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!