(approx) 2 Hours
Light to Moderate mechanical skill required.
- Flowmaster American Thunder Catback Exhaust System (99-04 GT/Mach 1/Bullitt) - Installation Instructions
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.
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Features, Description, Reviews, Q&A, Specs & Installation
Fitment: 1999 2000 2001 2002 2003 2004
CA Residents: WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov
April 27, 2020
Fit great and sound good
Easy to install everything fit good and sounds goodHelpful (3)
April 15, 2020
I had to send this back twice for chrome tips that were damaged or bent. American Muscle made the return process simple and I got the replacements right away. It took about an hour to install and was a perfect fit. Super happy with the sound, quite a nice difference from the stock exhaust.Helpful (4)
February 11, 2020
I have a 4.6 and put a flowmaster catback exhaust on my GT, it sounds great. Took just over an hour to install. Flowmaster American Thunder is a great exhaust.Helpful (3)
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 greatHelpful (5)
August 14, 2018
Worked out great. You will need a lift to install. Can't do it with just a jack and stands.Helpful (6)
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.Helpful (6)
June 26, 2018
If your looking for a deep muscle car sound this is for you.Helpful (7)
June 02, 2018
2004 Mustang Convertible
Very satisfied with this exhaust. Install was ok requiring some fine adjustments.Helpful (7)
May 31, 2018
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.Helpful (7)
March 02, 2018
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.Helpful (7)
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.
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!
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.
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.
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.
Flowmaster does offer a 3 year warranty on all of their aluminized steel products.
We would recommend contacting a local shop to check with current exhaust regulations to ensure it does not cause an issue when installing this on a vehicle that is operated in California.
Yes, this Flowmaster American Thunder Cat-Back Exhaust is going to work with an aftermarket x-pipe and headers.
This Flowmaster system will fit your V6 that has the aftermarket Mac H-pipe.
The Flowmaster American Thunder Cat-Back Exhaust would only come with the super 40s and you would need to swap your mufflers out for these.
Well, the piping size is identical between the two products, however they aren't necessarily designed to work with each other, so you may need a small modification to make it work if any at all, but it will be generally a simple install.
This will have no problems with fitment with your build as long as the BBK mid-pipe is a 2.5" tubing.
A catback exhaust does not have catalytic converters in it. The catalytic converters are located in the mid pipe.
Cats are for silly people. Lol I'm jk. Honestly any of the ones on here are fine! No mid pipe has ever fit perfect for me. I've had all brands! But they all do the same thing. Go with pypes. They are the cheapest. But also stainless steel.
This is for the V8 models. So without some type of dual exhaust conversion that's similar the the 99-04 gt & Mach 1, it won't fit . Make you have to correct mid pipe also.
(approx) 2 Hours
Light to Moderate mechanical skill required.
What's in the Box
|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|