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

Market Price $670.99 You Save 13% ($88.02)
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  • Item 18436
  • MPN# 817312

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    Features, Video & Details

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

    Review & Installation Video

    Review & Install Video

    Hi I’m Justin with and I’m taking a closer look at Flowmaster’s stainless steel American Thunder Cat-Back exhaust, fitting the 99-04 Mustang GT and Mach 1. The two biggest features with this system are of course the full 409 grade stainless steel constructions throughout, and also the super 40 series mufflers, which give you that classic Flowmaster throaty rumble. The stainless steel tubing features a 2.5” diameter and mandrel bends throughout, all of which exists through these 3” hand polished OE style tips. Due to the design of your 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, getting the American thunder bolted up, is a pretty straightforward that should take you roughly 2 hours using some standard hand tools and the included clamps. Now let’s hear how the stainless 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.

    Stainless Steel Construction. Constructed from mandrel bent 2.5" 16 gauge 409 stainless 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.

    Application. This Flowmaster American Thunder Stainless Steel 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: 409 Stainless 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

    Customer Photos (9) Submit Your Photo

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    6 Questions / 8 Answers

    Ask a Question

    Could this fit on pypes catted x pipes on a 2004 v6


    • Absolutely! Installing the Pypes Catted X-pipe for the V6 will allow you to install any GT catback, including this Flowmaster American Thunder. Hope this helps!


    Show 1 more answer Helpful (1)

    Will this exhaust system fit properly on a v6? I know the gt bumper has cutouts and the v6 bumper does not. So I'm wondering if it will work properly.


    • This Flowmaster American Thunder Cat-Back Exhaust is only intended for use on a 99-04 Mustang GT and Mach 1. This will not work on a V6 model Mustang due to different fitment.


    Helpful (0)

    Will this work with the MAC Ceramic Long Tube Headers #40062 and MAC Off-Road H Pipe #40064 on the V6?


    • The MAC Ceramic Long Tube Headers (94-97 V6; 01-04 V6), item # 40062, ( will bolt right up to the MAC Off-Road H-Pipe (99-04 V6 w/ Long Tube Headers), item # 40064, ( Though, this H-Pipe may not bolt directly to the Flowmaster American Thunder Cat-Back Exhaust. This Cat-Back is designed for a GT. We cannot guarantee fitment of this item on a V6. The H-Pipe and Cat-Back have the same piping size of 2.5", but you may need to do some modifications to make them work together.


    Helpful (0)

    Build Material The description states pipes and tips are stainless, what about the muffler itself?


    • The entire catback is stainless steel. Flowmaster's made of aluminized steel are typically painted black.


    Helpful (0)

    will this work with the pacesetter v6 h-pipe ?


    • Yes. The addition of the PaceSetter V6 Dual Exhaust Adapter Kit (99-04 V6) (Item #51063) allows any of the GT cat-backs to work for your V6, so yes you will be able to install the Flowmaster American Thunder Cat-Back Exhaust - Stainless Steel (99-04 GT, Mach 1)


    Helpful (0)

    Do these require Pypes Exhaust Hanger Kit I have a v6 & plan on getting the Dual Exhaust Adapter Kit to make the cat back installation possible. On the stock v6 bumper would you need the hanger kit?


    • Yes. My mechanic had to make his own hanger.


    Show 1 more answer Helpful (0)

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    Customer Reviews (27) Write a Review

    Overall Rating 4.8

    out of 5 stars

    Installation Time:Afternoon

    Installation Cost:$100-$250

    • Loudness / Volume 4.583333 4.6
    • Exhaust Note 4.708333 4.7
    • Fit & Finish 4.666667 4.7
    • Bang For Your Buck 4.541667 4.5

    • 5/5

      Installation Time: Afternoon

      March 30, 2013

      Sounds great

      Exactly what I wanted, not too loud but wanted more deeper rumble. I got it with this catback exhaust. Installation was easy, although everything fits so tight, it takes a bit of jiggling around to get it just right. Took 2 and a half hours for one side and one hour for the other. Real pleased with the look and sound. Would recommend this product.


      Year: 2002 Submodel: GT Canada

      Helpful (4)

    • 5/5

      Installation Time: Less than 1 Hour

      October 26, 2014

      American Thunder on a 4.6L 2V

      Car sounds awesome. No too loud. With the car on a lift, got it installed in less than an hr.


