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Flowmaster American Thunder Cat-Back Exhaust (99-04 GT, Mach 1)

Market Price $570.99 You Save 13% ($75.02)
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  • Item 18410
  • MPN# 17312

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    Features, Videos & Details

    • Aggressive Flowmaster Sound.
    • Super 40 Series Mufflers.
    • 3" Polished Stainless Steel Tips.
    • Mandrel-Bent Tubing.
    • Durable Aluminized Steel Construction.
    • Fits 1999-2004 GT, Mach 1, and Bullitt Mustangs.

    Review & Installation Video

    Hi I’m Justin with, and I’m breaking down Flowmaster’s American thunder Cat-Back exhaust fitting the 99-04 Mustang GT and Mach 1. A few of the big points with this catback are the 2.5” aluminized steel mandrel bent tubing, the 3” brightly polished OE style tips, and finally and most importantly, Flowmaster’s super 40 series mufflers which give this kit its very aggressive and unique sound. Flowmaster does offer a full stainless version of this same exact exhaust, which will be slightly lighter and better at resisting corrosion, however it does come with a slightly higher price tag. Due to the design of the factory exhaust you will need to cut it off in sections using a saw or something similar. Once you get that out of the way, the American Thunder bolts directly to your factory Mid Pipe, in about 2 hours using some basic hand tools and the included clamps. Now let’s hear how Flowmaster’s American Thunder sounds compared to our factory exhaust.

    Flowmaster Sound.The Flowmaster American Thunder Cat-Back exhaust system with its mandrel bent tubing will greatly improve your 1999 to 2004 Mustangs exhaust flow for an increase in power, and their included Super 40 Series Mufflers will improve your exhausts sound for a deeper throatier muscle car exhaust note.

    Quality Construction. Constructed from mandrel bent 2.5" 16 gauge aluminized steel tubing for it's corrosion resistant properties the Flowmaster American Thunder Cat-Back features their Super 40 Series chambered mufflers with 3.0 " diameter OEM style polished stainless steel tips for great muscle car looks and sound.

    Fitment. This Flowmaster American Thunder Cat-Back exhaust system is specifically designed to fit the 1999-2004 GT, 2001 Bullitt, and 2003-2004 Mach 1 Mustangs for both impressive looks and sound. This system will not fit the SVT Cobra Mustangs.

    Fitment: 1999 2000 2001 2002 2003 2004 Details

    What's in the Box
    • (1) Right muffler assembly
    • (1) Left muffler assembly
    • (1) Right tailpipe assembly
    • (1) Left tailpipe assembly
    • (2) Clamps
    • (4) Lock Washers
    Installation Info
    Tech Specs
    Cat-Back Exhaust Specifications
    Piping Material: Aluminized Steel Tip Material : Polished Stainless Steel
    Piping Size: 2.5" Tip Size: 3.0"
    Muffler Type: Chambered Fitment: 1999-2004 GT, Bullitt and Mach 1 Mustangs
    Will it fit my Mustang?
    • Bullitt - 01
    • GT - 99, 00, 01, 02, 03, 04
    • Mach 1 - 03, 04

    Fits GT, Mach 1, and Bullitt

    People Who Bought This Item Also Bought

    Most Helpful Reviews

    v6 mustang

    Very very happy with this exhaust, I first got the mac O/R H with SLP loudmouth 1 exhaust...big mistake, it was way too loud, and not a good loud. switch to this exhaust and it sounds perfect. good and ...

    Read More
    Helpful (76)

    American Thunder

    Just had this installed on my 2003 Mustang GT Convertible, this was exactly the sound I was going for. If you're like me, and don't want your car to sound ridiculous like some exhausts, then this is the ...

    Read More
    Helpful (29)

    Overall Rating 4.7 out of 5 stars
    Read All 100+ Reviews

    Customer Photos (25) Submit Your Photo

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    20 Questions / 9 Answers

    Ask a Question

    SOUND How is the American Thunder Catback sound going to be if we installed with BBK headers and BBK X pipe ? Because i have bought these headers and the x pipe and the cat back not yet so i am just asking before get that cat back any recommends for the cat back? Thank yall


    • that is what i have on mine, flowmaster/BBK headers/xpipe w cats... but calling them cats is a stretch. can get loud with your foot in it but what a great sound. its what a mustang should sound like. i daily drive and love every mile behind the wheel.


