There are a lot of parts to for 04 LS1 GTO and some parts now out for the
[email protected] 05 GTO.
I now have 398rwhp on a very mild setup on my 04 GTO (Close to 480hp at the crank).
All of the long tube (LT) 1 3/4 headers are going to flow pretty much the same. SW, Kooks, SLP, Dynatech - all of these are going to provide the same improvements in hp and tq. The drawback in going with SLP headers is they use a less quality stainless steel material, instead of the 304 SS SW, Kooks, and Dynatech use. They are coated, but coating really isn't needed when you use high quality stainless steel material like 304. All of these major headers have quality designs, but the Kooks appear to have a slight edge in fit.
I had 307HP and 328TQ SAE on the dyno with cold air induction and with a Corsa catback. LPE makes a nice cai and locates the IAT sensor away from heat - that is important because the factory sensor gets a lot of heat soak from the radiator and engine. If you go with an open element filter system, make sure the IAT sensor is located near the filter, away from heat, and has a divider between the hot underhood air and the air filter.
The bigest bang for the buck improvement is LT headers with high flow cats. I jumped up to 330hp and 348tq SAE adding Kooks setup.
My jump from 330hp to 398hp was AFR205 heads (complete stock cnc version, no milling), mild 216/220 custom camshaft made for the heavy GTO, ASP underdrive crank pulley, and slp throttlebody with coolant bypass (I wouldn't expect gains with the SLP tb, coolant bypass will give you a couple on the dyno).
If you have a 2005 GTO, the first modification I'd do would be LT headers and high flow cats. A friend of mine saw 21rwhp and 21rwtq without any tuning.
LS2 edit and HP Tuners 2.0 will both be out shortly, both in beta at this time.
We have 16+ 2004 and 2005 GTOs in our local club and a good portion of us autocross, drag race, or roadcorse them. 12 of us had a dyno day so we saw first hand what things worked and what didn't.
Aftermarket catbacks mainly improve/increase sound, they provide little performance improvement. The factory dual exhaust has some kinks, but does flow pretty well - you see much great gains from other improvements (headers, camshaft, aftermarket heads, quality tune specific to your combonation, etc).
I would not expect a large increase in hp with just a cold air induction (cai) open element system. We had 10 2004s on the same dyno within a few hours of eachother. Those with open element air intakes would see a loss in power when their IAT sensor would see high temps. I monitored this with HP Tuners and we compared the power lost on the dyno readouts with timing retard because of the higher intake tempts. Some of the higher HP outputs were complete stock air intake systems and one of the lowest was a cold air induction system that was sucking in a lot of underhood engine heat.
I would not expect much on a stock 05 GTO from an intake system. If it doesn't pull in hot air and it moves the IAT sensor away from heat, it will give you some gains, but they will not be 18hp. If they do, there was something wrong with your factory tune (too rich) or intake system that is not the norm.
Unless you get a custom tune specific to your vehicle, the computer will see you are leaning out and will richen up the fuel. This has happened to those that saw a 20hp increase with an air intake system. Once their car learned the lean condition, their hp increases were greatly reduced.