      Year: 2004 Submodel: GT Michigan, USA

      Helpful (3)

    • 4/5

      July 22, 2014

      Great Upgrade

      I bought this exhaust along with the pacesetter h pipe. Keeping in mind this was put on a v6 I am happy with the results. It seems to be as loud as my previous exhaust but it is not as high pitched and sounds smoother. It has only been in for a week so I am counting on some more improvements over time. Overall: The tone of the exhaust is good, nothing obnoxious. I did not install it myself but was told the fit was fine. I would recommend this product!!


      Year: 2000 Submodel: V6

      Helpful (3)

    • 5/5

      Installation Time: Less than 1 Hour

      May 16, 2014

      Classic Muscle Sound!

      Absolutely fantastic fit and finish, bolted right up and done the job in less then an hour. Love the sound, The exhaust tips fit in the bumper great!


      Year: 2003 Submodel: Mach 1 Hatboro, PA, USA

      Helpful (3)

    • 5/5

      Installation Time: Afternoon

      May 14, 2014

      2000 v6

      It isn't the loudest exhaust ever. But it isn't the quietest either. The idle isn't too loud, and it sounds really nice inside the car. With an H pipe, it sounds very nice within 3000 rims


      Year: 2000 Submodel: V6

      Helpful (3)

    • 5/5

      Installation Time: Afternoon

      April 21, 2014

      JBA H-pipe and Flowmaster American Thunder

      I did many hours of research to figure out this was the exhaust combo I wanted to go with. I was actually nervous that it would be too loud or drone really bad. I was mostly just concerned the ride wouldn't be enjoyable anymore with the drone inside and my neighbors hating me or my passengers not being able to have a conversation or enjoy listening to music. But I decided to pull the trigger on this combo as i just loved the exhaust clips I was listening to. I installed the whole system myself in my driveway, it wasn't a difficult install. And when i fired it up afterwards and took it for a ride. . . I was EXTREMELY pleased! all expectations were blown away! The exhaust sounds AMAZING! EXACTLY what I wanted. Nice rumble, crackles on down shift, highway driving is extremely peaceful running stock 3.27 gears. I can listen to music and enjoy the ride (better than my stock exhaust actually) but everyone around me still hears it. When I put my foot to the floor it SCREAMS like a monster on the outside, but still is nice volume on the inside when getting on it. And the only drone is when the tack touches 2,000 RPM. so there is NO DRONE. My stock exhaust droned more. THIS is how a muscle car should sound. I am 110% satisfied with this setup, it is perfect. SUPER LOUD outside, like a bat out of the lower infernal regions, loud when I roll the windows down, but nice interior residence with the windows up. I HIGHLY recommend this setup!


      Year: 2000 Submodel: GT PA

      Helpful (3)

    • 5/5

      Installation Time: Shop Install

      Installation Cost: $100-$250

      March 30, 2014

      Flowmaster Thunder stainless

      Better than I expected car sounds great like I have a mean engine under the hood. Wish I had done this sooner. Increased performance.


      Year: 2003 Submodel: GT Manahawkin, Stafford Township, NJ 08050, USA

      Helpful (3)

    • 5/5

      Installation Time: Afternoon

      March 04, 2013

      Perfect combo!

      Just got thru installing these bad boys along with an SLP catted X-pipe (AM part #23008) on my '02 GT, and I couldn't be happier. I'm 32 years old, so the days of being obnoxiously loud have come and gone. This set up is great. There's no real "drone" until you hit about 85 mph, and even then, it's not bad at all. The combo is fairly mild when you're not getting heavy on the skinny pedal... But when you do - everyone around you will know it. Great fit and finish, everything bolted up exactly like it should have. I've used cheaper products on other cars, and it's very true that you get what you pay for. I went with the stainless version simply to have all stainless from the headers all the way back. Great set up, couldn't be happier!


