It's bad, it's black, and most importantly, it's back. Fans of GM muscle have waited a long while for its return, and the new 2004 model of the legendary GTO does not disappoint. While this year's edition lacks the hood scoops and American roots of the 1960's version, anyone who drives one will understand that it sets a new standard for muscle cars in the 21st century.The 2005 GTO offers even more horsepower and hood scoops, but that doesn't mean 2004 owners will be left without recourse. The aftermarket is jumping on the Goat bandwagon with rave enthusiasm, as all types of bolt-ons for the 2004 model are coming to market left and right. Plus, owners of the new 2005 car will be able to benefit from the same or similar mods to the ones we showcase here on this GTO.
This particular car is a Phantom Black 2004 model equipped with an automatic transmission. A baseline test at Raceway Park in Englishtown, NJ yielded a 13.83 at 100.64 mph in the quarter mile, with a 2.04 60-ft. time. We thought this was about right for a humid, 65-degree summer evening. We weren't too surprised when the scales showed a 3,910-pound race weight, either.
Though the stock tires were providing adequate grip during that visit to the track, we knew that with more torque on the way, it wouldn't last long. We wanted something that was going to be stickier than the stock BFGoodrich g-Force T/As, but could still handle high speeds, traverse wet weather, and exhibit decent longevity. We went for the Nitto NT-555R Extreme Drag. This tire offers good all-around traction, as well as a super sticky surface when heated in a burnout. We chose the stock size, P245/45 R17--no way were we going to risk rubbing the GTO's notoriously tight fenderwell. As they only carry a 100 treadwear rating, they will wear more quickly than a standard road tire--but we can deal with a 15,000-mile useful life.
With traction duties taken care of, it was time to move on to the horsepower side of things. The GTO has one of the more pleasant-sounding exhaust systems of any production car, with a deep but subdued rumble. GM put a lot of time into its design, a true-dual setup that contains no crossover pipe. For our tastes, though, it was a bit too quiet. Enter SLP Performance Parts.
SLP has made up a new dual exhaust system for all 2004 GTOs made entirely from 304 stainless steel. It includes an "X" pipe to increase horsepower by balancing gas pressures between the two exhaust routes. Two options are available in this "modular" system: resonators, which have no packing whatsoever; or SLP's high-flow, straight-through mufflers. SLP is widely praised for the build quality of the products it puts out, and this system is no exception. The fit was better than we could have asked for, though some elbow grease was needed here and there due to some tight tolerances. This is by design of course, for the completed installation is tightly supported, has no rattles, and doesn't even come close to making contact with anything under the car.
With the "blow" side of the engine addressed, it was time for the "suck" side to get some help. LS1Speed (Speed Inc.) provided us with its new High Flow Induction System for all 2004 GTOs. This kit is overall a good piece, but there are a few things to be aware of. First, while the stock airbox is sealed to the area behind the headlight and away from underhood heat, LS1Speed's system has its filter hanging right inside the engine compartment.
Others claim to have experienced idling problems before their cars' engine computer learned the new flow of the LS1Speed intake, but we did not. Initially, the car had a bit of a hesitation to startup when the key was turned, but that issue went away as the computer learned. Probably the most noticeable effect the system had was that it made the intake snarl much more audible, which is always a plus.
To put the metaphorical icing on the cake that is our 2004 GTO, SLP offered to tune our computer. This mod alone yielded a ton of power on top of what the intake and exhaust gave us. And extra power puts dents in ETs--check out GMHightechperformance.com to see how quick we went.
Overall, we were very impressed with this first round of products on our project GTO. For a grand total of around $1,650, greatly improved looks, sound, power, and traction have resulted, and this is only the first step. Keep an eye on future issues because we guarantee more surprises real soon.
