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

Item 23017
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      Review & Installation Video

      Hey, guys, Adam here with Today, we're taking a closer look at listening to and installing the SLP Loudmouth II Catback exhaust available for the '99 to '04 GT and Mach 1 Mustangs. Now, if you're the owner of that 4.6 New Edge, you might be in the market for a new catback to not only give you a more aggressive tone but also a better appearance at the rear end and, overall, just a better sounding Mustang. Now, as you heard from our sound clips, I'm gonna give this one three-out-of-five on our loudness meter. It's definitely not the loudest of the loud, but it is absolutely aftermarket. It has a rich, deep, throaty aggression to it, and it's not too overwhelming. Now, inside the cabin, not a whole lot of drone, if any at all. At fifth gear on the highway, there was a little bit of drone, but it was pretty tamed. Nothing too overwhelming inside the cabin. Now, other than that, this thing is going to be pretty loud at wide-open throttle, and on deceleration, you can expect some backfire, which, personally, I really like. Now, the tips are three-and-a-half inch 304 stainless steel, which is a nice polished finish. Looks a little bit better than the factory but still very similar in design, only these are gonna have the SLP logo almost laser etched on the top end, which I personally think looks really good as well. Now, the rest of the system is completely modular, which is something to keep in mind. So if you pick up this system, and later on down the road maybe you want to change out those bullet-style mufflers, the modular system will allow you to do so. So what that basically means is all you have to do is unbolt those bullet-style mufflers or resonators from SLP and swap them out with another SLP product. Or if you have something of the same size that would fit, you can do so. The modular system allows you to change it up whenever you want. This one comes in right around the $500 mark, and it's gonna get two-out-of-three wrenches on our difficulty meter. Now, guys, there is a very small amount of cutting involved, very small amount of cutting. Some guys might be lucky enough to be able to slide this out without having to cut. But right behind your factory tips, you may need to cut that pipe in half. That's going to make it a lot easier for you to slide the over axle pipe over the axle to get your factory exhaust uninstalled. Other than that, very simple system, bolt-on, very simple hand tools required. I'm gonna take you through every step of the process. Takes you about two hours from start to finish. What do you say we get started? Tools used in this install include an impact gun, 14 and 15-millimeter deep sockets. Hanger removal tool is definitely recommended as well as PB B'laster or WD-40, and we used a Sawzall for cutting. This is a much bigger Sawzall than you need, but a cutting tool of your choice and a metal cutting blade. All right, guys, the first step of the uninstall here to get your factory catback out of the way is to grab a 14-millimeter deep socket. We're gonna remove the four bolts holding on your factory catback to your mid-pipe. As you can see there's two flanges, one on each pipe here, both of which has two bolts. I'd recommend grabbing some PB B'laster before you get started, hit those bolts with a little PB. These New Edges can be pretty old and sometimes pretty rusted and corroded, so having PB B'laster definitely makes life a little easier. We've already taken care of that as you can see, so let's loosen them up and get them off.All right, next up we're gonna grab a hanger removal tool and pop these hangers out of their isolators. Now, on each side there's two of them, one on the pipe and then one over here on the muffler itself. So, grab a hanger tool and pop them off. All right, do the same thing on the other side. All right, guys, at this point, I want to stop down and take a quick second to explain the cutting situation. Now, in some cases, guys, you may not need to cut if you're able to slide your over axle pipe out of the hangers by the tips all in one piece. But depending on your situation, you may need to cut right here where you see this clamp. Now, we've already taken care of the cutting in the past for previous videos so we have a clamp right here, but I still want to show you guys what that cutting is like. We still have footage of that, so I'm gonna show you guys some of that footage of us cutting this factory exhaust off. So, your Sawzall or something similar will do the trick. If you're not comfortable with that, of course, taking it to a professional is the way to go. But if you have the right tools, you can definitely get it done. I'm gonna remove this clamp, but let's see how that cutting is taken care of. All right, so once that's disconnected and cut, you can see the over