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Thread: $88 Rpm derivative spark control. What it's it?

  1. #1
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    $88 Rpm derivative spark control. What it's it?

    In $88, how does "rpm derivative spark control" work. I enabled it, but car bucked like a bull. I figured I'd learn more about it before i break something :)
    Ron

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    If idle RPM falls outside a specified window from desired, spark advance will be increased or decreased to correct idle speed. The delta advance and the upper and lower limits of the window are configurable.

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    Shannen, sounds like you're referring to the idle spark correction stuff, RDSC is a bit different... appears to be an arbitrary spark offset, used during periods of rapid RPM change, can be both additive and subtractive, depending on which way RPM is heading.

    I've only ever seen it used with manual transmission applications, and even then, not all of them.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    My apologies. Yes, I was referring to spark timing rpm control. If RDSC is an arbitrary amount of offset then it is likely a kluge to make a specific correction that affects (most likely) emissions during transient conditions.

    Most of the spark code I've looked at limits spark timing increase to a fixed amount per revolution so an "instantaneous" increase in advance would still be seen as a ramp, albeit a steep one. It would be interesting to see if anything different was done in $88. My memory is that the limit is still present.

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    going off of the code of it.... it's intended for short-duration periods of rapid throttle/RPM change without being in PE. I don't know if it's intended use is for emissions, drivability or if it is trying to be predictive.

    code flow:
    test for RDSC enable bit, bypass if not selected
    test for PE mode but, bypass if in PE
    test for EGR diagnostics active, bypass if active
    test coolant temp, bypass if below 49.1*F(hardcoded value, kind of weird)
    test for knock retard being applied, bypass if so
    test DFCO fuel ramp-out byte, if more than 3.1% of normally calculated fuel is being removed due to DFCO, bypass(hardcoded)
    test RPM for upper/lower thresholds, bypass if outside
    test N/V ratio, if above global limit, bypass

    and that's just the code for entering into it. after that:

    determine which "gear" range to use(1/2/3)
    determine if launch-mode active(A1 seems to bypass the rest of the RDSC code if launch-mode active, even with launch-mode specific value for RDSC, 88 likely similar)
    TPS thresholds tested against
    positive D-TPS threshold tested against
    RDSC allow timer is decremented if code has gone this far, if 0 is reached, jumps out of RDSC
    A/C vs no A/C value is determined for "part throttle" RDSC
    multiplier is grabbed for determined gear/AC/throttle scenario, as is the D-RPM X 12.5(positive or negative) since the last loop
    D-RPM and determined multiplier are multiplied, if MSB is anything other than 0, max spark change allowed for situation is used
    if MSB is 0, value is compared against max allowed change(and limited if necessary)

    and that's most of the magic, the rest is cleanup:

    compared against a minimum allowed RDSC change value, if less, everything just done is essentially undone
    spark change as determined in the RDSC code is then added or subtracted from total spark

    and that's it for RDSC. so, the faster RPM changes occur, the more advance change occurs. the multiplier values also play quite a role with the final amount of advance as well.

    a quick calculation/scenario:

    assume you have a vehicle that pulls a 0-60 time of 6 seconds and is a linear rise of RPM/MPH throughout, with no gear changes. 6000 RPM redline/shiftpoint/60MPH.

    RDSC loop is done 80 times per second(there will be 480 loops throughout the acceleration to 60), so let's do this calc at 3000RPM/30MPH: in 1/80th of a second, MPH should rise by (60MPH / 480 loops) .125MPH. RPM should rise by (6000RPM / 480 loops) 12.5RPM. convenient seeing how that is the resolution limit of the value used in the RDSC calcs. using BFUZ(91-94 L-body 3.1/5 speed) values since it has RDSC enabled:

    we're assuming 100% throttle and 1st gear, so KDSONS1 is being used, it has a value of 5. the 12.5RPM value will be a 1, multiplied with 5 is 5(and 5/(90/255) is 1.76*. D-RPM will be positive, so we check the max value of 13.06* and it passes. but then we look at the minimum allowed change of 2.12* and then all of this calculating gets undone. there would have been an additional 1.76* added to total spark, but it didn't meet the minimum change allowed.