    Show 1 more answer Helpful (7)

    V6 would this bolt onto a v6


    • The GT catback exhausts can be installed onto a V6 only when doing a dual conversion. You would need the V6 dual exhaust adapter in order for this to bolt up - without the adapter, the catback would not fit. I've included the adapter below so you can take a look. If you have any additional questions, please give us a call!


    Show 2 more answers Helpful (6)

    Do I need to buy a new rear bumper??? I have a V6 it’s just a single exhaust


    • The Flowmaster American Thunder Cat-Back Exhaust will require a GT bumper with the dual outlets.


    Helpful (0)

    Hey will this exhaust fit my v6 with the MAC off road h pipe with the MAC long tube headers?


    • This Flowmaster system will fit your V6 that has the aftermarket Mac H-pipe.


    Helpful (0)

    Is there a way to get this without the super 40's? When I got my mustang it had already had super 44's on there and I love that sound. Would I have to replace the super 40's with my super 44's? Or is there another way to go about getting this cat-back without the super 40's?


    • The Flowmaster American Thunder Cat-Back Exhaust would only come with the super 40s and you would need to swap your mufflers out for these.


    Helpful (0)

    Will this cat back fit perfectly with the Pypes off-road H pipe ?


    • Well, the piping size is identical between the two products, however they aren't necessarily designed to work with each other, so you may need a small modification to make it work if any at all, but it will be generally a simple install.


    Helpful (0)

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    Customer Reviews (100+) Write a Review

    Overall Rating 4.7

    out of 5 stars

    Installation Time:Full Day

    Installation Cost:$100-$250

    • Loudness / Volume 4.31 4.3
    • Exhaust Note 4.656566 4.7
    • Fit & Finish 4.4 4.4
    • Bang For Your Buck 4.61 4.6

    • 5/5

      January 10, 2010

      v6 mustang

      Very very happy with this exhaust, I first got the mac O/R H with SLP loudmouth 1 exhaust...big mistake, it was way too loud, and not a good loud. switch to this exhaust and it sounds perfect. good and deep, not raspy and not a tuner sound. I would recomend this for ANY v6 stang looking for duels.


      Helpful (76)

    • 5/5

      March 12, 2010

      American Thunder

      Just had this installed on my 2003 Mustang GT Convertible, this was exactly the sound I was going for. If you're like me, and don't want your car to sound ridiculous like some exhausts, then this is the one for you. I previously had Borla Stingers installed for about 15 minutes, and it was just stupid loud, didn't even sound cool to me, just stupid loud. This exhaust is one of the best I have heard, because I just wanted something louder than stock, but not obnoxiously loud. When you get on it it sounds great, but also sounds great just cruising, but you can put it up in a higher gear and it will get quieter, and there is some drone on the highway, but nothing too bad. I can't believe people actually like straight-through pipes, it would just be a constant drone, and annoying sound all of the time. I would highly recommend this exhaust, and I got the Stainless Steal version, great fitment, and a great quality product unlike others.


      Year: 2003 Submodel: GT

      Helpful (29)

    • 5/5

      Installation Time: Afternoon

      October 26, 2011

      True muscle car sound like no other!

      The American Thunder catback exhaust system is an easy bolt on kit that adds substantially more horsepower and an engine tone that is truly amazing. Installation is very simple, with only four bolts and a few exhaust hangers to deal with, meaning this mod will only take an hour or two. The result of the less restrictive 2.5" pipes (compared to stock 2.25") combined with a pair of Flowmaster Super 40 mufflers and 3" chrome exhaust tips is a sound that will turn heads! The deep, muscle car growl produced by this kit is loud enough to please any car enthusiast but not so loud that they annoy others on the road. The Flowmaster American Thunder catback exhaust system adds an excellent amount of horsepower and a sound that will draw a lot of attention to your Mustang with as little as a few hours labor and the use of common hand tools. A must have for anyone who wants to add a little more muscle to their muscle car!


