New to me 2008 Grand Prix GXP (LS4) with questions

[SgtMarshal]

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.

[chrsmi1]

join ls1tech.

http://ls1tech.com/forums/ls4-performance-108/

[SgtMarshal]

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

[chrsmi1]

check out this one too...

http://www.gmls4.net/index.php

[Fivefingerdeathpunch]

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.

[SgtMarshal]

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

[Fivefingerdeathpunch]

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.

[SgtMarshal]

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

[Fivefingerdeathpunch]

Yes it does.

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

[SgtMarshal]

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

[cheatah faheatah]

I'm sure ffdp will have the figures, but of the O.D./circumference of tires is the same, then no difference.

[SgtMarshal]

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

[Scottydoggs]

https://www.tacomaworld.com/tirecalc...0r18-225-60r16

[spazzz]

what size are the sixteens? 60/16?

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

[Fivefingerdeathpunch]

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

[ItHurtz]

yup

[SgtMarshal]

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

[ItHurtz]

I vote dont touch it

[91parkave]

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

[blueguy]

You'll like it till it breaks.

Then you'll fix it and sell it.

The LS4 isn't much fun.