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SLP Loudmouth Cat-Back Exhaust (99-04 GT, Mach 1)

100+ Reviews
Our Price


  • Item #23001
  • MPN# M31007

  • Market Price $513.95
  • You Save 4% ($21.00)
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Customer Photos
  • Rich, Throaty Exhaust Tone
  • Loudmouth "Shotgun" Race Resonators Included
  • Versatile Modular Design
  • 2.5" 409 Stainless Steel Tubing
  • 3.5" Polished 304 Stainless Steel Tips
  • Fits 1999-2004 GT, Bullitt, & Mach 1 Mustangs
More Power and Sound.   This SLP system includes SLP's infamous Loudmouth "shotgun" race resonators that offer virtually zero flow restriction to free up more power and to produce the most aggressive sound possible.

Modular Design.   SLP's unique modular design allows you to quickly and easily switch out the mufflers or resonators to suit your driving needs. Along with the included resonators, the Loud Mouth system is compatible with all SLP mufflers and resonators including the bullet-type Loud Mouth II and the traditional-style PowerFlo Muffler.

Superior SLP Construction.   This SLP Cat-Back system is made from mandrel bent 2.5" 409 stainless steel tubing for for its excellent non-restrictive flow characteristics and long life. The system ends with a brightly polished 304 stainless steel 3.5" tips, that are electro-marked with the distinctive SLP logo, giving a great finishing touch and a show quality look.

Application.   Designed to be a direct bolt on, this SLP Loudmouth catback will bolt straight up to the factory Mid-Pipe without any extra modifications required on the 1999-2004 GT, 2001 Bullitt, or 2003-2004 Mach 1 Mustang to give it a rich, throaty sound.

Fitment: 1999 2000 2001 2002 2003 2004 Details

What's in the Box
Hardware included:
  • (2) Intermediate Pipes
  • (2) Over-axle Pipes
  • (2) Resonators
  • (2) Tailpipes
  • (2) Tailpipe Tips
  • (8) Clamps
  • (2) Ball Flanges
  • Instructions
Installation Info
Tech Specs
Cat-Back Exhaust Specifications
Piping Material: 409 Stainless Steel Tip Material : Polished 304 Stainless Steel
Piping Size: 2.5" Tip Size: 3.5"
Muffler Type: Straight-through Fitment: 1999-2004 GT / Mach 1 / Bullitt 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

LOVE this catback!!!

So I just bought a 2004 Mach 1. Previous owner had a Bassani catted x-pipe with Magnaflow Street catback. Meh, it was ok, too stock sounding for my tastes. I've had SLP's before on older GT's, so I ...

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Helpful (1)

Sounds awesome!

Installed with pypes off road x pipe. Sounds great. Yes it's loud very loud. But I love the sound on my v6. It literally roars and bellows at hard throttle. It's got some deep sound to it. Overall I am ...

Read More
Helpful (1)

Overall Rating 4.9 out of 5 stars
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15 Questions / 9 Answers

Ask a Question

Will this sound ricey with my v6 mustang? Ill be using an off road x pipe, i also have a cai and 65mm throttly body installed.


  • I assume you have a dual exhaust system on your V6 or are building one. Part of what you can do to help get the growl is use an H-Pipe vs an X-Pipe. It changes the harmonics up. Check out the clip and keep in mind that this is a stock GT exhaust with Factory manifold and factory h-pipe. Find an exhaust with a sound like what you're after. The SLP Loudmouth sires is just that, Loud. If you want more of a rumble, you might try the magnaflows.


    Good Answer (6) (0)

Show 2 more answers Question Resolved

Will this catback work with the bbk off road x pipe you guys sell on the website? for a mach 1? im going to be ordering this catback on the 19th and want to get x pipes is well. are the bbk off road fine?


  • Yes it actually will fit because the GT and the Mach share the same rear bumper and 8.8 rear axle hope this helps, have any more questions don't be afraid to ask the staff of American Muscle...


    Good Answer (0) (0)

Show 1 more answer Answer This Question I need this answered (0) (0)

Which Will Be Louder With Bbk LT & Shorty Off X Pipe Borla ATAK Or This Setup I Want People To Know Its Me Before They See Me & This Is For A Mach 1


  • Probably the bbk LT but with the LT there will be a more difficult install than bolt on, if your bringing it to an exhaust shop it'll be no problem. Have fun!


    Good Answer (0) (1)

Answer This Question I need this answered (0) (0)

I want to know what headers will it be able to bolt up to?


  • Any of the headers that American Muscle offer for the 1999-2004 Mustang will work just fine with the SLP Loudmouth Cat-Back Exhaust! You will just need to purchase the proper X/H-pipe to go along with them!


    Good Answer (0) (0)

Question Resolved

With this kit and the pacesetter h pipe, is it all I need to bolt up to my 2004 v6 with single exhaust?


  • The SLP Loudmouth Cat-Back Exhaust (99-04 GT, Mach 1) and the PaceSetter V6 Dual Exhaust Adapter Kit (99-04 V6), item # 51063, ( will bolt up to your 2004 V6 Mustang. This will turn your single exhaust to a dual exhaust.