The stock exhaust also uses one of the most unique hanger systems this author has ever seen, utilizing six rubber hangers that simply slip off horizontally, held only by an interesting brass clip. The clips came off fairly easily with the use of a screwdriver and some pliers, but we were sure to not to bend them too much as they would be reused. | 
After unbolting the pipes from the flanges just aft of the catalytic converters, the entire system (minus tailpipes) comes off in one piece. We suggest a two-person team for this step. We actually had three people, but one was taking the photo and another "hadn't finished mowing the lawn." Hence, Gary "Fat Pops" Werner experiences the Pain of One. | 
Inside the stock muffler, one can see why it makes as much power as it does compared to an aftermarket system. It is a nearly straight-through design itself, far from your ordinary O.E.M. muffler. Remember though, that pipe going through there is definitely less than 2.5 inches in diameter and has a lot of perforations, so it does have some restriction to it. |
After loosely bolting SLP's "X" pipe to the cat-forward portion of the exhaust, the SLP mufflers slip right on the back. These mufflers are a straight-through design, unlike a lot of other manufacturers' mufflers that use baffles. These puppies have just enough packing in them to deaden the offensive sounds, yet keep the good tones. The available resonators look almost identical from the outside and are a good bit lighter. | 
This is a photo illustrating what NOT to do with the intermediate pipe. Here we are trying to slip the pipe into the rubber hangers before sliding the pipe onto the back of the mufflers. This will just not work, as the rubber hangers do not stretch very far (they are an especially tight fit to get onto the O.E.M. brackets--WD40 helps a bunch). The pipe must be put onto the mufflers first, and then the rubber hangers pulled down onto the pipe hangers by hand. | 
With the intermediate pipe in place, the tail pipe is next to go on. It must slide rearward through the SLP-supplied rubber hangars under the bumper first, then onto the end of the intermediate pipe. Note the reuse of the stock brass clips to hold the rubber hangers in place; one may have to bend them back into shape a bit to get them to stay on correctly. |
As we know, no job is complete without having to use a hammer, and this was no exception. The tail pipe needs some help getting onto the intermediate pipe, and another person must also hold on to the front of the tail pipe during hammering so that it does not pop off. This hammer also came in handy getting the mufflers onto the "X" pipe all the way. | 
To finish off the installation, SLP's sweet slash-cut tips are installed onto the end of the tail pipe. They can be adjusted individually a good amount depending on how you want them to look from the back of the car. These clamps, along with all others on the system, must be tightened as much as they will go, or else the system will leak at each pipe junction. This is not SLP's fault; it is just the nature of the clamps. | 
The finished SLP "Power-Flo" system looks so awesome, it is a shame it will be hidden underneath the car all of the time. The sound is substantially different from stock; expect a somewhat higher-pitched, sophisticated European tone. For even more noise and a deep bellowing grunt, go with the resonator-equipped "Loud Mouth" system, Part No. 31060, and save yourself $100. But be forewarned that the "Loud Mouth" is right at the borderline of attracting attention from the fuzz. |
LS1Speed's GTO High Flow Induction, Part No. IZM104, retails for $179.99. There are very few parts involved in the system, the main ones being a conical cotton-and-gauze filter and a 4-inch polypropylene tube. | 
As the first step in the installation, the stock tube leading from the throttle body to the mass air flow sensor is removed. Though not significantly smaller in diameter than the LS1Speed tube, this thing actually has a metal rib in it just before the bend to the throttle body. This rib protrudes into the path of incoming air significantly and surely costs a few ponies. | 
Before yanking out the stock airbox, we were sure to remove the factory wiring clip from the side of the unit. This was not a quick-disconnect by any means and just had to be delicately pulled and twisted until it came out. Oh well, at least Holden didn't just put it in there with zip ties. |
In order to get the stock airbox assembly out, one must first pull the coolant reservoir tank off of its mounts and then do the same for the airbox. GM uses some really neat, vibration-dampening rubber mounts for these that just pop in and out, which have heretofore been unseen. It makes the job that much faster and negates the possibility of screw stripping. | 
We just yanked the stock air baffle out of place from behind the headlight, but it makes it easier if you lift this end of the upper radiator shroud (just a few plastic pop rivets must be taken out)--there is much less risk of messing up your factory weatherstripping if you do it that way. This baffle sealed the factory box away from warm underhood air, yet lead only to a dead corner of the fenderwell with no real air passages leading to it. A design oversight, in our book. | 
With the airbox out, the mass air and stock rubber coupler are pulled off of the box for reuse on the LS1Speed kit, albeit flipped around to the other side of the meter. |
LS1Speed supplies a cool, washable 9-inch conical filter. It goes directly onto the end of the mass air flow sensor. 85mm MAFs require a silicone coupler andl larger filter. | 
We found that no matter how we twisted and turned the mass air flow sensor and swiveled the transmission cooler line, the black electronics box on the mass air flow sensor would contact this line. For this reason, we opted to wrap the line in some rubber hose to prevent damage to the meter from heat and vibration. | 
With the intake system already fully assembled, it goes onto the throttle body as one piece. We recommend tightening the silicone hose adapter onto the throttle body first, then inserting the induction tube. You need to really push on the induction tube and make sure the small lip on the very end of the tube gets past the hose clamp (but not so far that it is hung up within the throttle body), or the tube risks popping out of the rubber coupler. If this happens while driving, the engine will start sucking unfiltered air and it will throw the check engine light. |
In the final adjustment, some rather tough twisting of the mass air flow sensor will be needed to angle its plug away from the power steering pulley and to get it to contact the trans cooler line in just the right way. Here you see the interference between the trans line and the mass air. This may be a blessing in disguise, though, as it seems to help hold the system from falling. On manual cars, this line is not there, and the end of the air filter just rests on the fenderwell to prevent the whole system from moving. | 
The final installation of the LS1Speed intake system definitely looks cool, and we think it would be even cooler (both literally and figuratively) with a cold air enclosure box. This is particularly important because of the way the GTO's computer pulls out timing when it sees warm air at the intake air temperature sensor, though custom tuning can compensate for this. | 
With the install completed, it was back to SLP Performance Parts headquarters for a few more dyno pulls and some PCM tuning. |
SLP Performance Tuning: A Whole New Animal
To put it bluntly, we never expected such a huge difference from some timing and fuel adjustments, but SLP gave our Goat the biggest power increase you can get short of nitrous or a supercharger these days.
