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Flowmaster American Thunder Classic Axle-Back Exhaust (11-12 GT, 11-12 GT500)

Market Price $556.99 You Save 13% ($73.02)
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  • Item 18413
  • MPN# 817496

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Features, Video & Details

  • Aggressive Flowmaster Classic 40 Series Sound
  • Polished Stainless Steel 4" Tips
  • Durable 409 Stainless Steel Construction
  • Includes Two Flowmaster Mufflers
  • Fits 2011-2012 GT and 2011-2012 Shelby GT500 Mustangs

Review & Installation Video

Review & Install Video

Hey everyone I’m Justin with and I’m taking a look at Flowmaster’s American thunder classic axle-back for your 2011 and newer Mustang GT. The American Thunder series axle-back is designed to give your Mustang that iconic Flowmaster sound, while giving you a little more horse power at the rear wheels. The system is constructed from durable 3 inch, 16 gauge aluminized tubing, and features 2 40 series chambered mufflers, everything is finished off with two 4 inch polished tips embossed with the classic Flowmaster logo. Now the installation itself should only take you about an hour using standard hand tools. Now lets see how Flowmasters American Thunder sounds compared to your factory exhaust.

Classic Flowmaster Sound. Give your 2011-2012 Mustang GT or GT500 that unmistakeable Flowmaster sound with a nice power boost with the American Thunder Axle-Back exhaust system. This system improves exhaust flow using Flowmaster's patented chambered muffler design for that throaty, aggressive exhaust tone your high powered Pony deserves.

Quality Construction. Constructed from durable 3" 409 stainless steel tubing for its resistance to corrosion, the Flowmaster axle back features two special 5" X 10" chambered Classic 40 Series Flowmaster mufflers with 4" diameter polished stainless steel tips for muscle car looks and sound. As an added bonus the tip is embossed with the "Flowmaster" logo for true custom styling.

Application. This Flowmaster American Thunder Axle-Back exhaust system is designed to fit the 2011-2012 5.0 Mustang GT and 2011-2012 5.4L Shelby Mustang GT500 for both great looks and great sound.

Fitment: 2011 2012 Details

What's in the Box
Hardware included:
  • (2) Mufflers
  • (2) 3/8" Hanger Keepers
Installation Info
Tech Specs
Axle-Back Exhaust Specifications
Piping Material: 409 Stainless Steel Tip Material : Polished Stainless Steel
Piping Size: 3" Tip Size: 4"
Muffler Type: Chambered Fitment: 2011-2012 Mustang GT & 2011-2012 GT500
Will it fit my Mustang?
  • Boss 302 - 12
  • GT - 11, 12
  • GT500 - 11, 12

Most Helpful Reviews

Flow master American thunder

They sound great at cold start, and when you step on it. For the price they can't be beat . Took about an hour to install hardest part is factory cans are tacked. Other than that piece of cake. I would ...

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Helpful (1)

Awesome sound

Awesome sound I'm currently running an off road H-pipe with these axlebacks.Deep sound with no rasp, and there is minor drone then what everyone else is saying. A must buy for your stang!

Helpful (2)

Overall Rating 4.7 out of 5 stars
Read All 62 Reviews

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1 Questions / 3 Answers

Ask a Question

Rear Valance I am thinking about getting Flowmaster American Thunder Classic Axle-Back Exhaust (11-12 GT, 11-12 GT500), but will I also need to change the rear valance as well? I have a 2012 GT /CS with stock everything now and was just wondering if I would have to do more mods to get the system. Any help would be greatly appreciated!


Helpful (0)
  • Your current rear valence set up will be fine, as the single outlet mufflers will fit thru the same space as your existing mufflers. Had you gone for the quad set up (2 outlets per muffler), then you would need the gt500 rear valence.


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Customer Reviews (62) Write a Review

Overall Rating 4.7

out of 5 stars

Installation Time:Less than 1 Hour

Installation Cost:0-$100

  • Loudness / Volume 4.468085 4.5
  • Exhaust Note 4.680851 4.7
  • Fit & Finish 4.787234 4.8
  • Bang For Your Buck 4.521739 4.5

  • 5/5

    Installation Time: Less than 1 Hour

    September 25, 2013

    Awesome sound

    Awesome sound I'm currently running an off road H-pipe with these axlebacks.Deep sound with no rasp, and there is minor drone then what everyone else is saying. A must buy for your stang!


    Year: 2012 Submodel: GT West Palm Beach, FL, USA

    Helpful (2)

  • 5/5

    Installation Time: Less than 1 Hour

    April 04, 2013

    Monster Sound

    Overall, I love this exhaust. I was looking for something that had a really deep rumble at idle and this is it. It also sounds fantastic at wide open throttle. My only complaint is that there is a bit of a drone from 1300 to 2000 RPM. My stereo system is enough to take care of that though.