    Good Answer (0) (0)

Question Resolved

will this fit a off road h pipe?


  • The SLP Loudmouth Cat-Back Exhaust will fit to most aftermarket mid-pipes those outlets are in the factory location. If you were perhaps looking at a specific h-pipe, we would be more than happy to confirm fitment for you!


    Good Answer (0) (0)

Question Resolved

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Customer Reviews (100+) Write a Review

Overall Rating 4.9

out of 5 stars

Installation Time:Full Day

Installation Cost:$100-$250

  • Loudness / Volume 4.774194 4.8
  • Exhaust Note 4.83871 4.8
  • Fit & Finish 4.903226 4.9
  • Bang For Your Buck 4.806452 4.8

  • 5/5

    July 21, 2016

    LOVE this catback!!!

    So I just bought a 2004 Mach 1. Previous owner had a Bassani catted x-pipe with Magnaflow Street catback. Meh, it was ok, too stock sounding for my tastes. I've had SLP's before on older GT's, so I knew what I was getting into. But WOW, this catback paired with the Bassani x-pipe is insane! I never heard that amazing bubbly 4 valve idle on my car until I put this SLP catback on. If you've ever heard an 03-04 Cobra idle, this brings out that same sound in your 4 valve. Surprisingly there really isn't much drone considering how loud it is on the outside. But mash your right foot down and this baby SCREAMS! And of course, AmericanMuscle delivered FOR FREE within 3-4 days. Couldn't ask for a better price or better service. Thank you!


    Year: 2004 Submodel: Mach 1

    Helpful (1) (0)

  • 4/5

    Installation Time: Afternoon

    August 08, 2015

    Great Exhaust

    This is a great exhaust for the price! Paired this with an H pipe and it sounds amazing. Quiet at idle but when you get on it sounds great! I would recommend this SLP catback for anyone looking for an exhaust on their 'stang.


    Year: 2004 Submodel: V6 Arlington, Virginia

    Helpful (1) (0)

  • 5/5

    Installation Time: Full Day

    March 02, 2014

    Sounds awesome!

    Installed with pypes off road x pipe. Sounds great. Yes it's loud very loud. But I love the sound on my v6. It literally roars and bellows at hard throttle. It's got some deep sound to it. Overall I am very happy with this exhaust. Fits perfect,tucks way up and gives plenty of clearance. Highly reccomend drivers that want to be heard. As far as my full day install. I also installed a gt 8.8 rearend. So don't be scared to tackle this slp exhaust.


    Year: 2004 Submodel: V6 Savannah, GA, USA

    Helpful (1) (0)

  • 5/5

    November 26, 2007

    Get It

    This exhaust scares old people!


    Helpful (42) (0)

  • 5/5

    April 13, 2009

    Just Perfect

    Dont listen to anyone who tells you these are too loud. They are not.They are perfect. They do \'shout\' dont get me wrong and people will move out of your way (i love when that happens to me :-) but they are not obnoxious, I contemplated for the longest time on which catbacks to install on my 04 V8 GT. Got them put a week ago went down to the NJ shore this weekend...The drive was brilliant... Definitely recommended. PS they might be \'Too loud\' if you throw on and O/R pipe or something but i have the stock mid pipes and they are great. if you like something with a bite,these are it and AM ships right on time every time. I do all my \'car\' shopping out here. Kudos to AM and to my cat backs. cheers


    Year: 2004 Submodel: GT

    Helpful (34) (2)

  • 5/5

    January 31, 2010

    This exhuast = BA

    This exhaust was very easy to assemble, and very easy to line up. the tubing is already notched for easy clamping. Plus, the sound is very nice and loud. It gives my car a very muscular sound. So in all that is why i say this exhaust = Bad A**


    Helpful (19) (0)

  • 5/5

    August 13, 2009

    They Bark!

    I really like this exhaust. I also added a BBK ceramic coated long tube header and BBK shorty H-pipes with the cats and this system is the perfect addition. When you don't get on it, it's a "medium" mellow tone. Not too loud, but loud enough. When you get on it, it flat out barks! I mean it's loud! I love it!! It reminds me of my GTO with headers and glass packs from years gone by. I do recommend this combination, if you're into the sounds of the "old days".


    Helpful (17) (1)

  • 5/5

    Installation Time: Afternoon

    October 02, 2011

    It took a little over two hours to cut off the old exhaust and install the new ones

    It's a great product that sounds really good. I thought they would be too loud but ended up being just what I been looking for. These cat backs will make heads turn. I'm sure with headers it would sound even better. I had Borla Stingers on my old mustang, they sounded good but these sounds way better!


    Year: 2004 Submodel: Mach 1 Baton Rouge, La.

