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Thread: 408 tuning questions

  1. #1
    Potential Tuner
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    408 tuning questions

    Hey guys, I have been trying to tune a 408 and the runability just plain sucks.

    It is a 408, fast 102, : 242/250 .648"/.648" (with 1.8:1 Rocker
    Ratio), 114 LSA
    PRC heads
    T56

    Anyhow, this engine has been dyno tuned by a shop in washington and it put out awesome power, ran great on the dyno.

    Problem is now i get it to Montana, It doesnt want to start cold. Idle is all over the place, itll surge. Its almost impossible to start out. No driveability whatsoever. It bucks and jerks due to RPM

    Any thoughts on tune and log files?
    55chevy408 dtcedit.hpt55coldstart2.hpl55warmdrive.hpl

  2. #2
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    I tried adding a little more base air flow with no change. Brought down idle spark, and feel like it got worse. When it idles , it idles pretty decent.

  3. #3
    Senior Tuner Ben Charles's Avatar
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    This is something That’s going to be a little tricky to tune.. too much to post on a forum..

    But I see this a lot.. WOT pulls done on a dyno and everything else not even worked on...

    If I can help any please let me know

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  4. #4
    I wouldn't say that it is a very good tune at all. Timing table looks pretty stock. I would suggest getting it tuned by some one who knows their stuff. Not just on the dyno but driving it around and starting at different temps. It takes a bit of time to get stuff dialed in at all temps. Also the rev limiter is at 6000, I would raise that thing up above 7

  5. #5
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    Not what i wanted to hear haha. This thing pulled what i felt was a respectable 500 some horse on the dyno

  6. #6
    Senior Tuner Ben Charles's Avatar
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    500 is on the low end for a 408 PRC head and cam if that size in my opinion

    Email Tunes, PM me for more info
    96 TA Blown/Stroked, 4L80E/Fab 9
    15 C7 A8 H/C 2.3 Blower/PI
    14 Gen 5 Viper
    Custom Mid Engine chassis, AKA GalBen C

  7. #7
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    appears very rich at cold idle and start up. Agreed cold start and start up enrichment are important but it looks like this is too rich, hence the low idle and hunting after cold start, also supported by the high MAP reading in comparison to warm stable idle.

    VE table looks stock. edit that in the idle areas minimum, or ideally whole thing. it will influence cold start idle and cruise below 4k rpm significantly.

    disable STFT in open loop and lean out the cold areas of the OL EQ ratio closer to 1.00.

    try adding some base idle airflow and zeroing out the startup catalyst heating table.

    you may need to also mechanically add more air by either drilling a bigger hole in TB blade or adjusting the TB blade stop and re-setting that position to 0%TPS.

    if the engine can idle warm at desired RPM on 5-8 degrees only then you have enough air. that wil leave enough resoluton for the IAC and spark corrections to maintain cold idle. if it requires 20 degrees to idle warm there is no room left for cold corrections spark wise. dont disregard spark importance in start, idle and cold start corrections in combination with everything else