    if this pull were done in 2nd gear, and the same 0-60 time/etc, the multiplier is 7, which would have gotten us just above the minimum allowed value.

    if this pull were done at lower throttle and the same 0-60/etc, the multiplier is 16, which would have gotten us to 5.6*.

    also, "WOT" in this instance is actually less than 4% TPS.



    with BFUZ, it seems like RDSC is actually pretty difficult to get/stay in. something like BCFA(w-body 3.4/5 speed), it's actually appears to be difficult to stay out of RDSC.


    I wouldn't rely on it, it seems to add quite an element of extreme change that I can't really see a purpose for?
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    Discussions like this really make me miss having free time. There's probably something to be learned here.

    That code is very specific. I do not remember seeing it in applications that do not use a 60 deg V6. It's most likely introduced for a specific condition related to that engine family. The conditions are interesting and I suspect they're trying to match additional advance with conditions that degrade combustion. PE and EGR are usually mutually excluded, but an advance additive that works under "near PE" conditions might work with EGR to prevent a driveability issue and / or to help ensure most complete combustion by the time the exhaust valve opens. Disabling RDSC in a stock application might provide good feedback. Generally increasing spark increases NOx, not usually a goal, and increases how thoroughly the O2 is used. So if it's emissions related I'd watch for a lean reading on O2 and noticeable driveability changes. I'd also map EGR, PE, and RDSC to see the practical limits on when RDSC spark is actually added. What's really fun to think about is why the two different calibrations have such very different applications of the code. If both cals are latest releases it might suggest one calibrator using the code in a creative way that wasn't originally intended.

    I also agree with the practical evaluation. Without knowing what it's for it's probably best to leave it disabled.

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    Thanks for the explanation. I'm much dizzier now!
    For the n /v calc (which I've already done to get my shift lamp working correctly), a lot off the parameters are asking for "greater than" entries.
    My question now is:
    In the case of n/v ratios, Is greater than a bigger number, or smaller number?
    i.e "maximum n/v to allow rdsc"
    I.e "use 1st gear parameters if n/v is greater than this____ .
    Ron

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    The N/V ratio is rpm over speed. You can think of it as gear ratio. 1st gear has a higher N/V than 5th. If the definition is set up correctly then maximum n/v to allow RDSC should be lower than 5th gear to completely prevent RDSC.

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    another quick thought: RDSC operates in an inverted fashion as Burst Knock does, something I don't believe I've ever seen in a 60V6 application.

    launch mode, which also seems entirely or almost entirely 60V6-specific, seems to do a lot of the same, but I want to say it is generally/only used with automatic applications since it likes to reference the engine speed to calculated transmission input speed ratio a lot.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  10. #10
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    So, for 5th i have n/v of min/max 39/147. 4th is maxed (turned off because I have a four speed manual). 3rd is 60/158. I suppose the cutoff (maximum to allow rdsc) would then be 60?
    I'm confused by what constitutes a "minimum" and a "maximum" value in this context.

  11. #11
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    update:
    Ive got rdsc to work really well, with the exception of decel. it cuts so much timing that the motor pops. it keeps pulling down past idle rpm almost to stall. with rdsc turned off, everything is fine. what parameters in 88 effect rdsc decel behavoirs?
    ron

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    to change the amount of advance that is being added or pulled, you'll need to change the D-RPM multiplier for the gear you're in(effects both added and pulled spark), or change the maximum RDSC spark value for the gear you're in(D-RPM is Negative, to change only pulled spark).
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  13. #13
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    thanks robert!
    i had them set to 0*. i thought it would pull 0* timing out, not set to 0* on neg drpm! i found some 3.1 numbers from an auru bin and they work much better. ill play with it some more.
    ron

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