    Helpful (2) (1)

  • 5/5

    July 22, 2008

    hear me roar

    They are bad ass. I was pulled over by the Ohio state troopers just to tell me how bad ass my 2004 mustang gt sounds with the SLP loudmouth exhaust installed. It was and is an awesome system.

    p simms

    Year: 2004 Submodel: GT

    Helpful (7) (0)

  • 5/5

    August 16, 2010

    03 stang

    sounds nasty, but in a good way really you will love this cat back. took a wile to take off the stock exhaust, nuthing a hak saw cant take care of. the loud mouth went on very easy but i have a friend that has lift. good mean sound when you start it turns a lot of heads im the parkin lots. its not loud unless you want it to be cops dont even look at you. but when munch on that gas you will get chills. the only problem i have with it is when i drive down the interstate at about 2000 it has a lot of exhaust sound in the cab sounds good at first but gets a little old but all in all i love it. thanks AM


    Year: 2003

    Helpful (10) (3)

  • 5/5

    December 21, 2010


    Catback with catted x-pipes sounds awesome. Quiet when under 2k rpm but after it roars!!! Travel down highway and no annoying sounds. Hear radio just fine. Thank you A&M for good product to reach my door as promised.


    Helpful (5) (1)

  • 5/5

    June 13, 2009

    2004 V6

    I recently just put this cat back on my car along with a pypes catted X and BBK shorty headers and its louder than any other modded V6 I\'ve ever heard. No joke when i rev this B*TCH up she screams her ass off. GREAT PRODUCT THANKS AM!


    Year: 2004 Submodel: V6

    Helpful (4) (0)

  • 5/5

    April 07, 2008

    2003 3.8L V6

    Exhaust was easy to install once I got my v6-v8 adapter (H-pipe) welded on. I was a little unsure about the sound I was going to get...with my car being a v6, and with my friend and his MAC y pipe exhaust sounding nice, I didn\'t know what to expect. When the exhaust was done, his jaw was on the ground and he wanted my same SLP loudmouth setup.


    Year: 2003 Submodel: V6

    Helpful (4) (0)

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $250-$500

    April 06, 2013

    Loudest Exhaust Ever!!

    If you want semi-loud, try looking elsewhere. This exhaust is extremely loud and i love it. Some people do think its too much when paired with long tubes and off road x pipe. When we started the car after installation for the first time, I smiled from ear to ear and jumped around like an uncontrolled idiot.


    Year: 2000 Submodel: GT

    Helpful (3) (0)

  • 5/5

    December 11, 2010

    slp loudmouth 1

    this catback and an o/r xpipe is perfect if you want the attention! iam not goin to lie but you can hear my o3 Gt from blocks away! slp is great! AM is great

    jesus sanchez

    Helpful (3) (0)

  • 5/5

    Installation Time: Afternoon

    September 23, 2011

    awesome sound

    if you want to be loud and heard, this is by far the best system to go with. fitment was simple had no issues with installation.



    Year: 2001 Submodel: GT Aston, PA

    Helpful (2) (0)

  • 5/5

    February 16, 2010

    Woth Every Penny

    One word for this Exhaust system "Perfect". Give your car the sound that gives people shiver. TOP NOTCH SYSTEM!!!

    adam demarco

    Helpful (3) (1)

  • 5/5

    Installation Time: Less than 1 Hour

    June 14, 2016


    The sound is so crisp, clean and loud! Best sounding cat back only exhaust I have ever heard!!!! Love the product from SLP!


    Year: 2004 Submodel: Mach 1

    Helpful (1) (0)

  • 5/5

    Installation Time: Afternoon

    December 24, 2014

    Slp loudmouth's should come standard on all Mustang's

    Being OCD and selecting an exhaust is a hard thing to do. I spent weeks on YouTube listening to other mustang's exhaust systems. Spending $500 isn't an easy thing to do for me. But I am so glad I spent it on this system. It is what should come standard on all mustangs. It simply is what a muscle car should sound like. I installed it with my factory cats and H pipe on my '03 GT. Installation wasn't bad and you should lay out your old exhaust next to the new one to get a feel how it should go. It went in pretty easy in about 5 hours between cutting out the old, dealing with kids and fitting in the new. Overall the system is deep and loud. It has that Classic muscle car growl and the note is perfect. The slp loud mouths are loud. But they aren't over the top open headers loud. Many YouTube videos I watched the pipes seemed raspy. Mine aren't raspy at all. Mainly due to my factory cats. But I wont be deleting the cats not because of emissions in my state which there aren't, but because it sounds so good. I have had the system on there for 2 days and have had people stop me and ask me what it is. I love it and they do too! If your on the fence, well spend the money and get the SLP loud mouths!! You'll be glad you did!!!


    Year: 2003 Submodel: GT Jacksonville, FL, USA

    Helpful (1) (0)

  • 5/5

    Installation Time: Shop Install

    April 13, 2012

    Be Heard!

    I like it when i hear other American Muscle cars rolling down the street!. Now with this new cat back system paired with the matching X-pipe, i get a lot of attention. It sounds wonderful and looks awesome with the SLP logo on the tips! And for that added thrill factor hit up a parking structure, rev it up and let your loudmouth set off EVERY car alarm in the garage! Tested and Proven to give you chills!!!


    Year: 2001 Submodel: GT Las Vegas, NV, USA

    Helpful (1) (0)

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