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Thread: Slight stumble in rev range

  1. #1
    Tuner in Training
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    Slight stumble in rev range

    Just wondering if anyone has any ideas on what could be causing a bit of a stumble or surging around 1500 revs while gently accelerating. I know there could be heaps of things, but any ideas would be appreciated.

    I have tried to attach a log and tune that shows where this happens. Hopefully that attached itself below! I am wondering if it's to do with the IAC valve opening and closing, as this seems to change up and down by 1 step or so between 1500-1550rpm. I have also attached my tuned that was loaded in while this short scan was done as I can't find anything else that would be enabling the IAC to be opening and closing while driving. I have noticed that it's still opening to 100 steps or so while travelling around 60KM/H, but I have got my throttle follower settings set to disabled as far as I can tell.

    Anyway, please have a look at the attached files and throw out any ideas you may have. I still have a fair bit of logging and tuning to go for my VE table to be right, then play with timing again as I feel like I have taken too much out cause it doesn't have the same pull as before, but I just need to get the low rev range smoother so I don't have my wife complaining that it's rough now that I have changed the stato from auto to manual!

    09.12.2018 7.hplWH2 Manual 06.12.2018.hpt

  2. #2
    Tuning Addict 5FDP's Avatar
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    Why is your timing so low?

    You should be well up into the 20-30's at that light of throttle.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
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    sort your spark table out, its all backwardsfinal 395rwhp.hpt

  4. #4
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    Thanks for the suggestions guys. I recently dropped all the timing values in the area up to 2000 revs to 13 degrees to match what I had in my idle tables. This was only done to make the timing stay at one value as I thought it could have been jumping large values in the transition from idle to main table. This made no difference, except dropped the power output!
    I will have a look at the file you attached over the weekend and see where mine is wrong. I did edit this table years ago by increasing and decreasing the timing via VCM scanner while driving along till I either didn't feel it increasing in pull or it started to ping. I then backed it off a degree or two to keep it safer.

  5. #5

  6. #6
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    copy the spark tables from attached tune above

  7. #7
    Those spark tables are pretty choppy in the 395 tune if it doesn't work for you, you can try this one.
    Attached Files Attached Files

  8. #8
    Senior Tuner kingtal0n's Avatar
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    if the motor was almost not in motion, 0.01rpm approaching zero
    what timing value would we choose?

    something like 20-60* after top dead center, right?

    So now speed the motor up.
    0.1rpm, 1rpm, 10rpm, just move the decimal around

    0.1rpm optimal timing: 35-45* atdc
    1rpm optimal timing:28-30* atdc
    10rpm optimal timing: 20-24*atdc
    100rpm optimal timing: 5-17* atdc
    1000rpm optimal timing: ...
    10000rpm optimal timing: ...

    generally, as rpm increases optimal timing moves to before top dead center
    in order to time pressure peak at optimal 4-stroke position in degrees
    the angular velocity of engine is compared to the speed of chemical reaction of combustion at high rotating frequency

    there is a crossover point from atdc to btdc and the fuel/temperature plays a role because that affects the rate of combustion reaction

  9. #9
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    They’re only choppy in the idle and coast down areas. Not in the initial throttle hit or cruise areas. I wasn’t getting the response I wanted from throttle cracker and follower table edits so I’ve used the idle and main spark tables to remove some timing in the areas populated in coastdown to a stop. The car is manual and this was what I needed to do to stop rev hang on coast down. It makes the table look horrendous, but it worked. It doesn’t reference the choppy areas at any time other than coast down

  10. #10
    I wasn't trying to bash your spark tables Luke. Just offering up some options for the OP.

  11. #11
    Tuner in Training
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    You shouldn't need to change the spark tables to get rid of the hanging rev on coast down. This can be done via the cracker/follower tables: multiply follower decay (gear) X 4, zero out follower delay (gear), divide cracker airflow by 3-4 (just enough to keep it running), zero out cracker airflow decay delay.

    My editor can't open your .hpt file, but you might try toggling the "M6 spark smoothing" setting.

  12. #12
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    I have finally gotten around to updating this thread! The timing tables haven't fixed my issue, but they have improved the smoothness of my acceleration a bit.
    One thing I have noticed is that I sometimes have a small amount of pinging around 1800 revs under light throttle. This is a bit more pronounced now that I have the manual in as I am always revving through this range. The Auto never seemed to go through this much unless I was at cruise, but I did notice a similar feeling around 80km with the auto, but I thought it was something to do with the auto or TCC lockup. Now I am thinking I have had it all along since putting this engine in.
    Another idea I have been thinking of is O2 sensor switch point tuning, as I have never looked into this. I am wondering if it's leaning out the 1600-2000 range causing it to hunt for the right AFR as well as ping due to not enough fuel in the cylinders.
    I will search around for how to log and tune the O2 switching, but if anyone has any links they know are really good on this subject it would be much appreciated.