      Year: 2004 Submodel: GT Mobile, Alabama

      Helpful (23)

    • 5/5

      January 22, 2010

      great sound

      This is a great sounding exhaust not overpowering loud but just loud enough. Sounds great when you get on it.


      Helpful (14)

    • 5/5

      June 29, 2010


      wow i got this system at 10 in the mornin, stalled a little then finally started assembling it at 3pm and was done by 7! wouldve been done at 5 if i didnt have to take the mufflers back off and drill some of the hole out on the right muffler connecter! overall im more then happy with this exhaust, its amazing and i would reccomend this to anybody that likes a nice smooth, deep tone but not overly loud car!


      Helpful (12)

    • 5/5

      January 18, 2011

      more that satisfied

      Just installed on my 2000 vert gt, more than happy with the low rumble and subtle backfire, noticable slight gain in power (untuned) in mid rpm range. Could be a tad louder, can easily be fixed with a midpipe.


      Year: 2000 Submodel: GT

      Helpful (9)

    • 5/5

      May 01, 2011


      The only thing that I am disturbed with this product, is they do not come with bolts to connect the joints. So when you take off the stock exhaust and your bolts break because of rusting you will be going to the nearest hardware shop.

      Brandon Morris

      Year: 2004 Submodel: V6

      Helpful (6)

    • 5/5

      Installation Time: Afternoon

      December 15, 2011

      Sound Great in the V6

      i have a 00 V6, and wanted to get some muscle car tone out of her. I took a chance and bought the pace setter h pipe and these bad boys... I was shocked with the upgrade in take off and sound.. My V6 doesn't sound like a V8 would with a full exhaust, but if deff sounds like a muscle car. Much better than i expected.. deep tone, and pretty throaty.. nothing like a ricer, so any V6 owner looking for an upgrade.. here is the answer to all your problems.


      Year: 2000 Submodel: V6 CT

      Helpful (5)

    • 5/5

      June 22, 2010

      2000 rpms = :D

      Awesome exhaust. Perfect sound. Intalled myself and i wish i would have lined everything up perfectly before welding, it hangs a little low but thats my fault. Glad i went with these.


      Year: 2000

      Helpful (5)

    • 5/5

      Installation Time: Afternoon

      October 09, 2011

      01 V6

      My exhaust fit perfectly, and installed in a few hours with no tweaking at all. Pipes hang flat and level. Tips stick out about a half inch on each side, nice and even. I welded my exhaust at the muffler. I am not quite sure I like the sound while cruising, BUT OMG it sounds awesome above 2,000 rpms and under hard acceleration! I also, as other have, strongly recommend this for any V6 owner considering a dual exhaust conversion


      Year: 2001 Submodel: V6 Watertown, SD

      Helpful (4)

    • 5/5

      Installation Time: Afternoon

      September 27, 2011

      the pipes were good sound like thunder!!!!!!

      I will recomennd the pipes to anyone. Great fit only took me about 1 hour to put in.....


      Year: 2002 Submodel: GT Minnesota

      Helpful (4)

    • 5/5

      April 29, 2011

      V6 Mustang

      This is a great exhaust for any of you with a V6 looking to put a true dual exhaust system on your Mustang. It sounds great, and has a low deep rumble. The install wasn't bad at all. I pulled it up on ramps and put jack stands under the rear. Just make sure you buy the pypes exhaust hanger kit available from, if you are putting this on a V6 because you will need the GT style hangers for this exhuast. I took it to a muffler shop to have the pipes welded to the mufflers because it was hard to get the clamps tight enough. If you have a V6 Mustang, I would definitely recommend this exhaust for anyone looking to upgrade.


      Year: 2002 Submodel: V6

      Helpful (4)

    • 5/5

      January 18, 2011

      Sounds great!!

      This is the perfect exhaust if you're looking for a nice deep rumble! Sounds great inside and out. Took less than 1 hour to install without any problems at all. Very happy with this purchase and would recomend to anyone looking to get more noise out of their stock v8!


