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

100+ Reviews
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  • Item #18410
  • MPN# 17312

  • Market Price $544.63
  • You Save 12% ($62.66)
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Product Videos Customer Photos
  • 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 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
  • Mach 1 - 03, 04
  • GT - 99, 00, 01, 02, 03, 04

Fits GT, Mach 1, and Bullitt

Most Helpful Reviews

Great sound

I had the magnapacks but decided to go with the flowmaster cat back. I feel that flowmasters make the best sounding exhaust for any mustang. Love the color black, installed with no problems. Thanks am ...

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Helpful (2)

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 ...

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Helpful (4)

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

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.

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14 Questions / 7 Answers

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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.


    Good Answer (5) (0)

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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!


    Good Answer (8) (1)

Show 2 more answers Question Resolved

How would this sound on a V6? With all of the correct items in place for install how would this sound on a V6? I understand the difference but looking for any kind of insight or details as to what to expect. These items are kind of expensive so its hard just to throw a dart in the dark and hope for the best sound. Kraig


  • I've heard a set of Flowmasters on a V6 with a True-Dual, and I was surprised at how aggressive it sound. If you have the correct set up I would equate it to just above a stock 99-04 GT as far as volume and overall sound. It still won't have the rumble of a V8, but it does sounds surprisingly aggressive.


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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 4.275862 4.3
  • Exhaust Note 4.639535 4.6
  • Fit & Finish 4.367816 4.4
  • Bang For Your Buck 4.586207 4.6

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $100-$250

    September 26, 2015

    Music To My Ears!

    I'm thrilled with the sound and look of the Flowmaster American Thunder Catback exhaust system! The first time I turned the key and heard that melodius growl, it was Heaven on earth! I had spent quite a bit of time reading all the reviews of all the systems AM had to offer and was looking for a setup that wouldn't be ridiculously loud, and still derive some pleasure knowing that the "Beast" had awakened. Needless to say this system met all my expectations. It arrived within a few days of ordering, and couldn't wait to have it installed. Normally, I wouldn't hesitate to have installed it myself, as a great amount of reviewers mentioned how easy it was to install. However, age wise my back no longer permits me to crawl underneath. So the repair work I undertake is above ground level. All in all, I couldn't be more happier with this system. Looks great and sounds great! Thanx AM.


    Year: 2001 Model: GT Driving Style: Daily Driver 'Stangs I have owned: 2 Tewksbury,MA

    Helpful (1) (0)

  • 4/5

    Installation Time: Afternoon

    December 04, 2011

    Not as loud as "American thunder" would have you think

    I was expecting it to be a bit louder. The sound is very plain. But i believe it is worth the money. I would rather something a little throatier but it is good enough. I did not notice a difference in power.


    Year: 2002 Model: GT Driving Style: Daily Driver 'Stangs I have owned: 1 Louisiana

    Helpful (2) (0)

  • 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 Model: V6 Driving Style: Daily Driver Watertown, SD

    Helpful (3) (0)

  • 5/5

    August 04, 2011

    Great sound

    I had the magnapacks but decided to go with the flowmaster cat back. I feel that flowmasters make the best sounding exhaust for any mustang. Love the color black, installed with no problems. Thanks am for another great service.

    Anthony E

    Year: 2002 Model: GT Driving Style: Daily Driver

    Helpful (2) (0)

  • 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 Model: V6 Driving Style: Daily Driver

    Helpful (4) (0)

  • 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 (75) (8)

  • 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 Model: GT

    Helpful (29) (3)

  • 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) (1)

  • 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 Model: GT Driving Style: Daily Driver 'Stangs I have owned: 1 Mobile, Alabama

    Helpful (23) (6)

  • 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) (1)

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