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Thread: Big off idle stumble, whatt first? IFR? calc.vet? Something else I'm missing?

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    Default Big off idle stumble, whatt first? IFR? calc.vet? Something else I'm missing?

    I had a post in the COS forum, but not sure if it goes there or here. If I'm wrong, please advise. I just swapped a marine intake and Whipple supercharger onto my 1998 GMC with 5.7 Vortec. The intake allowed me to run proper injectors, Bosch 0280155811 at 3.5 bar. Stock they flow 36# at 43.5psi, so using this calculator

    http://users.erols.com/srweiss/calcpchg.htm

    and some googling I arrive at 38.888 lb/hr and an IFR of 4.8988. I plugged that in (or something close to it) and the truck fired right up. Trims are off which I wasn't expecting at low load plus I have this NASTY off idle stumble (all other transients are fine) that's worst when the engine is cold. I don't want to go changing a bunch of crap in the wrong order. The truck seems to run better if I fudge the IFR down I believe in the 3.55 range but I assume that's just covering up the real problem. So should I have faith in my IFR math and just go do a calc.vet or is there more than meets the eye when it comes to nailing down a new IFR? Or is there some super simple idle setting I'm totally missing here? My only thought is the addition of some real estate between the engine and the MAF with the supercharger being in the mix now. Physically its location has remained the same as stock.

    411 Whipple V1.tun
    411 Whipple V2 IFR Change.tun
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  2. #2
    Joe (Moderator) joecar's Avatar
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    Get VE/MAF correct.

    Make sure injector data correct.

    I'll look closer later tonite.

    ~ posted by phone ~

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    Quote Originally Posted by joecar View Post
    I'll look closer later tonite.
    There's a lot to look at in this tune file.

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    Quote Originally Posted by Supercharged111 View Post
    I have this NASTY off idle stumble (all other transients are fine) that's worst when the engine is cold.
    Post a log.

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    Here's a log.

    Whipple Log 2.efi

    I guess my biggest fear is committing to the mathematically correct IFR, doing a calc.vet, and then realizing that for some reason the IFR is wrong and having to start over again. Sometimes I have KR, sometimes I don't. I really need to get the fuel right on this thing so it's more consistent. I thought I'd pulled enough timing, but I suspect now that since the bypass valve closes before atmospheric pressure is achieved, the IATs are higher at a lower MAP than I'd anticipated. I don't really want to monkey with that until the fueling is correct. I was about to do a calc.vet yesterday, but decided to wait on what I heard in here first. So can I trust that 4.8988 IFR and should do the calc.vet with it and report back? The PIDs that I have currently selected to log are the calc.vet PIDs, but I don't think any of the logs have enough data to try and correct anything. Plus I'm running the wrong IFR to fudge more fuel into it before trims now in a ghetto effort to help the off idle stumble. It's there with a warm engine too, just less bad.

    I'll add that I didn't do the quasi-closed loop operation because the truck is only making 3.XX# of boost which gets me up to 100 kpa on a cool night. I think I'm going to do it though to add a little knock resistance to the range right where the bypass valve closes but I may or may not be exceeding atmospheric.
    Last edited by Supercharged111; June 19th, 2017 at 04:22 AM.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Quote Originally Posted by statesman View Post
    There's a lot to look at in this tune file.
    Could you please elaborate instead of being a douche about it?
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  7. #7
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by Supercharged111 View Post
    Could you please elaborate instead of being a douche about it?
    I think he's referring to me looking at it...

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    It sounds like he's implying that I have a lot of things wrong. If that's the case, why not lay it out instead of making you do it all? If not, I gotbmy panties in a twist over nothing.
    Last edited by Supercharged111; June 20th, 2017 at 12:45 AM.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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    Quote Originally Posted by Supercharged111 View Post
    Could you please elaborate instead of being a douche about it?
    It's Aussie humor, and it looks like some people just don't get it.

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    Quote Originally Posted by Supercharged111 View Post
    It sounds like he's implying that I have a lot of things wrong. If that's the case, why not lay it out instead of making you do it all? If not, I gotbmy panties in a twist over nothing.
    If I knew what was wrong, I wouldn't need you to post a log.

    Nice panties.

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