Well...like superv6 said...
180* Thermostat (Local Auto Store) - $10 if that
Autolite 606's (Local Auto Store)- $10? maybe...
Custom Fenderwell Intake (Filter, $30; Intake parts @ Lowe's/Home Depot...maybe $20?)
2.5" Offroad Downpipe (ZZPerformance) - $119.99 plus shipping
Front Powerlog Manifold (ZZPerformance) - $119.99 plus shipping
ZZPerformance HV3 Upper Insert - $199.99 plus shipping
Tuned PCM (Intense Racing, Overkill Performance, or 3800performance.com) - Around $100 after you return your stock PCM to them
THEN...if you still want more out of your car...sell the PLOG and DP...get headers. Headers really depend on your budget and can be had from $300 or so to around a grand. I have Pacesetter's and love them...once you get past sealing up all the little leaks and such they are great for the price (I got mine with the 2000* Coating for about $400 and something brand new back in the day).
You can either take a couple routes here.
1) Turbo/Intercooler Kit
Some guys that know more about turbo-ing the L26's can jump in here if they'd like...I think TDC is pretty knowledgeable about the turbo setups in the 3800 powered cars. I know there are Cartuning, ZZPerformance and maybe someone else that I can't think of right now...
2) ZZPerformance High Ratio Roller Rockers and Headers
Very good driveabilty and gains are close to that of a smaller cam upgrade without the hassle of opening up the motor and not sacrificing mpg's. They will also really help on your top end power since the 3800 is lacking that department from the factory anyways. ZZPerformance sells 1.90:1 Modded Stock Rockers, but you will need upgraded valve springs if you plan on shifting higher than 6,000rpm's. You can get stock LS6 Corvette Springs and Retainers from GM. 1.9 Modded Rockers from ZZP are $329.99 plus shipping.
3) Cam and Headers
Again, MANY choices here and you can get even a custom grind cam from PRJ Performance but since you are not going real balls to the wall that won't be needed but incase...he does them for $500 IIRC. ZZPerformance sells the GT line-up of camshafts for the 3800. They offer the GT1, GT2, and GT3 Cams. The GT1 is the easiest going of the three and you'll see gains with the right tune of 20-30+hp. The GT2 is more aggressive and will require upgraded valve springs. It is also a higher powerband cam where you'll be making power up to 6,500rpms. Upgrading the pushrods is recommended but not NEEDED. You'll see gains anywhere from 25-35+hp depending on your other mods. Then there is the GT3 cam. ZZP Says this about their GT3 camshaft upgrade:
"The GT3 cam was designed for use with ported heads and added displacement. Upgraded pushrods are recommended. For use with stock heads mod'd retainers are required.
This cam was designed for naturally aspirated applications using our stroker kit. For use on a highly modified 3.8 liter, shift points will need to be 6800 - 7k for maximizing the powerband. It will set the P0300 code unless programmed out of the PCM. Idle is lumpy, especially with headers. Gears and a higher stall torque converter are recommended.
Look for gains of 30-40 HP depending on your car's current configuration."
Then you have the same kind of options from
INTENSE-Racing.com - G8, Grand Prix, Bonneville, Regal, L67 3800 Supercharged Performance Products.
Hit them up too. Hope you can get a bit of knowledge from this read
