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Thread: DCFO Minimum idle airflow desired Coastdown speed

  1. #1
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    Default DCFO Minimum idle airflow desired Coastdown speed

    Hi people, been busy getting a demo car ready for our local Holden Dealer. A brand new L77 VE V8 2012 Auto trans.
    Mods to engine include AFM Delete kit the big Texas Speed LS3 camshaft, FAST intake manifold, supporting Valvetrain mods, Airspeed OTR Mafless, the large X-force full exhaust twin 3 inch.

    In an effort to produce a car with drivibility near to standard yet have a good cold start, good cold running, excellant "car park" manners, good fuel economy, good overrun characteristics etc I've been spending more time road tuning than I have dyno tuning
    I would like to share a few "discoveries" I've made and a few questions to ponder and perhaps get a few answers back.

    Info on these settings in the thread title has been hard to find in the E38 section. Some of you may already know the information below some may not.
    Point 1:
    I have found that "desired coastdown speed" appears to be the same as the desired idle speed in gear. Important to know to be able to set coastdown spark table, coastdown high and low spark tables and most importantly Minimum Idle Airflow which is essentially minimum blade angle on initial throttle pedal return.
    Point 2:
    The e38 is very clever in its idle and coastdown strategies and easily learns between a maximum and a minimum blade angle. As long as there is a broad enough range the targets can be met. Observation on overrun of ETC TP , RPM and spark from 100km/hr in 6th gear down to 6km/hr in 1st gear had allowed me to first dial in the minimum idle airflow table. The idea seems to be to get the rpm to return to as close to target idle rpm as you can. If its too high the spark will show this by learning down as far as it can set by Coastdown spark table high. But only when NOT in DCFO. So in 6th and 5th gear I've gone for about 1200rpm coasting in DCFO and -5 degrees. Set this rpm by simply reducing minimum idle airflow in 6th gear until you reach the rpm that feels right to you the driver. Reduce by 1g/s lots until you get close then by 0.5g/s after that. You will see the ETC TP getting smaller on overrun each time.
    Point 3:
    In 4th, 3rd, 2nd & 1st gears you dial it in via your idle RPM target watching spark learning. If spark's lower the the desired spark in the coast down table you need to keep lowering minimum idle airflow in the appropriate gear until you see the spark learning up towards desired spark . I've set this desired spark at about 14deg but you could change this for your application. I can run this car up a slight incline driveway at 6km/hr cold with foot off pedal with NO Surge. Blew me away really.
    Now the other thing to remember is that the ECT TP will start to learn upwards from the minimum together with spark learning once you are close to target so don't worry about that when you see it happening. All this happens in the background giving a uniform feel to the overrun in each gear but can be set up through tuning to suit any application/combination of engine specs.
    Point 4:
    Because 6th & 5th gear provide more glide the minimum idle airflow in these gears ends up being much lower than the other 4 gears and you can end up with quite a different "looking" table to the factory Holden one.
    Low speed surge is addressed here too. Coastdown spark is the target spark when driving in car parks, up driveways etc so you need to dial in this table to achieve an idle spark of between 14 to 18. The ETC TP will have learned up from minimum by this stage so minimum idle airflow 1st gear doesn't come into it unless you are miles out. Set up rows 600 & 400 rpm with higher settings like 20 then up to 30 at row 400rpm . This acts like a back stop and ups coastdown spark instantly should it dip below target for any reason. This technique works really well.
    Point 5:
    The feel of the down change on overun is affected by minimum idle airflow as well. In particular 4th 5th 6th gear, when a down change occurs the minimum idle airflow sets ECT TP for the next gear. The feel of the down change can vary from jerky to barely perceivable by adjusting these settings. Setting the next gear down to run more or less ECT TP on minimum idle airflow.
    Point 6:
    DCFO only stays on with the torque convertor locked (just like in a LS1) once it unlocks DCFO turns off ( I didn't know that )
    Point 7:
    Observing IPW during DCFO and as you exit DCFO in search of fuel economy I've noticed that:
    >With DCFO "on" IPW does not equal "minimum pulse width" as defined in the tables. I assumed it did.
    >STFT's go to zero.
    >That when DFCO turns "off" the IPW actually decreases as STFT trims the fuel back ( I tune the VVE table slightly rich like most people do )
    >So I'm actually using less fuel with DCFO off than I am on...
    Which leads me to the question > when in DCFO does it just run with the VVE table with no fuel trims? It certainly does not appear to be turning off the injectors or going to some defined ms pulse width.
    Any comments welcome.......

    thankyou.

    Mike
    Last edited by The Alchemist; March 31st, 2012 at 10:53 PM.
    "Just a tune > yeah right !!!! "

  2. #2
    Joe (Moderator) joecar's Avatar
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    Hi Mike, thanks for your analysis/writeup

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    Senior Member Dieselman's Avatar
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    Default

    Great write up Mike

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    Senior Member mowton's Avatar
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    Default CTSV appears confused between normal and coastdown operation ??????????

    Is the explanation the same for an E67 2009 CTSV LS9 cam setup? Have a case where the PCM seems to be confused as to whether it is in coastdown (closed throttle) or normal (HO Table) and seems to cycle back and forth thus producing a "cruise control" situation at rpms like 1300 and 2100 (2 examples we have seen and reducing min air dies not correct). For some reason when APP% goes to 0%, TPDES and ETCTP% both drop to 13% (TP%=18.8%) and the spark decays from 45 degrees to 17 (believe this to be the coastdown target) when instantly the TPDES% jumps to 27% (APP% still at 0%) and TP% jumps to 29.8%. Timing begins to climb (reaches 28.5 degrees in .3 secs) and than instantly drops to 0 degrees.

    Attached is a log for this phenomenon. We have tried this in MAF only as well as Speed Density and it occurs in both approaches. We have replaced the TB twice. Another bit of interesting information is we completed the car in March and all was perfect. No issues. Customer changes out TB with a new ported version and then returns the car with this cruise control situation. We tried for a bit to stop the hanging rpms and finally said put the old one back on, worked perfect. Re-flashed his original tune and sent him on his way.....seqway to today where he went back to the original TB but the hanging rpm situation remained. Initial assumption his he messed up something during the swap which prevailed when the original unit was re-installed.

    Looking to better understand how the E67 determines between closed throttle, coast down and norm as well as any insight as to the details related to the TP PIDs mentioned above so we can best look for a root cause to this.
    Thanks

    Ed M


    jay_clsd_1.efi

  5. #5
    Senior Member mowton's Avatar
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    So being one who hates threads that just end, looks like I was guilty...even if no one else elected to jump in and help :-)

    Turns out customer went and changed tire sizes and apparently that threw the ABS/TC system into a tizzy. Put the correct tires on and all was right with the world again.

    As a note they were his drag radials which at the the track, this phenomenum wouldn't even be noticed and he doesnt drive on the street with them .... for the most part!

    Thanks,

    Ed M
    2004 Corvette Coupe, LS2 Shortblock, MN6, Vararam, American Racing Headers/CATs, Magnaflow, 4:10's, Trickflow 215 Heads, 30# SVO, Vette Doctors Cam, Fast 90/90, Performance Clutch, DTE Brace, Hurst shifter, Bilsteins etc.....480 rwhp and 430 rwtq

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  6. #6
    Joe (Moderator) joecar's Avatar
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    Hey Ed,

    No worries, some things take time.

    I looked at the log but I had no comment (I didn't understand why it was happening).

    Ha... tire size... I'm surprised he didn't jump to the conclusion that the tires were different.

    Thanks for posting the conclusion.

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