Installation
There aren’t many cars that one can swap a pair of mufflers out with a couple wrenches and no cutting or welding. In fact the 05’s mufflers are so easy to R&R that we swapped ours out in under 20 minutes, twice. Once in the track parking lot where we were shooting the before and after videos and again on the dyno. Unless you’ve been eating too many donuts there is no need to raise the car to access the mufflers. Here is a quick run down of the very simple process:

The factory 2005 Mustang exhaust system is fairly optimized, consisting of 2.5″ mandrel bent tubing.		Located at the very back of the car, the 05’s muffler pose a challenge for controlling resonance and part throttle “drone”.
Ford must have anticipated aftermarket muffler swaps because the tail pipes are simply clamped to the over-axle tubes. Loosen the clamp nut first.		There are two hanger brackets at the each end of the muffler. Don’t try to wrestle the hanger from the rubber, simply unbolt the entire bracket. It’s held to the chassis with two bolts, 10mm socket.
With both front and rear mounts unbolted the muffler can be pulled off the axle pipe and slid out from under the car.		Match up the Flowmaster mufflers with the old, then transfer the mounts onto the hangers of the new muffler.
Place the supplied clamp over the rear pipe and then position the new muffler into place on the over-axle pipe.		Reattach the hanger brackets to the body. Due to tight space above the muffler, we found it easiest to get the lower bolt on the rear bracket tightened first.
Using a 15mm wrench tighten up the clamp connecting the muffler pipe to the over-axle pipe.		Step back, check the position of the tips and adjust if necessary. Fire it up and enjoy.

Video 1: Listen to the deeper, more raucous tone of the new Flowmaster American Thunder system on a 2005 Mustang GT. The polished 4″ rolled tips look stunning, especially with the embossed lettering.


Video 2: For comparisons sake, here is a clip of the bone stock 2005 Mustang exhaust system. It sounds better than Mustang’s of years past but still lacks authority. The slash cut 2.5″ tip however looks hideous, and reason enough to replace.

Power
We haven’t seen any egregious claims from ’05 exhaust kits just yet, though we anticipate they’ll show up soon. Mclelland explained to us that modern engines, especially those with variable cam timing like the ’05 Mustang, are not going to respond to a reduction in backpressure like older engines do. This is largely in part due to the variable cam timing and wide lobe separation angles used on modern cams. As a result the exhaust system plays very little role, if any, on scavenging and drawing the intake charge to the cylinder. It is no surprise then that we saw no significant power gains on the dyno. The factory ’05 mufflers apparently flow well enough to support as much as 425 horsepower. Once you add a blower or increase displacement is when you’ll see some gains from the less restrictive Flowmasters. Either way, the sound and looks alone are worth it if you’re looking for reasons to set your new ’05 apart from the rest.