Though it no longer has an on-site installation and customer dyno center, SLP continues to offer mail-in tuning of your 1997-up GM engine computer, whether it's a GTO, Corvette, Hummer H2, or Cadillac Escalade. A few levels are available depending on exactly what you are looking for, with prices starting at $149.95 for the "PCM1" calibration. See SLP's web site for details.
Of course, since we were present at the facility, we got a full custom-tailored treatment, roughly equivalent to a $449.95 "PCM3" calibration. Before even touching the GTO's computer, the SLP boys knew big increases were to be had, as the data acquisition equipment was showing only about 18.5 degrees of full timing advance above 4900 rpm and a rich air-fuel ratio that dipped to as low as 11.85 to 1 during dyno runs. In addition to this, the computer was pulling out more timing than was necessary to compensate for increased intake air temperatures. The SLP guys decided to bump things up to 25 degrees of full timing and stop the computer from pulling out any timing until the intake air temperature was over 110 degrees, both of these values being very safe. In fact, we were advised we could still even run 87 octane fuel if we wanted to--the computer would sense engine knock and retard timing until the next gas fill-up, just as it does from the factory.
Though the air-fuel ratio improved somewhat as we threw on the intake and exhaust systems--into the low 12 to 1 area--the SLP guys decided to push us a bit further, into the high 12 to 1 range. This is leaner than stock but still quite safe. It should be noted that the tuning SLP offers to customers does not make any adjustment to air-fuel ratio because every car is so different (they vary from between 11.5 to 1 and 12.5 to 1 from the factory). This is why other over-the-shelf tuners often end up making cars too lean when, say, the mixture is leaned a full point. This discrepancy between what we got and what SLP customers will get should not be much of a concern, though, as the vast majority of our extra power came from ignition timing adjustment alone.
While the power increase from SLP's custom tuning is great in and of itself, automatic-equipped cars get an added bonus in a full transmission recalibration. Complaints of sluggish or non-responsive tranny performance will disappear in a heartbeat as shift firmness, shift mph, warmup time, torque converter lockup speed, and a myriad of other parameters are modified. Customers can choose from increases in shift firmness starting at 50 percent and going all the way up to neck-jarring race shifts. We chose the 50 percent increase, and it was a huge improvement in feel without being annoying and head-snapping during part-throttle driving. The factory torque reduction was also eliminated, which is GM's attempt at getting all of its cars to go 200,000 miles without a transmission rebuild. This feature can do a bunch of things you don't necessarily want in a performance car, including shift delay, pulling engine timing during shifts, and so on. Though the system does improve transmission longevity, eliminating it allows firm shifts no matter what. In addition, our tranny shift points were bumped up to 6200 rpm, which is where the engine rev limiter used to be (now 6400).
As a final adjustment, SLP changed our cooling fan settings. This helps the engine run as much as 10 degrees cooler, even with the stock thermostat. All said and done, SLP worked an absolute transformation on our GTO with its engine and transmission recalibration, and we'd recommend it to anyone who wants an even better driving experience with their GM car or truck.
Special thanks to SLP's Brian Reese and Hank Daniecki for their super job on our GTO's tune, and for the dyno time donated for this story.
SLP's Hank Daniecki delves into the GTO's stock computer program and works some magic with both the engine and transmission. His efforts really transformed the car in ways that no other mod is ever likely to. SLP's tool of choice is LS1Edit, and it is continually updated to provide access to the latest computer functions that GM comes up with. For example, the intake air temperature-timing compensation table only came into existence very recently on production cars. | 
From a 290.4/297.6 baseline, the SLP exhaust brought the dyno numbers up 6.3 ponies to 296.7 horses and 297.5 lb.-ft. The 0.1 lb.-ft. peak loss is deceptive because the car gained as much as 5 lb.-ft. or more in the upper rpm range. LS1Speed's intake gave an additional 7.6 ponies and 3.3 lb.-ft., with 304.3 horses and 300.8 lb.-ft. resulting. SLP's tuning yielded what can only be called an awesome increase in power and torque throughout all rpm ranges, adding 23 horses and 26.8 lb.-ft., for a grand total of 327.3 horses and 327.6 lb.-ft. showing at the rollers. | 
The Drag Pak gave the GTO an extra 36.9 horses and 30 foot-pounds at the wheels, and a trip to the track on a 75-degree day rewarded us with a 13.20 ET at 104.9 mph on a 1.97 60-ft. time, an improvement of over 6 tenths and 4 mph! |