GT500-Swapped Fox Body Mustang Hits the Dyno and Goes for a Rip: Video

By Brett Foote - September 18, 2025

Things went a little too well for this GT500-swapped Fox Body Mustang, as it made so much power it can’t seem to find traction.

The S550-generation Ford Mustang Shelby GT500 bowed out of the lineup a year before that version of The Blue Oval’s pony car gave way to the new S650, but it remains an incredibly popular vehicle, for some pretty good reasons. That’s pretty much to be expected for a machine that looks great and comes equipped with the supercharged 5.2-liter Predator V8, cranking out 760 horsepower and sounding amazing in the process. We’ve even seen a handful of GT500-swapped Fox Body Mustang builds out there, which are arguably even cooler.

This particular GT500-swapped Fox Body Mustang comes to us from the folks at Motion Performance, and since it’s a fairly fresh build, it’s just now getting around to hitting the dyno – in this case, a hub dyno. As it sits, the Predator sitting in the engine bay is pretty much stock, save for a couple of different pulleys that will result in a bit more boost, at least. Things start off rather promising with a baseline run that results in just under 770 horsepower and 626 pound-feet of torque, too.

After swapping out pulleys and extracting a bit more boost from the blower, this Fox Body Mustang winds up improving to 780 horsepower, but torque dips to 615 pound-feet – and that’s with some belt slip occurring. After a little trouble shooting, our hosts discover that the throttle body was actually closing a bit because of a fuel injector duty cycle warning, a safety mechanism that’s built in to the factory ECU.

There are a few more issues that pop up along the way as well, which is to be expected for a modern powerplant running modern software. After hitting 788 horsepower, our hosts find out that the system was pulling ignition timing out at the top end due to the fact that the engine was getting a bit hot. Thus, they let it cool down and gave it another go, only to be rather shocked when the pony car lays down an incredible 865 horsepower and 658 pound-feet on 14 pounds of boost.

That’s so much power, in fact, that our host says that it would be best to have two tunes – one for the street and one for the track – with a more conservative 800 horsepower output for regular driving, since it’ll be hard to hook on the street, anyway. Even in spite of that, a little test drive reveals that the car is “basically useless in second” gear, as it just won’t hook at all. Regardless, to us at least, that just makes it even more fun to tool around in – if not a bit sketchy.