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

Item 18410
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      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.

      Product Information

      Features, Description, Reviews, Q&A, Specs & Installation


      • 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.


      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

      Flowmaster 17312

      CA Residents: WARNING: Cancer and Reproductive Harm -


      Customer Reviews (100+)

      Write a Review

      Ratings Summary

      4.8 out of 5 overall rating from 100+ customers

      Installation Time: Afternoon

      Installation Cost: $100-$250

      • April 27, 2020

        5 2000 GT - Coolkevin Installation Time: Less than 1 Hour

        Fit great and sound good

        Easy to install everything fit good and sounds good

        Helpful (3)
      • April 15, 2020

        5 2000 GT - Brian Installation Time: Afternoon

        Flowmaster exhaust

        I had to send this back twice for chrome tips that were damaged or bent. American Muscle made the return process simple and I got the replacements right away. It took about an hour to install and was a perfect fit. Super happy with the sound, quite a nice difference from the stock exhaust.

        Helpful (4)
      • February 11, 2020

        5 1999 GT - williyjoe Installation Time: Afternoon

        Great sound

        I have a 4.6 and put a flowmaster catback exhaust on my GT, it sounds great. Took just over an hour to install. Flowmaster American Thunder is a great exhaust.

        Helpful (3)
      • December 11, 2018

        4 2002 GT - RickP Installation Time: Less than 1 Hour

        Love the sound

        Had them installed and glad I did. The pipes to the muffler was not a tight fit the clamps that came with the system did not really work so the tech welded the pipes Other than that sounds great

        Helpful (5)
      • August 14, 2018

        5 2004 GT - BryanF


        Worked out great. You will need a lift to install. Can't do it with just a jack and stands.

        Helpful (6)
      • July 11, 2018

        5 2003 GT - JohnM Installation Time: Afternoon

        D.S tail pipe to short

        P.S muffler and tail pipe fit great . D.S. Was to short. Looks like it was made to weld together not clamp. I only could get tail pipe pushed in about 3/4 inch. Barely hangs in rear hanger. Also also fancy tip on ds tail pipe dented in. I pushed in ps tail pipe to match. Can’t really see tips anymore.

        Helpful (6)
      • June 26, 2018

        5 2004 GT - AndreaW Installation Time: Afternoon

        Good sound!!

        If your looking for a deep muscle car sound this is for you.

        Helpful (7)
      • June 02, 2018

        5 2004 V6 - EricW Installation Time: Afternoon

        2004 Mustang Convertible

        Very satisfied with this exhaust. Install was ok requiring some fine adjustments.

        Helpful (7)
      • May 31, 2018

        5 2000 GT - KennethM Installation Time: Afternoon

        Great Sound!!!

        Flowmaster American Thunder Cat-Back exhaust sounds like America on a Mustang!!! Just replaced 18 year old Flowmaster mufflers with the complete catback. AM delivery was super fast and product was in perfect condition. Install was a breeze with exceptional fit. Performed install in garage on jack stands. A little quiet when first installed but give it a week and the sound starts to open up. Plan to add electronic cutouts for more aggressive sound with my stage 2 cams at car shows and cruises. Sound is perfect for me for daily driving as is.

        Helpful (7)
      • March 02, 2018

        5 2003 GT - JoeyS Installation Time: Afternoon

        Great sound

        This exhaust definitely brought my stock gt to life with a louder deep growl that sounds like a mustang should. I am very pleased and it was very easy to install. A direct fit to the stock h pipe. A must have for any new egde mustang.

        Helpful (7)

      Questions & Answers (23 answers)

      5 Top Rated Questions

      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.

      Helpful (7) Show 1 more answer

      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!

      Helpful (6) Show 2 more answers

      I just installed these onto my bone stock 03 gt. Now in the 2k-3k rpm range escpecially I'm noticing alot of vibration within the cabin. Is this because part of the exhaust is touching part of the car? I seem to really feel it on the accelerator and the shifter.

      • The Flowmaster American Thunder Cat-Back Exhaust should not be in contact with the body of the car, however, if it is, it could indeed cause a vibration. I would recommend taking a look and try to tweak the exhaust to avoid contact with the body.

      Helpful (3)

      Will this system fit a 2002 GT Convertible ?

      • This system was not designed for convertibles (interior drone), and Flowmaster has another system with Super 50's designed for GT Verts. However, it will work if you install 12" resonators between the mid-pipe and the mufflers, which I did on my 2003 GT Vert, and am very happy with the results.

      Helpful (3) Show 1 more answer

      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 (1)
      10 More Questions
      Do these mufflers have any warranty? I bought these from AM over a year ago and now there making a loud vibration noise at mid to high rpm. There's also no physical damage to mufflers. 1 answer
      Is this legal in california? 1 answer
      I have bbk long tube headers and bbk cat delete x pipe. Can I use this cat back with that combo? 1 answer
      Hey will this exhaust fit my v6 with the MAC off road h pipe with the MAC long tube headers? 1 answer
      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? 1 answer
      Will this cat back fit perfectly with the Pypes off-road H pipe ? 1 answer
      I have an 01 GT with BBK long tube headers and a BBK Offroad X-PIPE. My question is, will my setup bolt right up to the cat back exhaust without any cutting or welding? I'd really like to just put my GT on some jack stands and finish the job myself. 1 answer
      I need to install catalytic converters into my exhaust, would i be able to make the adjustment with this catback? or what are my options? 1 answer
      Hi I have a 04' mach 1 with stock exhaust manifolds and manual transmission. I wanted to have your recommendation for a x pipe with cats to go along with this flow master american thunder catback? Im looking for real easy install that won't require any modifications. Thank you 2 answers
      Fit with long tube headers on a V6 Will this exhaust fit on my V6 if I have the MAC ceramic long tube headers along with the MAC off-road shorty H-pipe? Or do I still need the Pace setter dual exhaust adapter? Thanks 1 answer

      Installation & What's in the Box

      Installation Info

      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

      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

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