Frank H. Cicerale
December 1, 2008
Thankfully this New Jersey State Trooper liked the heat our Shelby GT500 was packing. After all, it had 600 hp on tap thanks to the addition of SLP's 600HP Performance Pac.

It's been said that you never want to bring a knife to a gunfight. After all, if you do, you will be at a serious disadvantage. The same can be said for waging battle on the streets and dragstrips of America. With the new Dodge Challenger now roaming the tarmac, and the Camaro about to hit showrooms with a V-6 purported to lay down 300 some-odd horsepower, the Mustang is going to have to step up its game. Thanks to companies like SLP Performance Parts, adding the necessary go-power is as easy as making a phone call.

Known to produce parts for the dark side-a.k.a. the F-body cousins, SLP also has some stellar parts and pieces for Mustangs, including the GT500-namely its 600HP Performance Pac for the SVT special. After speaking with SLP's Director of Engineering, Hank Daniecki, at length about the package, we hopped in our beater Taurus and cruised down to Toms River, New Jersey, where we checked out the parts and how easy it was to install them. We then borrowed an SLP-equipped GT500 that just had the 600HP Performance Pac installed, and not only hit the dragstrip with it, but cruised the roads around central and southern New Jersey to see just how the road manners would be.

It may look stock, but human hands have touched this GT500's 5.4L Four-Valve mod motor. The 600HP Performance Pac from SLP is the same as the one the Toms River, New Jersey, company offered for the Terminator Cobra. It consists of a high-flow air filter, a smaller upper blower pulley, long-tube headers, an x pipe system, a full-on exhaust, and a Diablo Predator programmer stocked with a custom tune.

While the 600HP Performance Pac comes with three different options, all of the basic parts are the same. The package is made up of an SLP high-flow Blackwing Panel air filter, a 2.70-inch upper blower pulley, a Diablo Predator programmer stocked with a custom SLP tune, SLP long-tube headers, and a Powerflow-X x-pipe assembly. The options lie in the type of exhaust system you want, as you can get the 600HP Pac with either a Loud Mouth exhaust, or a PowerFlow exhaust with either 3- or 4-inch tips. The Pac is a bolt-on system, and the smaller upper pulley still retains the usage of the stock belt. All told, the Pac is good for 600 SAE-corrected horsepower to the rear tires on the SLP SuperFlow chassis dyno.

"The GT500 Pac was derived directly from the Pac we had developed for the '03 and '04 Terminator Cobras," Daniecki explains. "We knew how much power we developed with those cars, and applied the same type of changes to the GT500. The added power in the Cobra was more than enough for the average enthusiast, and the goal on the GT500 Pac was to obtain the same gains, with any more than that being icing on the cake. Once we got close to the 600hp level, we made some minor adjustments to the package to make sure every car would reach that level."

The most difficult part of the installation of the 600HP Pac is the addition of the long-tube headers. Included with the Pac is a header installation kit that allows you to reconnect the O2 sensors to the headers. The new pipes are made of stainless steel, and look quite nice.

Obviously the exhaust improvements offered by the high-flow filter, long-tube headers, and better exhaust system, would undoubtedly help the 5.4L supercharged Four-Valve inhale and exhale easier, but the majority of the power gains seen in our test vehicle came from the swap to the 2.70-inch upper blower pulley and the addition of the custom tune. "The most important changes in the tune for any modified supercharged engine are ignition, timing, and fueling," Daniecki says. "Both are modified for the extra boost and added flow, keeping the engine safe, while at the same time adding horsepower and torque."

The results speak for themselves. While the header install is the most difficult portion of the overall installation of the Pac, for a competent shop with a lift, the install can be performed in a day. Our test car spun the rollers to 520 hp and 510 lb-ft of torque in bone-stock trim. With the addition of the 600HP Performance Pac, the same Shelby rang the dyno to the tune of 601 rwhp and a stump-pulling 604 lb-ft of torque. That's a solid gain of 84 hp and 94 lb-ft of torque at the rear tires.

Thanks to companies like SLP Performance Parts, adding the necessary go-power is as easy as making a phone call.

The SLP 600HP Performance Pac for the Shelby GT500 is a surefire and easy way to add some venom to the snake's bite.

Of course, a true MM&FF evaluation wouldn't be complete with out a thorough dragstrip flogging. The GT500, which was equipped with the Powerflow exhaust system with the 4-inch tips, was driven to Old Bridge Township Raceway Park in Englishtown, New Jersey, our resident test facility. We promptly opened the hood, and while a pair of portable fans were blowing air onto the blower to cool it down, we jacked the rear end up and swapped the stock rear sneakers for a pair of 26x11.5-inch Mickey Thompson tires mounted to 17-inch Mustang GT rims. This was done due to the fact that the added power and torque would make it difficult at best to get the beast to hook upon launch.

Since this car wasn't tested in bone-stock trim, we had to look back into the MM&FF archives, where we found a best elapsed time of 12.38 seconds at 115 mph for a stock GT500. After perfecting his launch and shifting techniques, MM&FF's own Evan Smith promptly ripped off an 11.43 at 122 mph. This was with a 1.72 best 60-foot time, and on the same day MM&FF tested the new GT500KR. We have to say, the SLP-prepped GT500 held its own against the new Ford super Stang. Not too bad for some added bolt-on parts.

To install the new headers, you will have to remove the steering column intermediate shaft and move it out of the way. This will allow you to sneak the headers in with a minimal amount of knuckle scraping, tool throwing, and cursing.

In addition to ripping it up at the dragstrip, we also took the car for a weekend to evaluate its road manners. After all, with the added horsepower, we wanted to see how tractable the car was when performing street duty. After putting on a fair amount of miles over the course of a few days, the verdict was one of happiness and, as expected, success. There was no surging when it came to the revised tune, and even though the fuel trims were changed, fuel mileage was still respectable, as we knocked down 24 mpg on the highway during one stretch. Over the course of three tanks of 93-octane pump gas, the Shelby averaged 18.5 mpg both on the highway and on roads around town. The exhaust tone emitted by the long-tube headers, x-pipe system, and exhaust system was not overly abrasive, though there was a drone at 80 mph that made us have to crank up the stereo just a bit to overcome. Overall, though, the sound and added power not only allowed the car to make a statement, but back it up. Just ask the patrons in front of a local eatery who witnessed the Shelby roll up in authority and scare a pesky 350Z into the depths of the parking lot.

The stock airbox and throttle body is also taken out to aid in installation of the headers. While the airbox is out of the car, the stock air filter was replaced with the high-flow piece that comes with the Pac.

All in all, the SLP 600HP Performance Pac for the Shelby GT500 is a surefire and easy way to add some venom to the Snake's bite. The best part of it is the fact that the parts have been tested and approved by SLP, and are all available separately from one another, just in case you don't want the whole shebang. "The advantage to the Performance Pac is that it is thoroughly tested to not only guarantee the horsepower and torque levels advertised, but it is also guaranteed to be safe for the engine for the long haul. By creating a package with components from one supplier, we know exactly how the vehicle will react, and can create a tune that comes with each package so that each package performs the same on every car, every time. This allows the customer to install the package and the tune and hit the road quickly without any worries or need for added dyno time."

While SLP has no plans to add anything to the package as of now, with how well it works and what it does for the Shelby GT500, would you really want any more? I guess by asking that question, it means we're bringing a loaded weapon to a knife fight. Let's get it on!