1999-04 Mustang Flowmaster American Thunder Stainless Catback Exhaust Kit GT/Mach 1 by Flowmaster

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Flowmaster Mustang American Thunder Stainless Catback Exhaust Kit  (99-04) GT/Mach 1 817312

Improve throttle response, sound, and power with this 1999-2004 Mustang Flowmaster American Thunder Stainless Steel catback!

Item #FLO-817312
MPN# 817312

Your Price: $582.97

Market Price:$763.24
You Save 23.62 % ($180.27)

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  • Signature Flowmaster Sound
  • Increased Performance
  • Improves Throttle Response
  • Lifetime Warranty


  • 2.5" Mandrel Bent Tubing
  • American Thunder Mufflers
  • 409S Stainless Steel
  • Made In The USA

What's in the Box

  • 2) Muffler Assemblies With Flow Tubes
  • (2) Tailpipes with Stainless Steel Tips
  • (2) Exhaust Clamps
  • Install Instructions
  • Flowmaster Stickers
  • Warranty Card

Made in the USA

Product Description

Flowmaster American Thunder Catback
Add Flowmaster's signature sound to your Fox with this 1999-2004 Mustang GT American Thunder catback! With easy fit in mind, these American Thunder systems from Flowmaster are mandrel bent for maximum performance and flow! American Thunder systems deliver a moderate to aggressive interior and exterior tone and are suited to the Mustang enthusiast who wants their performance to be heard!

Flowmaster Quality
Flowmaster mufflers are proudly made in the U.S.A. and the quality shows in this American Thunder catback. Constructed with 2.5", mandrel bent tubing, this catback flows much better than your factory exhaust. Flowmaster utilizes 409s Stainless steel to ensure long lasting performance! Designed for an easy fit, these Mustang exhaust systems include all necessary parts and hardware for easy installation.

-Fits 1999-2004 Ford Mustang, GT, MACH1 and Bullitt models, (not Cobra), with a 4.6L SOHC engine.

Flowmaster Exhaust Warranty - Click Here

Product Videos

Review: 1999-2004 Mustang Flowmaster American Thunder CatBack Exhaust

Check out our YouTube channel for even more tech tips, installation videos, how-tos, and more. The best place to go for anything Mustang related!

Customer Photos (0)

Product Reviews (3)

out of 5 stars
4.5 stars
3 reviews
5 stars    1
4 stars    2
3 stars    0
2 stars    0
1 star    0

3 of 3 REVIEWS
3 of 3 REVIEWS

4 Stars   Exactly As Advertised
By on 4/20/2015
Bought this Cat-Back as part of a total exhaust overhaul, BBK long tubes and BBK catted H-Pipe as well. Probably deserves 5 stars as it bolted right up, adjusted into position and sounds just like a set of Flowmasters, but I have an unrealistic expectation. This set up gives a noticeable growl at start up, a quiet rumble at idle and really opens up at full throttle. The only downside is a strong resonance under load between 2,000-2,800 RPM. This wouldn't be that bad, but it's between 55-65 MPH with my stock gears and that's where I spend most of my commute. Potential solution: Sub box and some sound deadening in the trunk is next.

Yes, I would recommend this item to a friend!

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4 Stars   Fit not so good 04 GT
By on 4/11/2017
Received my system yesterday and installed it last night in the garage. The tailpipes could have been an inch longer at the muffler, barley in the tip hanger and not engaged fully in the muffler (leak), and the clamps are useless, bottomed out and the pipes still turn in the muffler, have to get the tailpipes welded in the morning. Other than that I'm back to normal levels with the O/R X-pipe with this system

Yes, I would recommend this item to a friend!

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5 Stars   Great Looks and Fit
By on 2/23/2017
I had an auto technician at my local shop install these cat backs on a Pacesetter H-pipe on my '04 V6. He did a great job installing them and they look great. I did not have the bumper cover cutout to make it like a GT. The tail pipes hang about an inch below the bumper cover and LOOK GREAT.
Two of the guys in the shop comments to me about how it "rumbles." I love it. No "Honda" drone with these.

Yes, I would recommend this item to a friend!

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Vehicle Applications

Ford Mustang - 1999 (99) - 2000 (00) - 2001 (01) - 2002 (02) - 2003 (03) - 2004 (04)

About This Brand

Flowmaster - The Exhaust Technology Company
Flowmaster operates the only exhaust Research & Design facilities of its kind in the world, centered around the SuperFlow SF-7100 engine dynamometer complete with E.C.A. (engine cycle analysis) equipment and also SuperFlow chassis dyno's.

Through use of these dyno's, Flowmaster's engineering staff can track the complete engine cycle in millionths of a second and can obtain exact technical data of each degree of cam rotation, from induction to the tip of the exhaust system, while also showing the power numbers that really matter, the power numbers "to the ground."

Flowmaster also utilizes Spectrum Analyzers to fine "tune" the sound of each of the eleven different series of mufflers that Flowmaster currently offers, both for peak performance and just the right sound levels and tones. Flowmaster's dedication to quality is what makes them the exhaust industry leader for Latemodel Mustangs and SVT Lightnings!

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