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Carrying PN 30164 and retailing for $1,199.95, SLP offers this long-tube header system for all 2004 GTOs. The headers are made from aluminum-ceramic coated 409 stainless steel and feature 13/4-inch primary tubes and 3-inch collectors. Although many other header manufacturers use uncoated 304 stainless steel as their material of choice, SLP favors the luster and appearance of a coated and polished 409 header. High-flow catalytic converters are also included in this kit; you can save $60 by ordering the kit with off-road pipes instead (PN 30165). |
After disconnecting the negative battery terminal, we're ready to get started. A lift will make this install much easier, though it can still be done on jack stands. Once underneath the GTO, we can see the stock system, which measures 2.25 inches in diameter and has a total of four catalytic converters; clearly not the optimum setup for good exhaust flow. The SLP system replaces everything starting from the exhaust ports back to where the stock Cat-Back system bolts up (far bottom of photo). Either the stock exhaust or, in our case, SLP's Loud Mouth system will bolt right up--as will any other system designed to work on these cars. |
Since the Cat-Back system on the 2004 GTO is so well-supported by rubber hangers, we were able to leave our SLP Loud Mouth system in place throughout the entire installation. Four bolts are disconnected where the stock cat pipes meet the Cat-Back. |
There are a total of four oxygen sensors on this vehicle, which need to be unplugged at this time. This is easy for the aft-cat sensors, but the two front ones are a bit of a challenge as the harness plugs are tucked atop the transmission. Some dexterous hands and slim arms are needed to access and unclip the plugs. |
We left you last time with our 2004 GTO making 471 rwhp ("Back in Black," Part 6, May 2006), a good 54 rwhp increase over the prior installment. The additional grunt had resulted almost entirely from custom-tuned fuel and ignition maps from East Coast Supercharging. In order to make this increase safe, however, we had also keyed the harmonic damper/crank pulley assembly and installed an SLP low-temperature thermostat, NGK TR6 spark plugs, 60 lb/hr Mototron fuel injectors, and a Vortech-supplied fuel pump.
The Goat was running an internally untouched LS1 that was still spewing gases through the stock exhaust manifolds and catalytic converters. Before even thinking of pulling a valve cover, we wanted to see just how much more we could squeeze out of our 346-inch Gen III motor.
SLP Performance Parts is a well-known name in the GM aftermarket community. Just because they've crossed the thin blue line and begun making hop-up parts for Mustangs, it doesn't mean they've forgotten about us here on the good end of the dark side. To that end, SLP offers headers for many a late-model GM, and the '04 GTO is no exception. We chose a set of the company's long tubes and decided to see what kind of a power increase we could get. Check out the install photos for a step-by-step evaluation of this system.
Upon post-install startup, we were greeted to a much deeper exhaust note than before which, although relatively unobtrusive at part-throttle driving, made the GTO sound like Warren Johnson's pro stock car at full throttle. Hey, horsepower makes noise, and in this case it was music to our ears! At idle the car really sounds like an old school, big-block musclecar. Surprisingly, highway-speed drone was actually reduced versus the pre-header install; this has to do with the system exhibiting different resonant frequencies thanks to long header tubes, collector design, and catalyst placement. Still, full-throttle blasts will get the law's attention real fast. Note however that at this point we still had SLP's Loud Mouth resonators installed in the Cat-Back system; a swap back to the Loud Mouth II mufflers we still have will yield a subtler note with negligible power loss.
Sad to say it, but as you read this, our project GTO has already been sold and is now in the hands of Mr. Will Carpenter, an avid GM enthusiast with a few mint late-models in his garage. We hope he enjoys it for years to come ... and he tells us the mods aren't over yet! Space permitting, we'll keep you updated on his progress.
For now, though, a couple of final notes on the vehicle. You'll recall that in the November 2005 issue, we mentioned some strange noises coming from under the hood, especially around right-hand turns with the throttle closed. We concluded it was simply the supercharger changing speed as the engine basically idled. But it also made this noise in reverse gear. As it turns out, the noise was 100 percent the fault of the installers (i.e. yours truly): I had adjusted the supercharger intake hat such that it was within a millimeter of contacting the driver side fenderwell! In reverse gear and around right-hand turns, the engine leaned toward the driver side enough that the aluminum intake hat began rubbing the steel fenderwell, making the odd noise we were hearing. A simple loosening and rotation of the hat, and we were noise-free.
Finally, you'll note that we've been saying all along how Vortech's supercharger system makes between 7 and 8 psi. Well, a few months back we hooked up a boost gauge during an interim dyno session and were surprised to see 9 to 10 pounds of boost at 6,000 rpm, with a touch more at the very top. This told us that upping the rpm limiter would be a not-so-good idea at this point, as this would cause the Vortech V-2 to really get into its efficient boost-making speed range and start dumping some serious air into the engine. Thus we have, and will continue to, stick with the stock rev limiter and shift points for the time being. ECS recommended that before doing anything else to this LS1 engine, a water/methanol injection system would be in order. This would add power by cooling the incoming charge of air (and likely allowing more ignition timing to be used), but the primary idea of installing such a system would be to stay safe with pump gas, as it would increase effective fuel octane to well over 100. (Who knows whether that's really 93 you're getting at the local fuel stop?) The proof is in the pudding, as ECS's record-holding, mid-9-second supercharged C5 runs such a water/methanol injection system--and as such is able to burn ordinary 94 octane Sunoco fuel.
Needless to say, there's more power and performance left in this project car, but we're happy with the way it has turned out. Next time you see a black GTO prowling the mean streets of Jersey, beware--it just might be Will Carpenter's 688hp sleeper!
A brace holding the rear cats to the transmission crossmember will not be reused. Just disconnect the bolts holding the brace to the tranny; leave the nuts attaching the brace to the cats alone, as the whole thing will be tossed anyway. | 
Two nuts hold each cat pipe to each manifold; some of these nuts are more difficult to access than others... | 
...With the nuts removed, the driver and passenger side sections of cat pipe come out individually, each with two cats in it. |