Equipped with a 400-horse, all-aluminum, 364ci LS2 powerplant, an '05 or '06 GTO is a sophisticated modern-day musclecar. Dubbed a "Gen IV small-block" by GM, the LS2 took the LS1's bore of 3.90 inches and brought it out to 4.00 inches while retaining the same 3.62-inch stroke. Compression was raised from 10:1 to 10.9:1 by the use of flat-top pistons and the more efficient LS6 cylinder heads that debuted on the Z06 Corvette. Conservative camshaft specifications of 204/211 at 0.50 and 0.520/0.520 lift on 1.7:1 roller rockers and the 116-degree lobe-separation angle ensure a smooth idle and crisp performance. With 400 lb-ft of torque on tap, stock GTOs routinely rip off low-13-second e.t.'s while simultaneously providing one of the most balanced ride-and-handling packages ever offered by a domestic manufacturer. So why mess with success? Because there is no such thing as too much horsepower.
LS2 owners looking for more are doing exactly what many original GTO owners did: replace the cam and exhaust system with the best the aftermarket has to offer. Although 500 hp was obtainable with a built 389, along with the power came a radical idle and poor street manners. Since the LS2 idles as well as any modern car of its generation, what would happen if you swapped out the cam for a more aggressive design and freed up the exhaust? How much more power could be added by putting on an LS2-specific nitrous-oxide system?
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Here is the heart of the GTO Package-a custom Comp cam and the associated valvetrain components required to optimize its impact in the '06 GTO. | 
The stock LS2 exhaust manifold is dwarfed by the ARH long-tube header. In addition to the much larger collectors (3 versus 2 inches), the tuned long-tubes of the headers work to scavenge the exhaust from the engine, allowing more efficient combustion. |
Enter Horsepower Engineering (HPE) of Houston, Texas. Founded in 2003 by owner Chuck Anders and currently housed in a 13,000-square-foot facility, HPE has been pushing the envelope on late-model GM performance ever since.
According to Chuck Anders, "Substantial power gains can be achieved with the LS2 by optimizing the camshaft and the exhaust system. Our GTO Package has at its heart either a 'C' or 'S' series camshaft. The 'C' series cam was custom-designed for the owner desiring stock-like driveability, moderate lope at idle, and noticeable power gains. Owners looking to maximize the power of their LS2 with an aggressive cam choose our 'S' series cam.
"Although the LS2 exhaust manifolds are an improvement over the LS1 design, a highly efficient exhaust system that utilizes either American Racing's or Kooks 171/48-inch stainless headers and 3-inch connection pipes will really allow the heads to flow to their capabilities. Combine the camshaft package and enhanced exhaust flow with custom PCM tuning, a Lingenfelter cold-air kit, a ported throttle body, and an underdrive pulley, and we routinely get gains of 80-100 rwhp.
"Owners looking to make over 500 rwhp can opt for the Nitrous Package, which features a Cold Fusion Nitrous plate system that is jetted to safely add another 100 hp. Optional equipment, such as the Cold Fusion billet-aluminum remote bottle opener and bottle heater, adds great functionality to the system."
Follow along as HPE searches for big power in an LS2 by installing a GTO and nitrous package on an '06 GTO owned by Corey Poiencot of Houma, Louisiana. In stock configuration the Goat belted out 344 rwhp at 5,800 rpm. Let's see what it does after the mods. Technician Dan Taber of HPE will handle installation of the components, while HPE tuner Chuck Anders will dive into the powertrain control module and retune the Pontiac to maximize the power potential.
GTO Power Package
HPE's GTO package retails for $2,499 and consists of matched cam and exhaust components and a few other items.
The "S" series custom-ground Comp cam used in the test car has a duration of 238/240 at 0.050 on a 112-degree LSA with a 0.605/0.609 lift using 1.7 rockers. | 
Patriot Performance's Gold dual valvesprings are capable of up to 0.650-inch lift and feature an open-spring pressure of 380 pounds, with 135-pound seat pressure as compared with approximately 310 and 110 pounds for the factory LS2 springs. | 
A 3-inch off-road connection pipe from ARH or Kooks is included in the GTO Package and mates to the header's 3-inch collector and 2.50-inch after-cat inlet. The connection pipe is also available with high-flow metal matrix catalytic converters for those concerned with emissions legality. |
Each customer has a choice between either a "C" or "S" series camshaft custom-ground by Comp Cams. The "S" series cam used in the test car features an aggressive duration of 238/240 degrees at 0.050 on a 112-degree LSA and 0.605/0.609 lift with 1.7 rockers. Conversely, the "C" series camshaft, though it's also a strong performer, specs out at a more mild 232/234-degrees duration at 0.050 on a 112 LSA and has 0.595/0.598 lift with 1.7 rockers.
Patriot Gold dual valvesprings, titanium retainers, Super 7 locks, locators, Viton rubber intake and exhaust valve seals, and Trick Flow 7.40-inch-length, 51/416-inch one-piece chrome-moly pushrods complement the cam.
American Racing Headers (ARH) or Kooks Custom long-tube headers with 171/48-inch primaries are also utilized. Each header is fully mandrel-bent T304 stainless steel with heavy-duty 31/48-inch flanges and 3-inch collectors for excellent flow. A 3-inch off-road connection pipe from ARH or Kooks is included as well. Each header kit has O2-sensor extensions to take care of the revised O2-sensor placement on the header and connection pipe.