      Year: 2002 Submodel: GT Cypress, TX

      Helpful (3)

    • 4/5

      Installation Time: Afternoon

      October 25, 2012

      Great looking and sounding Catback

      I bought this catback after listening to all the audio files. It sounds great. With my Catted BBK x-pipe it is a little louder but very nice sound. Does drone a little at 2k but not as bad as my stock system did. The sound is much more mellow now but sounds like a true muscle car. The installation was very easy. I was also very surprised at the weight difference. The Flowmaster is much lighter then stock. I am not a big fan of the non rolled tips. I felt my Mach 1 looked better with rolled tips. Otherwise very happy with the product. Had great welds and sounds great. Thanks American Muscle you guys offer great products!


      Year: 2003 Submodel: Mach 1 St Louis, MO, USA

      Helpful (3)

    • 5/5

      Installation Time: Shop Install

      Installation Cost: $250-$500

      June 13, 2017

      Good to Amazing

      Love the sound this exhaust puts out, i have Flowmaster shorty headers, Magnaflow X pipe, and this cat back system. It creates a beautiful low growl (almost like thunder) at lower RPM's. When you get up to the higher RPM's it sounds like a dream.


      Year: 1999 Submodel: GT

      Helpful (2)

    • 5/5

      December 13, 2016

      Flowmaster shipped quickly

      Ordered my Flowmaster stainless exhaust on cyber Monday. Couldn't pass up the deals. Too bad I have to wait till spring to install it. I'm sure it will be nice to hear my Mustang come to life with the new exhaust


      Year: 1999 Submodel: GT

      Helpful (2)

    • 5/5

      August 09, 2016

      Sounds like it should

      This is the one thing that has been missing from such a cool looking car is the cool sound, Now it is complete. I wish I would have had it done when I bought the car but now it seems kind of like a different better car.


      Year: 2000 Submodel: GT

      Helpful (2)

    • 5/5

      July 28, 2016

      American Thunder

      Another fine product from Flowmaster. Easy install with the right tools and a little patience you will have an amazing sound coming from your Mustang.


      Year: 2003 Submodel: GT

      Helpful (2)

    • 5/5

      July 10, 2016

      great exhaust

      I like the craftsmanship they are a good value for me they are just a little loud but a very good value


      Year: 2004 Submodel: GT

      Helpful (2)

    • 5/5

      Installation Time: Afternoon

      September 17, 2015


      After reading reviews and viewing multiple videos of systems online, I liked the sound of this catback right from the start. I didn't want a sound that would break windows, my ear drums nor that would resonate too deeply inside the car. After a very smooth installation that took about 3 1/2 hours (we took our time, cleaning, painting, greasing) I was stunned when I turned the key. The sound was perfect. A solid, clean rumble at idle. A very nice hard rumble purr upon acceleration. Heads turn constantly as I'm driving down the road. I can't say good enough things about this system.


      Year: 1999 Submodel: GT Rochester, NY

      Helpful (2)

    • 5/5

      Installation Time: Afternoon

      August 02, 2013

      great product

      amazing difference from stock. deep and more aggressive. would definitely recommend this for anyone looking for a deep tone.


      Year: 2001 Submodel: GT Aston, PA

      Helpful (2)

    • 5/5

      Installation Time: Full Day

      July 26, 2013

      Happy with this product

      Main problem was getting a flange nut off the old exhaust on one side due to corrosion. Flowmaster exhaust fits tight in a few spots, but otherwise installation went well. It looks good and sounds great. Not too loud... just right for me.


      Year: 2001 Submodel: GT Illinois, USA

      Helpful (2)

    • 4/5

      Installation Time: Full Day

      June 26, 2012

      S S Flows

      The flowmaster stainless steel American thunder cat back is a very nice system . Fit and finish is very good . Took me about five hours by myself to remove and install new exhaust. This cat back definitely woke the car up but when driving it is loud inside the car when going through the gears, quites down on the highway some drone.


      Year: 2000 Submodel: GT Ridley, PA, USA

      Helpful (2)

    • 5/5

      Installation Time: Afternoon

      September 11, 2017

      Found my sound!!

      Sounds great ..Took some time to put it on..But well worth the effort ..I finally found my sound..Not sure if I gained any horse power but it definitely sounds like it ..teehee


      Year: 2002 Submodel: GT

      Helpful (1)

    • 5/5

      Installation Time: Afternoon

      September 07, 2017

      Very Nice catback exhaust system

      This Flowmaster exhaust made a big difference in sound. Sounds good on acceleration and no "drone" when cruising.


      Year: 2000 Submodel: GT

      Helpful (1)

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