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  1. #1
    GrandPrix Junkie SgtMarshal's Avatar
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    Default New to me 2008 Grand Prix GXP (LS4) with questions

    My first question is which spark plug are people running and what gap is good. I assume to go with oem plug wires.

    Is there a "safely modding guide" for the LS4?

    If I wanted to add some E85 to a tank, like a couple gallons occasionally, could I do that with just a maf adjustment or is there an easier way? or even just add it and not make any adjustments.

    Another thing I noticed when looking at the high octane spark table is that wot timing is set to 7 degrees. that seems pretty low to me.

  2. #2
    Awesome Level Member chrsmi1's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Comp G. E85, 2.8, Headers, Tune, Intake, S1X, 105# springs, 60 # injectors, 255 walbro,etc.

    *NEW* 2002 Camaro Z28- Lid, Headers, Catback


  3. #3
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    I'm on there already, I figured I would ask here first

  4. #4
    Awesome Level Member chrsmi1's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    check out this one too...

    http://www.gmls4.net/index.php
    Comp G. E85, 2.8, Headers, Tune, Intake, S1X, 105# springs, 60 # injectors, 255 walbro,etc.

    *NEW* 2002 Camaro Z28- Lid, Headers, Catback


  5. #5
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    The GEN 4 computer is far more advanced than the 3800's or even a P01 or P59 computer. Just changing the MAF settings for running E85 is not correct. Either install a ethanol content sensor or run ONLY E85 with no mixing. There is no inbetweens. You'll have to start tuning in lambda if you wish to make the switch because tuning in AFR is no longer going to work for you.


    Stock plugs work, many switch to like a TR55 or similar plug.

    I think you need to drive the car and actually see where the engine runs at for full throttle so you can see where they run for timing. That LS4 will never see over .85-.90 cylinder volume without boost. You'll be in the .60 to .75 for the most part and that timing is in the 20's. They also use spark correction tables to add timing in certain parts of the power band.

    SMGPFC Member #1

  6. #6
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Overkill has a couple e85 kits fit the LS4, would I need a tune for it to work?

  7. #7
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    It always have to be tuned for, you couldn't just plop it in there and get it working and expect the computer to know what to do 100% of the time. His website even says it will need tuning.

    SMGPFC Member #1

  8. #8
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Does the 08 GXP have a catalytic converter diagnostic? I looked around but didn't see anything conclusive that it did or not.

  9. #9
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Yes it does.

    Engine Diag > Exhaust. There is a cat test and post o2 test that can be enabled or disabled.

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  10. #10
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    If I take the 18's off and put 16 inch wheels on, how much will the speedometer be off, is there something in the tune I can change to adjust for different wheel size?

    Will the 16's even fit in the gxp?

    I have good snow tires that I would like to run

  11. #11
    GTP Level Member cheatah faheatah's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    I'm sure ffdp will have the figures, but of the O.D./circumference of tires is the same, then no difference.
    not what it looks like

  12. #12
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Going from a ~245/40/r18 to 225/60/r16

  13. #13
    Killa Bee Scottydoggs's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions


    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, 2.8, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW

  14. #14
    GrandPrix Junkie spazzz's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    what size are the sixteens? 60/16?

    It's the front brakes that wouldn't let you

  15. #15
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Pretty sure you need a minimum of 17's to clear the brakes.

    SMGPFC Member #1

  16. #16
    GrandPrix Junkie ItHurtz's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    yup

  17. #17
    GrandPrix Junkie SgtMarshal's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    I'm setting up my scanner and am wondering what else I need to scan for in addition to the standard L67 stuff

  18. #18
    GrandPrix Junkie ItHurtz's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    I vote dont touch it

  19. #19
    Donating Users 91parkave's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    Ngk tr6ix by far. Don't fiddle with cat settings if it's still on the car.

    Sure you could add about 2 gallons of e85 if not more butt most places are in winter blend so you can get away with a little more . Scanning is your friend.

    I always recommend a couple things first

    Tune the transmission take off dod. Disable tcc lockup to to 4th gear
    remove all bank 2 b.s from the scanner.
    Install catch can
    get
    trans cooler
    Install shift kit if you want it to live.


    No, 16s will not clear
    06 GXP | 222/227 cam/cartuning turbo kit on 8psi/meth/e85 coilovers/ still on stock trans at 130k

  20. #20
    Admin blueguy's Avatar
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    Default Re: New to me 2008 Grand Prix GXP (LS4) with questions

    You'll like it till it breaks.

    Then you'll fix it and sell it.

    The LS4 isn't much fun.
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
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