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.
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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. |