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

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

MPN# 817312

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: 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?

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Questions & Answers (7)

5 Top Rated Questions

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!

Helpful (1) Show 1 more answer

Is this street legal and will it pass the Emission?

  • The Flowmaster American Thunder Cat-Back exhaust - Stainless Steel (99-04 GT, Mach 1) will not affect emissions however this system will be louder than stock. We recommend checking local laws as to legality in your area.

Helpful (0)

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, (http://www.americanmuscle.com/mac-ceramic-longtube-0104v6.html) will bolt right up to the MAC Off-Road H-Pipe (99-04 V6 w/ Long Tube Headers), item # 40064, (http://www.americanmuscle.com/mac-or-shorty-hpipe-9904v6.html). 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)
Post Your Question
2 More Questions
will this work with the pacesetter v6 h-pipe ? 1 answer
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? 2 answers

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Customer Reviews (29) Write a Review

Overall Rating 4.7

out of 5 stars

Installation Time:Afternoon

Installation Cost:$100-$250

  • Loudness / Volume 4.461538 4.5
  • Exhaust Note 4.653846 4.7
  • Fit & Finish 4.576923 4.6
  • Bang For Your Buck 4.423077 4.4



  • 5/5

    Installation Time: Less than 1 Hour

    April 13, 2018

    Flowmaster

    Nice Fit sounds Great Great Value Big Difference from stock

    ClaytonC

    Year: 2004 Submodel: GT

    Helpful (3)

  • 2/5

    Installation Time: Afternoon

    April 01, 2018

    Quality isn’t what it used to be.

    On my 02 gt I ordered the aluminized catback from flowmaster when I was on a budget and really never had any issues with install and it being my daily driver. Now that I have my 04 which is a garage Queen I figured I’d spend the extra money on the stainless steel system. It’s good quality but the production must have changed because fitment was ridiculous. Definitely poor quality and zero inspect upon completion before it’s sent out to be sold.

    AndrewM

    Year: 2004 Submodel: GT

    Helpful (4)

  • 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

    Ghostdog

    Year: 2002 Submodel: GT

    Helpful (4)

  • 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.

    JasonP

    Year: 2000 Submodel: GT

    Helpful (4)

  • 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.

    Sally

    Year: 1999 Submodel: GT

    Helpful (4)

  • 5/5

    Installation Time: Afternoon

    April 08, 2017

    American Thunder cat back exhaust

    This product is amazing. A grand slam on sound ! I finished install and drove the car 4 hours just loving the sound. Slight drone at about 2 thousand rpm. Below that is pretty quiet so cruising in fifth gear you can easily have a conversation. Fit is great, well planned out. 110 satisfied. No other exhaust should be made for 99-04 mustangs, you WON'T be disappointed.

    TravisO

    Year: 2004 Submodel: Mach 1

    Helpful (5)

  • 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

    John

    Year: 1999 Submodel: GT

    Helpful (5)

  • 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.

    Lance

    Year: 2000 Submodel: GT

    Helpful (5)

  • 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.

    frank

    Year: 2003 Submodel: GT

    Helpful (6)

  • 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

    shaker56

    Year: 2004 Submodel: GT

    Helpful (4)

  • 5/5

    Installation Time: Afternoon

    September 17, 2015

    Perfection

    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.

    Tilly

    Year: 1999 Submodel: GT Rochester, NY

    Helpful (5)

  • 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.

    Derek281

    Year: 2004 Submodel: GT Michigan, USA

    Helpful (4)

  • 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!!

    PC26

    Year: 2000 Submodel: V6

    Helpful (7)

  • 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!

    AMCoalK

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

    Helpful (5)

  • 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

    Kev1029

    Year: 2000 Submodel: V6

    Helpful (7)

  • 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!

    JoshR

    Year: 2000 Submodel: GT PA

    Helpful (5)

  • 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.

    Celery

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

    Helpful (5)

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $100-$250

    January 12, 2014

    Good Sound

    This system had great fitment and was the easiest part of my exhaust system upgrades for the local shop to install. Hangs at a good height. Sound is great. Mine went on with a catted x-pipe and the result is pretty good. Sounds how a Mustang should sound!

    Brandon7733

    Year: 2002 Submodel: GT

    Helpful (3)

  • 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.

    AlexAM

    Year: 2001 Submodel: GT Aston, PA

    Helpful (4)

  • 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.

    MR11

    Year: 2001 Submodel: GT Illinois, USA

    Helpful (4)

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