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

Market Price $529.95 You Save 3% ($15.00)
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  • Item 23001
  • MPN# M31007

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Features & Details

  • 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

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Most Helpful Reviews


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.

Helpful (3)


This exhaust is soooooooooooo loud. My neighbors hate my car even more than before.

Helpful (1)

Overall Rating 4.9 out of 5 stars
Read All 100+ Reviews

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32 Questions / 8 Answers

Ask a Question

Are these legal in WA


  • This exhaust does not affect emissions so it will pass inspection and emissions, however, depending on your local land ordinances in WA this exhaust may violate a sound ordinance if they are strict in your area.


Helpful (1)

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.


Show 2 more answers Helpful (1)

Is this catback too loud for a daily driver gt?


  • The SLP Loudmouth Cat-Back Exhaust will live up to its name, being the loudest cat back we offer. The Loudmouth resonators are essentially open chambers with no baffles or sound deadening materials.

    Rob D

Helpful (0)

Is this exhaust louder than a straight pipe system?


  • Hello! No this is not louder than a straight pipe exhaust. Having a true straight pipe setup is pretty much the loudest that you can possibly make a car. Although the SLP Loudmouth is definitely a loud exhaust, it will not compare to a straight pipe exhaust.

    Doug P

Helpful (0)

Will the exhaust tips have slp brand name on the top of them like how other tips have "flowmasters" or "pypes" labeled on them ?


  • The top portion of the exhaust tips will have a powder coated "SLP" branding. You can reference the SLP exhaust tips from the customer submitted images of the product.


Helpful (0)

I know convertibles have different structural integrities than the coupes. Will I have any issues with this cat-back with my convertible?


  • These will not have issues when trying to fit them on a convertible between the years 1999-2004.


Helpful (0)

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Customer Reviews (100+) Write a Review

Overall Rating 4.9

out of 5 stars

Installation Time:Afternoon

Installation Cost:$100-$250

  • Loudness / Volume 4.782258 4.8
  • Exhaust Note 4.822581 4.8
  • Fit & Finish 4.879032 4.9
  • Bang For Your Buck 4.814516 4.8

  • 5/5

    Installation Time: Afternoon

    December 09, 2016


    This exhaust is soooooooooooo loud. My neighbors hate my car even more than before.


    Year: 1999 Submodel: GT

    Helpful (1)

  • 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)

  • 4/5

    Installation Time: Less than 1 Hour

    May 13, 2016

    slp loudmouth 1s

    As far as product and sound goes.. this exhaust sounds awesome! Very throaty and very loud. But when just cruising has a nice little rumble to it. I was missing a few bolts for the exhaust but had an extra couple bolts so I'm not to worried about it. I have bbk 1-5/8" headers with a bbk offroad xpipe and now the slp loudmouth and I get constant compliments. Great product from AM!


    Year: 2002 Submodel: GT

    Helpful (1)

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $100-$250

    September 09, 2015

    Best sounding exhaust i've had on a mustang yet

    Had this done by a shop as my stock was rusted on, the install wasn't that expensive at $125 and the exhaust sounds amazing! i couldn't be happier with it. doesn't drone to badly but sounds amazing at the high end.


    Year: 2000 Submodel: GT Downingtown, PA

    Helpful (1)

  • 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)

  • 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)

  • 2/5

    Installation Time: Shop Install

    March 29, 2012

    Not what I was expecting...

    After reading through all of the reviews and seeing how happy people were with this exhaust I was not hessitant to buy it myself. After the install however I was blown away by how LITTLE the sound of my exhaust actually changed, It's barely louder and at higher RPMs I think it sounds terrible. Through about 1-2.5 Grand It sounds ok But after that the tone and volume drop substantially. Maybe it's just my Mustang but if your expecting a loud exhaust, it's not here.


    Year: 2004 Submodel: GT South Beloit, IL

    Helpful (2)

    • Response from AmericanMuscle

      I'm sorry you're disappointed in the overall sound of this catback after installing. This system is one of the louder options available and I've never heard feedback on the lack of sound produced. Adding the SLP Catted X-pipe, SKU 23008, might help you obtain the sound you are looking for and provide you with better airflow and power gains at the same time. I'd recommend contacting SLP directly as well and chat with them about your install to see if they have any insight as to why your system isn't performing as expected. Hope this helps!

      Lisa , Content Specialist

  • 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 (3)

  • 5/5

    June 05, 2008

    woo hoo

    The way a muscle car should sound, with stock cats, deep tones and added burble. Awesome! Not too loud at any RPM.

    Randy O.

    Helpful (1)

  • 5/5

    April 26, 2008

    Sounds great

    After listening to numerous Mustangs and pulling my hair out over making the right choice, I landed on these. All stainless was a huge selling point, and cheaper than the other brands. I wanted a deep, old school muscle car sound; and with my stock H, I got just that. Wow, did it wake my car up. They are not too loud at idle, but when you go, look out! No drone on the highway, just pure, hot, steamy, exhaust, awesomeness.


    Helpful (1)

  • 5/5

    November 26, 2007

    Get It

    This exhaust scares old people!


    Helpful (42)

  • 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 (4)

  • 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)

  • 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)

  • 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)

  • 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)

  • 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)

  • 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)

  • 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)

  • 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)

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