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

    People Who Bought This Item Also Bought

    Most Helpful Reviews

    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 ...

    Read More
    Helpful (15)


    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 ...

    Read More
    Helpful (9)

    Overall Rating 4.6 out of 5 stars
    Read All 67 Reviews

    Customer Photos (8) Submit Your Photo

    View All 8 Photos

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    3 Questions / 5 Answers

    Ask a Question

    I own a 2013 v6. Will these fit? Thank you for your time and assistance.


    • Unfortunately, this Flowmaster American Thunder Classic Axle-Back Exhaust is specifically designed for use on 11-12 GT and GT500 model Mustangs, so it will not fit your 2013 V6.


    Helpful (0)

    i have the outlaw series cat back will these mufflers bolt up to the outlaw exhaust the outlaw series mufflers are too loud with the long tube headers


    • Yes, The piping will step down to allow for the axle back to be interchanged from the Flowmaster Outlaws to the Flowmaster American Thunder Classic Axle-Back Exhaust.


    Helpful (0)

    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!


    • 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.


    Show 2 more answers Helpful (0)

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    Customer Reviews (67) Write a Review

    Overall Rating 4.6

    out of 5 stars

    Installation Time:Less than 1 Hour

    Installation Cost:$100-$250

    • Loudness / Volume 4.442307 4.4
    • Exhaust Note 4.653846 4.7
    • Fit & Finish 4.692307 4.7
    • Bang For Your Buck 4.392157 4.4

    • 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 (15)

    • 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

      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

      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

      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

      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: 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

      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

      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)

    • 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

      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: 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

      June 11, 2011

      not impresses

      i installed these yesterday and thinkin they are going on craigslist. they drone so loud i cant hear my radio or have a conversation. borla or ford racing. no drone is the way to go. 6th gear 1500 rpm they drone up till 2200 rpm thats about 50-75mph they are useless


      Year: 2011 Submodel: GT

      Helpful (2)

    • 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)

    • 2/5

      Installation Time: Less than 1 Hour

      May 14, 2015

      Not What I Wanted

      I have a 12 with an automatic. At 1700 to 2100rpm the drone is so loud you can't ave a conversation in the car. Shift point and interstate speed falls right in this drone range. At cruise speeds around town the deep rumble is awsome and the idle rumble takes me back to the day. I just can't handle the drone so I'm planning on replacing these mufflers with something with less drone. Flowmaster missed it with these mufflers. I have had two other cars with Flowmaster mufflers and enjoyed them both but not these.


      Year: 2012 Submodel: GT Louisville KY

      Helpful (1)

    • 2/5

      Installation Time: Afternoon

      July 19, 2013

      Product gives too much interior drone

      I expected a great exhaust note based on my 2008 GT that had the same system. The exterior sound on my 2011 is excellent but the interior sound is unbearable. I am switching to a Ford Racing GT500 Axle back hoping to tone down the interior drone. If not, its back to stock.


      Year: 2011 Submodel: GT North Carolina, USA

      Helpful (1)

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