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

Item 23001
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      Review & Installation Video

      Product Information

      Features, Description, Reviews, Q&A, Specs & Installation


      • 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

      MPN# M31007

      CA Residents: WARNING: Cancer and Reproductive Harm -


      Customer Reviews (100+)

      Write a Review

      Ratings Summary

      4.9 out of 5 overall rating from 100+ customers

      Installation Time: Afternoon

      Installation Cost: $100-$250

      • February 03, 2020

        1 2002 GT - LaShaun Installation Time: Shop Install Installation Cost: $100-$250


        I like the sound but it gave me hearing damage and now I'm paying for it today. I had to get rid of these things. I couldn't even joy it on my car. Also, I had a hard time selling it. It took me three months to get rid of it. I went by all the good reviews on this website before buying but has I found out some people are sensitive to loud noises. I also spent money on another kit and 2 resonators that's supposed to be less loud but it was still too loud. So I decided to buy the factory pipes and put those back on as soon as I get more money. When looking for exhaust pipes I'll never go by reviews. And I'll never go by sound clips ever again.

        Helpful (0)
      • December 09, 2019

        5 2003 Mach 1 - Juggernau7

        Showed up quick and in perfect shape.

        Unfortunately wasn't what I needed but came on time and was a good price.

        Helpful (0)
      • November 18, 2019

        5 1999 Cobra - btylerb123 Installation Time: Less than 1 Hour

        Great Tone and Performance

        Great sounding System. Quick and easy install. Overall completely satisfied with the SLP’s.

        Helpful (0)
      • November 18, 2019

        5 2001 GT - Mtk Installation Time: Less than 1 Hour

        Buy it!

        This item came well packaged,on time and it sounds amazing. If your looking for a nice throaty sounding exhaust, look no further. Well worth the money

        Helpful (0)
      • October 29, 2019

        5 2001 Bullitt - Johns01bullitt Installation Time: Afternoon

        She's loud

        Received all the hardware, install went pretty smooth did it solo in my garage. Sounds mean now compared to stock. Nothing looked or felt cheap and fitment was well just about perfect.

        Helpful (0)
      • October 16, 2019

        5 2000 GT - Fauxbra Installation Time: Shop Install Installation Cost: $100-$250

        Old school sound

        I wanted to make my 2000 Mustang sound as much like a 70's hot rod as it could, and the SLP Loudmouth did the trick. Plus, the American Muscle videos of each exhaust system were critical to picking the right parts. It gives you that old school sound under hard acceleration, but when you're cruising at 70, I hear more road noise than exhaust. Perfect for me.

        Helpful (1)
      • September 05, 2019

        5 1994 GT - BlackMustang Installation Time: Afternoon

        SLP Loudmouth Cat-Back Exhaust

        This exhaust is awesome! Totally changed the way the car sounds and it is awesome. It is not too loud at idle, you'd never know it is a monster waiting for the gas pedal to be mashed. And when you do it sounds amazing. It fits perfectly and was easy to install. The polished tips with the SLP logo are the finishing touch to a really nice exhaust system.

        Helpful (1)
      • September 04, 2019

        5 2003 GT - Garcia03 Installation Time: Shop Install Installation Cost: 0-$100


        These things are loud. I'm not sure if anyone is thinking these aren't but just tapping the gas peddle slightly already creates a nice deep growl. I'm in love with this system. If you're looking for deep and loud aggression. I would definitely recommend these pipes. They are no joke. Once you hit 3k RPMs is when you really open up those pipes.

        Helpful (0)
      • September 02, 2019

        5 2000 GT - ArealCustomer Installation Time: Afternoon

        Loud and Deep

        These sound great, I have stock headers and H-pipe. at idle its quiet but when you hit the gas it turns heads, and when you punch it all #(* breaks loose. I've always had FM with the deep rumble everyone loves, but these are a different deep sound, more modern. High quality also would def. recommend!

        Helpful (1)
      • June 11, 2019

        5 2002 GT - HenryG Installation Time: Shop Install


        bought these from american muscle and they arrived on time and in good condition. They sound amazing and am very happy with the product! they sound way louder in person

        Helpful (0)

      Questions & Answers (60 answers)

      5 Top Rated Questions

      Will this bolt up to a Mac off road h pipe on my 99 gt?

      • The SLP Loudmouth Cat-Back Exhaust will not bolt directly up to the Mac off-road H-pipe. The cat-back and the h-pipe use different couplers to connect.

      Helpful (4)

      Will this exhaust pass the emission inspection in California?

      • The SLP Loudmouth Cat-Back Exhaust will not alter emissions and therefore will not cause your Mustang to fail emissions.

      Helpful (3)

      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 (3)

      Does it require a tune up after. reason I ask is because I've been told because it is basically straight pipe I would need to get a tune up after or it would damage my engine from lack of back pressure

      • This will not require a tune no.

      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.

      Helpful (1) Show 2 more answers
      10 More Questions
      Will this system work on a 95 gt? 0 answer
      Is this cat back system compatible with the cat less bassani h pipe 0 answer
      do these sit lower than the stock pipes ?? in other words will I bottom out on my current set up after the install 0 answer
      With stock headers, and cats, how many decibels does it make? 0 answer
      Will this work with bbk long tube headers and bbk h-pipe? 1 answer
      Will this bolt up to a JBL off-road H pipe. 1 answer
      Will the slp loudmouth exhaust fit and sve off road x pipe 1 answer
      Will this fit on a stock H pipe 02 Mustang Gt? and does it require a tune up after. reason I ask is because I've been told because it is basically straight pipe I would need to get a tune up after or it would damage my engine from lack of back pressure. 0 answer
      Is it road legal 1 answer
      Will this hook up to a pypes off road x pipe? 1 answer

      Installation & What's in the Box

      Installation Info

      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

      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
      • GT - 99, 00, 01, 02, 03, 04
      • Mach 1 - 03, 04

      Fits GT, Mach 1, and Bullitt

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