    Year: 2012 Submodel: GT Nova Scotia, Canada

    Helpful (1)

  • 5/5

    Installation Time: Less than 1 Hour

    July 16, 2012

    Flow master American thunder

    They sound great at cold start, and when you step on it. For the price they can't be beat . Took about an hour to install hardest part is factory cans are tacked. Other than that piece of cake. I would highly recommend these if your on a budget.


    Year: 2012 Submodel: GT Tx

    Helpful (1)

  • 4/5

    Installation Time: Less than 1 Hour

    May 30, 2012

    Not the Loudest / But Sounds Good

    I installed these in about an hour taking my time. I installed them in my drive way with the car barely jacked up off the ground. The hardest part was getting the stock exhaust off. They are in a very tight area, the bolts were easy to loosen, it was physically getting them out of the rubber bushings and out from underneath the car was the problem. Installing the Flow-masters was a lot easier. The Mufflers aren't near as bulky and slide right in (with some lubricant spray), except for one of the front hangers on the drivers side ( I just cut about a half inch off the one hanger and it fit perfect). The Exhaust isn't as loud as I was hoping for. But I was use to the SLP Loudmouths on my 2008 GT. They were a little annoying at times ( at least that's what my Wife said). So this time I was going more for the tone, especially when down shifting, that's one thing I didn't get with the SLP's. And for the most part I am happy with this Axle Back. It sounds great when your on the throttle and when you are down shifting. As for the drone, I get a little drone going 70 up the interstate. So I just do 80 and it goes away. The only real complaint is when the car is at idle it isn't as loud as I would like. Overall I am a satisfied customer. But if your wanting the loudest exhaust ( not so much the tone or note) go with the SLP Loudmouth. They will wake up the neighbors, and believe me literally wake up the neighbors. If you want a great sounding exhaust then these Flow-masters are top notch. Tim L.


    Year: 2013 Submodel: GT Rock Hill, SC, USA

    Helpful (2)

  • 5/5

    Installation Time: Afternoon

    December 02, 2011

    These muflers are great, and easy to install.

    These muflers are great, and easy to install. The fit is real good (great)!


    Year: 2012 Submodel: GT Huntsville Alabama

    Helpful (2)

  • 5/5

    Installation Time: Less than 1 Hour

    November 13, 2011


    have been wanting to get for months, much better sound in person than in videos.. one thing never mentioned in reviews i read was how much more you can feel the rumble of the car once installed. Sound is perfect. install is easy.


    Year: 2012 Submodel: GT

    Helpful (2)

  • 5/5

    Installation Time: Less than 1 Hour

    October 08, 2011

    Great sound easy installation

    Flowmaster sticks to its quality of sound with this one. Easy install great sound. Bolts right on couldnt believe the quality of this axelback exhaust numerous compliments and thumbs up.


    Year: 2012 Submodel: GT Charlotte, NC

    Helpful (2)

  • 5/5

    Installation Time: Less than 1 Hour

    September 14, 2011

    Sounds Great!

    Just put them on my 2012 mustang gt 5.0. took less then an hour and they sound great a lot better then the stock. the clamps were spot welded onto the old pipes but just had to make one little cut then they came right off and went on the new ones great.


    Year: 2012 Submodel: GT Hanover, PA

    Helpful (2)

  • 5/5

    March 18, 2011


    Sound friggin awesome!The way my 5.0 SHOULD sound.Easy 45 min. install.Follow instructions and you can't go wrong.Nothing but compliments on how awesome my baby sounds now!!There is a little drone but not too bad in 6th at 1800-2100 rpms.Really nice when I drop the top and let my pony run.Drone is avoidable by shifting down to 5th or just leave it in 6th and ACCELERATE.


    Year: 2011 Submodel: GT

    Helpful (9)

  • 5/5

    March 17, 2011


    This exhaust has the deep rumble in low RPM's and that hollow mustang tone that we all love in the upper RPM's. In cab noise isn't bad at all, does drone a little bit around 2000rpm in 6th gear (3.73) but no complaints, just compliments from everyone who hears it. Install was very easy, 30 min tops. Its so simple you will try to over think the install. You will have to use the factory clamps so don't tear anything up while removing the factory mufflers. Wish i could give it more than 5 stars.


    Year: 2011 Submodel: GT

    Helpful (9)

  • 5/5

    March 24, 2011


    1.5 hr install time. mufflers tuck under car out of view. sound is deep but not 2 loud. upon start it rattles my garage door until i back out. u can feel it inside but can have a conversation easily. there is drone @ 2k but install jlt intake & tune then drone is gone. stomp the throttle on freeway and you will turn heads. hope this helps

    jason bradford

    Year: 2011 Submodel: GT

    Helpful (7)

  • 5/5

    August 08, 2011

    great exhaust system

    love the black as it hides the muffler from being seen from the back and underneath. I also think the quality and workmanship of the welds are great. i installed these in 1 hour and fifteen minutes and i am 66 years old laying under the back of the stang. i had to have another person help fit the drivers side into place because of the pan hard brace but still was not a big deal. i have an automatic and i really dont have any drone with these but they do sound great at idle and super at wot. btw the noise does not override my sync as a lot of aftermarkets do. i would highly recommend these to anyone for street use

    dan raab

    Year: 2011 Submodel: GT

    Helpful (7)

  • 5/5

    Installation Time: Less than 1 Hour

    September 23, 2011

    Real Mustang sound.

