. #47


    Can you show the setup and activation settings you're using for your Throttle Cracker table?

  8. #48


    There's nothing special in the setup, it's just always on.

    Last edited by Qwktrip; 06-05-2020 at 11:54 PM.

  9. #49


    Thank you. I'm going to be testing this out on a troublesome Sniper.

    What temperature are you setting your baseline IAC setting at and what are you setting the IAC at for your baseline?
    Last edited by Moparmatty; 06-06-2020 at 02:05 PM.

  10. #50


    My IAC Hold position is 15% because that's what my engine can tolerate in cooler weather without aid from the Throttle Cracker table. I already had that figured out in the past. Yours might be different.

    I do all the tuning with the engine fully heat soaked (coolant, oil, transmission). Don't even start tuning until after a 15 minute drive. Engine oil takes a long time to come up to temp (about 50x slower than engine coolant). You'll be surprised how much IAC behavior changes as the engine gets heat soaked.

    First rough pass at filling out the Throttle Cracker table was pretty simple. I datalogged a 30-40 minute drive while throwing all kind of situations at the IAC. I drove highway to do testing at lower MAT temp, and drove on 25 MPH road to get MAT up high. I studied all the overshoots in the datalog and kept track of how much IAC Position was needed to catch the engine during overshoot at various MAT. I took half the difference between that and my current Hold Position, and that number went in my Throttle Cracker table in the left column (RPM where hand-off occurs to the IAC Ramp Down strategy). For example, let's say IAC went to 25% to control overshoot. My IAC Hold position is 15% so, (25%-15%)/2 = 5% --> that number went in the table. Pretty decent starting point it seems.
    Last edited by Qwktrip; 06-07-2020 at 10:30 PM.

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