Shelby GT500 Performance

Package for the 2007+ Shelby Mustang

In our continuing efforts to offer performance improvements for the new Shelby GT-500’s… Alternative auto is now offering a 13psi boost upgrade by changing the stock blower pulley and adding a upgraded higher flowing air box assembly.

For further gains we’ve also worked with large 1 ¾” diameter semi full length Kooks headers, matching 3” X-pipe equipped with hi-flow cats, and a pair of better mufflers too.

In the summer of 2006 we developed a custom tune for the GT-500’s that was worth about 60+ RWHP, upping a typical GT-500 to about 475 RWHP from stock which is usually low 400’s RWHP.

Now with the addition of our smaller blower pulley, and improved air box with larger MAF and again with our tuning, we’re now seeing over 530 RWHP with totally stock exhaust. This package with tuning and potentially a 3.73 will make for a very hard accelerating and fun GT-500.

We’ve also worked with one of the most impressive looking and fitting headers on the Shelby that we’re manufactured by Kooks. The Kooks 1 3/4” header and X-pipe system came with a 3in X-pipe that also has two hi-flow cats built into it.


The 3” X-pipe is only 3” diameter till about half way through the system. Then from about the middle of the car and back, the exhaust systems diameter stays stock at 2.5”. We did not go out of our way to make the last half of the system 2.5” because we’ve learned that 2.5” is really not a problem till your trying to get well over 600-625+ RWHP.

And also when using a exhaust system that’s entirely 3in, it gets rather difficult to make the system sound reasonably quite and tasteful over all. So in our efforts to retain a good, modern and tasteful sound we went out of our way to also install two resonators in the system to keep the Flo-Master 3-chamber mufflers from droning and getting overly loud as you go deeper into the gas pedal, when the power really starts to come on with the higher revs.

In a nut shell the well thought out exhaust we offer for the Shelby ends up sounding very good with out any annoying rasp as you start to go up in the revs under load.

And the entire system was worth about 20 more RWHP above our 530ish RWHP that is achieved with the pulley change, air box & tuning… Ending up with about 550 RWHP.

All of this combined with a 3.73 makes for a very incredible feeling GT-500 regardless of what the car weighs which is rather heavy over all with one or two people in the car.