    This Flowmaster system sounds great on my 2012 GT. It makes it sound like a mustang should sound. Fits up under car alot better than the stock system. Stainless tips fill up the cut out area much better. Ford should sell Mustangs this way. Installed in about 45 minutes. Great sound, great looks. A++++++++++++++++


    Year: 2012 Submodel: GT Boise, ID

    Helpful (5)

  • 5/5

    November 23, 2010

    Great Stainless Kit!!!

    Kit # 817496 compliments my 2011 5.0L to the fullest. It sounds amazing and the pricing is unbeatable. I cant believe I only paid 374.99 for a stainless kit!!! I would totally recommend this kit to any fellow 5.0L owner. The installation was very simple and painless, another great feature is the black paint. It allows the mufflers to blend in with the undercarriage of the vehicle. I was under the impression this kit was not stainless but it is, it's constructed out of a 409 grade stainless steel. Great American made product for a great price!!!


    Year: 2011

    Helpful (14)

  • 5/5

    Installation Time: Less than 1 Hour

    October 30, 2011

    A Classic Sound for a Classic Car

    I installed my flowmasters in just under an hour. After reading some of the reviews of previous users, I had my game plan set prior to install. NO Need to disconnect any type of bars underneath. I trimmed approx. 1/2" off one of the two rear hangers. (most inside hanger). I lubed the hangers prior to removal of old mufflers, and install of the new ones. The sound is iconic. Deep purr at idle. Nice and loud at WOT. The spot weld on the old mufflers?? No worries!! just bend them repeatedly to remove them. I jacked the car up maybe 4" to make it easier. I absolutely love these mufflers!!!! Great product!!!!


    Year: 2011 Submodel: GT FT. Bragg, NC

    Helpful (3)

  • 5/5

    April 13, 2011

    It's all about the sound!

    My neighbor sees the box and says with a disgruntled look, “new exhaust huh?!" I'm thinking heck yeah! Got to love that old school rumbling sound! Well the installation was really easy and it looks and sounds great! This should be everyone's first purchase for their new Mustang, besides a lowering kit.

    Franciscus Ladage

    Year: 2011 Submodel: GT

    Helpful (4)

  • 5/5

    Installation Time: Afternoon

    January 06, 2012

    Great sound

    Sounds good, looks good, easy to install. The stock ones are a pain to get off there, at least I had issues. Great deep sound. May MPG has gone way down since getting this because I can't keep my foot out of it to hear the sound. I have some drone at cruise speed, but radio handles that just fine. Combined with a Heartthrob straight pipe and Hpipe.


    Year: 2012 Submodel: GT Indiana

    Helpful (2)

  • 5/5

    Installation Time: Less than 1 Hour

    September 21, 2011

    2012 GT Excellent!

    Had to have them installed, due to one small detail left out of instructions, that being the original clamps were welded to original pipes! Cost $92.00 for install. Sounds absolutely awesome. Deep, low rumble at idle, sings nicely at WOT. Highly recommeded. Better have ability to cut the factory spot welds off clamps, though, if you attempt. About 45 mins. for shop install. First Mod for mine. Turns heads everywhere. Thanks AM!


    Year: 2012 Submodel: GT Euless, Texas

    Helpful (3)

  • 5/5

    May 24, 2011

    Great Product; but ask questions before you install

    First off the pros: Great sound, easy of install; approximately 1 1/2 hours. Read the directions thoroughly. You might need a helping hand installing the driver's side muffler due to the panhard bar. It has a definite drone in 6th gear between 1800 & 2000 rpm; just adds more notice to the car. Now the Cons; BTW this is not really a negative comment, but something to think about. Are you going to be heavily road racing your car; street driving, drag racing, etc. I spoke with an authorized Ford/Shelby Mod-shop regarding several other upgrades to my car and found out that, the Flowmasters "Could Possibly" have some issues with the baffles inside the mufflers. No disrespect to Flowmaster, they are on top of the game, however, the baffles "could" separate from the welds and "could" cause restrictions within the muffler at "some point". Everything does wear out over time and I am quite certain that Flowmaster researched long before they ever started building them. I give the product 5 stars. I am a simple street driver with occasional access to road course driving. I don't think I will have a problem with the mufflers, but just think about what use you are going to put them through.

    Keith Stolarek

    Year: 2011

    Helpful (2)

  • 1/5

    Installation Time: Less than 1 Hour

    September 21, 2012

    Flowmaster American Thunder Axle back exhaust

    These mufflers have a horrific hollow bellowing tone inside the car on a 2011 convertible. I took them back off after one day and threw them in the trash, This product was a $380.00 dollar mistake.


    Year: 2011 Submodel: GT Bangor Maine

    Helpful (0)

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