      Helpful (4)

    • 5/5

      December 11, 2010

      Nice System

      Very nice system with the high quality you would expect from FloMaster. Not too loud but a very nice sound and no drone at highway speed. I highly recommend this system.


      Helpful (4)

    • 5/5

      Installation Time: Shop Install

      Installation Cost: 0-$100

      June 28, 2013

      How to enhance your Stang!

      Just get this Flowmaster American Thunder Catback Exhaust matched with a Magnaflow Tru-X X-Pipe, and you'll relive that pleasure you had the very first time you drove a Mustang! You are gonna fall in love a second time with your car. You can't get a better sound, quality or satisfaction than that. Simply the ultimate Mustang sound. And if you need any help, call the AM team... tell them what you need, and the result you'll get is a PERMA-PRESS smile on your face.


      Year: 2003 Submodel: GT Montreal, QC, Canada

      Helpful (3)

    • 5/5

      April 13, 2011


      I installed this catback system on my 03 mach 1 and I couldn't be happier. Install was extremely simple, anyone could do it. It has a nice deep rumble like a mustang should. Not raspy and annoying. Would recommend this system for anyone who wants to turn heads.

      jacob petersen

      Year: 2003 Submodel: Mach 1

      Helpful (3)

    • 5/5

      March 20, 2011

      That Flowmaster Sound!!!

      Great Sound..not simple,but Easy if you get the car high enough in the air,you'll need the height to rotate the old/new tailpipes on/off..HINT..the factory hangers have a "nub" on them,makes it a Bit*% to get the rubber hangers off..I trimmed the nub off with a cutoff wheel(or hacksaw)..only cutoff the "bump". New push-on keeper clips come in the kit,I used those once all was lined up. Take your time,and it will be easy,Well worth it!! Not nasty loud, but nice,throaty rumble..after these I did a Cold air kit and an SCT Tune..and Holy Sh*T..sound changed ALOT! mileage went down!..but only 'cuz I have my foot in it more to hear "That Flowmaster Sound" will enjoy...Buy 'em !!


      Year: 2003 Submodel: GT

      Helpful (3)

    • 3/5

      Installation Time: Less than 1 Hour

      August 16, 2013

      this product has bad finish

      This exhaust is a great exhaust other than the finish. Bought the flow master catback for my sons 03 GT because I had on my 92 foxbody and my cobra. The sound is good and the fitment is good but the mufflers and pipes finish are disappointing. The exhaust has only been on the car for 6 months, and is already rusting out. The hangers are all rusted, all the welds are rusted and the mufflers are rusting already not even 300 miles on the car since installation. Would not recommend with these issues so soon.


      Year: 2003 Submodel: GT Media, PA, USA

      Helpful (2)

      • Response from AmericanMuscle

        This is definitely a first with this catback system as we haven't heard of any rusting issues in the past. Flowmaster does have a 3-year warranty on this kit. I would highly suggest contacting them direct right away so they can get a replacement sent out for you. If you have any other questions or need anything else from us, please don't hesitate to give us a call. We'll do what's necessary to rectify the situation for you.

        Lisa, Customer Care

    • 5/5

      Installation Time: Afternoon

      July 27, 2013

      Sounds mean!

      I previously had a BBK O/R X pipe with SLP Loudmouths and they were stupid loud. I wanted to choose a system that wasn't too loud but had a nice, mean tone... and this was the one! Sounds mean with a nice tone! Even with the o/r x I have its not too loud. If you're looking for an exhaust that sounds great,looks great and has the famous flowmaster sound then here it is!


      Year: 2001 Submodel: GT Chicago, IL, USA

      Helpful (2)

    • 5/5

      Installation Time: Afternoon

      January 05, 2013

      American Thunder

      I purchased this exhaust for my 2000 v6 along with the pacesetter h-pipe. The change was shocking! Difference in tone was 100 percent better. Recommend this exhaust to anyone with a v6!!!


      Year: 2000 Submodel: V6 West Virginia, USA

      Helpful (2)

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