I know everyone one will have a different opinion of if my thought is a good idea or a bad idea, but let me just lay down some background info... I am a Combat Vet with PTSD and one of the few things that truely helps me is working on vehicles, not for repaired but to upgrade, improve, and modify to be cooler. I actually purchased my GXP during my mid tour leave in March of '08 (it had 24k on it now it has 106k). I was going to pick up another LS powered car (RWD) and make it my project, but I thought that I should be different and work with what I already have an attachment tool, Pontiac's but in particular the one I already have.

My goal is around 425-500whp (for now, maybe more later). I know I could do it easy with a Turbo set up. But at 106K and climbing I would want to rebuild the engine. So my thought is to build the engine and tranny (it would be done with what ever route I go) first. I know anything is possible with enough money and custom parts but I dont have tons of money so its a build with as many off the shelf items as possible. So my thought is to re-sleeve the LS4 to 4.125 bore. I have already talked to the guys at ERL Performance and they say that it can be done... If I did my math correctly while I was supposed to be busy at work it would take the 325ci to 404ci (correct me if I'm wrong). The 4.125 is a common size since the LS7 so reasonably priced pistons are available.

With that size bore I can step up to L92/LS3 cylinder heads (or possibly LS7?). Now I come to what is the most cost effective head and intake combo to use. The Fast 102 thread makes me want to go that route, but would there be a more cost effective intake to go with?

I'm almost sure that there is not going to be an off the shelf cam that I will like so I'm ready to have to spent time talking with a cam company to get it spec'ed out. The car will be a street car, so most of the time it will be either cruising or between 3k-5.5k rpm.

I passed on the 06 GTO for the 06 GXP because I wanted something that people did not normally see, something cool, and admired buy those who know what it is and those who don't really know.... Love to run 12.0x at the track N/A (little faster with a small shot from the a bottle if i get the itch).

 

The re sleeve alone would put your power where you need it, your talking a 5.3 bored out with LS3 heads would put you at or above 500 crank with the cam and and f.a.s.t im thinking youll hit about 450whp easy now the ultimate issue with the w-body platform in general is.....putting down the power to wheels....not trying to **** on your brainstorm but just ask guys like david gxp and the turbo guys getting this beast to really hook without drag radials will be your ultimate hurtle

 

Oh I know traction will be an issue... I have put 82k miles on this car so I know the short comings of traction. Any playing around I do is always from a roll. Even how it sits right now on Kumo Platinum's I have to take it easy off the line when its damp. But once this it done it will no longer be my DD and I will plan on having drag radials or a very good summer performance tire on it.

 

oops... re did my math the displacement sleeved at a 4.125 bore is 387ci... I mixed up what the bore and stroke was stock.

 

You are doing exactly what I intended to do. I have spoke with ERL. We can sleeve to 4.185" bore, JE 311973 pistons, with the stock 3.622 crank. The pistons will require a 6.125" rod, and I would recommend comp star I beams. I was planning on using Trickflow GenX 235cc 70cc heads http://www.trickflow.com/partdetail....S-3061T003-C03 with a .045" head gasket. Compression would be around 11:1 Slap on a FAST 102 LSXR manifold, combined with a Nick Williams 100mm TB, and DT headers with a modified 3.5" collector. Ditch The AC compressor and install a vacuum pump to evacuate crankcase pressure. Obviously a custom cam would be in order with some DOD delete hardware. Good luck with your build, and welcome home!

ARP main studs and head studs of course.

The 4T65E with a 300M Input shaft, and Z-pack clutches should take the abuse with ease. I would stick with 3.29 gears.

 

Don't forget to put lots of pics

 

Best bang for the buck intake would be an LS3 or L92, which can be had for about what you'd nearly pay in sales tax and shipping for anything from FAST.

Not knocking FAST, but their stuff could never be considered "best bang-for-buck".

Of course, this assumes you go with the rectangular port heads.