From an article on FordMuscle.com:
http://www.fordmuscle.com/archives/2005/03/05Mufflers/

New Flowmaster American Thunder muffler specifically designed for the 2005 - 2006 GT's. (If I'm reading the article right.)

When exhaust technology company Flowmaster went to develop a system for the '05 Mustang they faced a challenge. How to improve upon a factory muffler that flows as well as their popular 40-series muffler, and how to do it without creating a resonance frequency that will annoy the driver during part throttle driving. Even though the 05 Mustang has a very simple and easy to replace muffler system, Flowmaster deliberately waited until after the release of the car to come out with their product.

In fact, while at a glance one might assume the Flowmaster '05 mufflers are just their 40 series muffler with '05 specific hangers and piping, the reality is the internals of this 5" x 10" muffler are completely unique for the new Mustang GT.

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.

I've listened to both several times, and while I do notice a difference in the sound, I don't hear really any big changes. Especially not enough to justify plunking down several hundred bucks.

 

the flowmaster is more raspy,i like magnapacks and an o/r H, thats what my bro has on his 06, sounds awesome

 

I think if you want to see actual hp gains then go with the magnapacks....flowmaster sounds pretty good, but they aren't exactly the leaders in gaining hp....just my opinion.

 

lol, you know if theres a difference in hp it would barely be enough to see on a dyno, much less feel.

 

Yeah your probably right, but every little bit counts

 

the flow is clearly louder by a tad thats about it. The stock cats hold alot of sound back.

 

The upshot of the flowmasters for 05+ is (by all accounts) they went back to the drawing board and actually tried to cut the resonance. That's really hard to do given the extremely short tail pipe length, so all the sound is pretty much being developed in the muffler instead of being formed there and tuned with the exit pipe size, shape and length.

I think flows sound tinny like you're tapping a metal plate with each exhaust pulse, so I really have a distaste for them. Borla, Bassani, Magnaflow, etc.. are all fine sounding mufflers as well.

 

Damn, them 05's leave pretty good.