1999-04 Ford Mustang Flowmaster American Thunder Stainless Cat Back Exhaust Kit GT-Mach 1 - New Edge by Flowmaster

Mustang Flowmaster American Thunder Stainless Cat Back Exhaust Kit (99-04)

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

Flowmaster American Thunder Exhaust Systems add a signature growl that's best described as the "Flowmaster Sound" - A muscular, throaty rumble that announces your engine's presence. Flowmaster is the true tone of classic American muscle and performance.

Item #FLO-817312
MPN# 817312

Your Price: $582.97

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

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  • Gives Your Mustang That Signature Sound
  • Increased Performance
  • Improves Exhaust Tone


  • Fully Welded Construction
  • Chambered Muffler Design
  • 409 Stainless Steel
  • Lifetime Warranty
  • 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

For the Mustang owner looking for a complete cat-back dual exhaust system with an aggressive tone. With easy fit in mind, these American Thunder systems from Flowmaster are mandrel bent for maximum performance and include specially selected mufflers. 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. Like all Flowmaster systems, the American Thunder benefits include improved throttle response, power and mileage. Designed for an easy fit, these Mustang exhaust systems include all necessary parts and hardware for easy installation.

Since Flowmaster mufflers use sound energy cancellation technology, you don't have to worry about "packing material" deteriorating and getting louder after as little as a few months. The interior case volume of the inside of a Flowmaster muffler is far superior to that of a "straight-pipe" or "straight-through" muffler design. Imagine filling a "straight-pipe" or "straight-through" muffler with water vs. filling a Flowmaster muffler with water. Which one do think would hold substantially more volume? The deflector in a Flowmaster muffler acts as a one way check valve eliminating reversion back to the engine and creates a low pressure area to help scavenge the exhaust. Flowmasters unique style of H-Chamber designs help control interior resonance inside the cab area of your Mustang. "Straight-through" muffler designs cannot. Flowmasters use fully welded construction for added strength and durability to last even when installed on high horsepower engines.

Flowmaster mufflers are proudly made in the U.S.A.

- Fits 1999-2004 4.6L Mustang GT, 2001 Mustang Bullitt, & 2003-04 Mustang Mach 1
- 2.5" Inlet / 2.5" Outlet
- Super 40 Series Mufflers
- 409 Stainless Steel Construction
- Mandrel Bent 16 gauge Tubing

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!

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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