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

AmericanMuscle no longer carries the Flowmaster American Thunder Cat-Back Exhaust (99-04 GT, Mach 1). Please check out 1999-2004 Mustang Cat-Back Exhaust for an updated selection.

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 AmericanMuscle.com, 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

MPN# 17312

CA Residents: WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov

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?

    Customer Photos (25) Submit Your Photo

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      Questions & Answers (23)

      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 (2) 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)
      Post Your Question
      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

        Customer Reviews (100+) Write a Review

        Overall Rating 4.7

        out of 5 stars

        Installation Time:Afternoon

        Installation Cost:$100-$250

        • Loudness / Volume 4.302752 4.3
        • Exhaust Note 4.648148 4.6
        • Fit & Finish 4.40367 4.4
        • Bang For Your Buck 4.59633 4.6



        • 4/5

          Installation Time: Less than 1 Hour

          December 11, 2018

          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

          RickP

          Year: 2002 Submodel: GT

          Helpful (2)

        • 5/5

          August 14, 2018

          flowmaster

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

          BryanF

          Year: 2004 Submodel: GT

          Helpful (2)

        • 5/5

          Installation Time: Afternoon

          July 11, 2018

          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.

          JohnM

          Year: 2003 Submodel: GT

          Helpful (3)

        • 5/5

          Installation Time: Afternoon

          June 26, 2018

          Good sound!!

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

          AndreaW

          Year: 2004 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          June 02, 2018

          2004 Mustang Convertible

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

          EricW

          Year: 2004 Submodel: V6

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          May 31, 2018

          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.

          KennethM

          Year: 2000 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          March 02, 2018

          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.

          JoeyS

          Year: 2003 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          February 10, 2018

          Flowmaster American Thunder

          This is the perfect exhaust for my 1999 Mustang GT Convertible. Now my Mustangs sounds as it should!!

          StevenS

          Year: 1999 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Less than 1 Hour

          February 07, 2018

          Love it

          The product purchase was the right fit and sound I was looking for.

          KimberlyW

          Year: 2002 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          December 11, 2017

          Mustang Sound

          Installed this behind a catless X pipe and sounds really mean. Quiet at idle but loud and nasty under full throttle. I wish the tail pipes were bigger around.

          JamieF

          Year: 2003 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          November 13, 2017

          True muscle car sound !!!

          This was a gift for my sons 16 th bday. And American muscle did a great job getting it shipped fast and arrived Perfect. Only took a little over an hour to install. Went smooth and the sound is great !!!!!! My sons excited and can’t wait to drive. Next thing is Pypes h pipe. Can’t wait

          SethM

          Year: 2004 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          November 09, 2017

          Awesome

          Easy install perfect fit. Sounds great when u get on it, no droning sound at cruise.

          MichaelS

          Year: 2004 Submodel: GT

          Helpful (5)

        • 5/5

          Installation Time: Less than 1 Hour

          October 13, 2017

          Cat-back

          Awesome exhaust, great fitment and amazing sound! Fits beautiful and changes the over look and sound of the car. Very impressed

          EricR

          Year: 2001 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          June 11, 2017

          Easy install, quality sound

          Installation was simple and smooth. It is not super loud but people know when you are driving by. Good deep and quality sound

          AdamS

          Year: 1999 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          May 12, 2017

          Great Exhaust

          The sound is amazing. Best purchase I have made for my car.

          DM

          Year: 2004 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          April 20, 2017

          Awesome Service and Product

          As always American Muscle was quick on the delivery and the product was great quality. So I want to say thank you, thank you for your great service and products. Ill be purchasing again very soon. Thank you.

          JamesB

          Year: 2001 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Less than 1 Hour

          April 04, 2017

          Good sound!

          Nice deep flowmaster sound and easy to install!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

          JordanF

          Year: 2000 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Afternoon

          March 23, 2017

          Easy install

          Had a friend install it for me and they could not believe how easy it was to install! Good product at a good price. Very satisfied once again. Could not believe how quickly it arrived, less than 24 hours from order to front door!

          RosemaryM

          Year: 2001 Submodel: GT

          Helpful (4)

        • 5/5

          Installation Time: Less than 1 Hour

          March 09, 2017

          Great Exhaust

          Bought this item with the BBK O/R X Pipe and goes together like a dream it sounds mean and aggressive, loud but not obnoxiously loud. Definitely let's people know you're coming I'd recommend this setup to anyone with a 99-04 GT. Very pleased with the results

          NickM

          Year: 2002 Submodel: GT

          Helpful (3)

        • 5/5

          Installation Time: Afternoon

          March 04, 2017

          Flowmasters

          Along with my BAMA tune and off road bbk x pipe they really opened the car up and sounds great. I could really feel the difference in the torque curve. My daughter runs it at the local strip and with a few extra bolt ons she just ran 8.7's in the 1/8 mile now to get it to hook up should be about flat 8's for a stock engine and tranny thanks AM.

          ShawnM

          Year: 2002 Submodel: GT

          Helpful (4)

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