axle pipe with the muffler hangs down on the axle itself. You can just pull it out in one piece. All right, guys, we finally got our factory catback off of our '99 GT sitting on the ground next to our SLP Loudmouth II. I want to take you through some of the similarities but mainly the big differences between the two kits, and there are quite a few to mention. Your factory catback here, as you can see, over the years gets pretty rusted, corroded. Doesn't look very good. Some of the hangers might be looking a little bad as well as the flange, so it's good to update all the systems with something of a better quality. The SLP Loudmouth here is using two-and-a-half inch 409 stainless steel mandrel bent tubing, mandrel bent meaning it is going to be a lot smoother, better for airflow, less restrictive than your factory catback that can have some kinks and some of those bends. 409 is also a better quality material than your factory catback, so it's not going to end up looking like this after a few years. It'll definitely hold up a lot longer. Now, the rest of your system here includes bullet-style mufflers, which are the small muffler almost resonators that you see, which is a huge difference from the factory option. That's really what's gonna convert this to almost like a straight-pipe-style design. There's not a whole lot of baffling involved here. You can pretty much see right through them, which is creating that sound you heard. It's creating that noise, the tone, little bit of drone, the raspiness of this catback. It's also going to create a little bit of that backfire. Now, toward the end of the system, you can see that it has a similar tip style to the factory, just a little bit more polished up. Those are three-and-a-half inch polished 304 stainless tips. 304 is the highest quality material on the market, so the tips are gonna hold up a lot longer, which is good because that's really what you're seeing from the exit style of this catback. So the dual exhaust here, the tips coming out the rear are gonna hold up a little bit longer. And, that's gonna be a good thing because they're good to be polished up and have that show car finish. Now, they do have the SLP name embedded in the materials on top. I like when brands put their logo on that tip so you can really represent. Now, we're going to show you guys how to install this. You're gonna use the same isolators from the factory. If your isolators are looking pretty beat up, now is a good time to swap them out. You can pick those up at your local auto parts store. Our's looking pretty good, so we're still going to reuse those. All clamps are included in the kit. You're also going to use a new flange for your connection to your mid pipe, in this case is our H-pipe. So we'll grab that and start there at the flange. All right, grab one of the flanges included in the kit as well as your exhaust pipe that will connect to the flange. Now, there's two different sizes. The smaller one is going to go on your driver side, the longer pipe is going to go on your passenger side. So we're going to show you guys how it works here on our driver side, so we're gonna slide the flange on the exit down to the stopper. Now, that will connect to your factory studs. So, slide those over, and align the studs. They're gonna seat properly inside of each other, then you're gonna grab your factory nuts, and thread those on to the studs just like that. Now, when we tighten these down, you want to leave a little bit of room to make adjustments so we can angle it properly. But, for now, we're gonna have this angled toward the outside of the vehicle. All right, from here, grab your 14 socket, and you can tighten this guy down. But, again, just nice and snug, not too tight. Leave room for adjustment. All right. Now, you can do the same thing on the other side. From here, you can take your SLP muffler, and you're gonna slide that over the exit with one of the clamps installed over the inlet. Once you have that taken care of, grab a 15 socket and tighten down that clamp. All right, do the same thing for both sides. All right, next up is our over axle pipe. Now, you'll know which side's which depending on the hangers. The taller hanger is gonna be on the left side going up to that taller or higher up isolator. So, line that guy up, slide it over the axle, connect it to your muffler with a clamp installed on that inlet. And, before we tighten anything down, slide these hangers into those isolators just like that, and the same thing on the top guy. All right, once that guys in that isolator, you can slide that guy in and then reconnect your pipe. From there, tighten down your clamp. All right, we can keep going with that over axle pipe with the next one for our driver side. Now, that, again, is very specific. You'll figure that one out because it'll go straight toward the isolator where your tip will go. You want to make sure you're using the pipe that makes the tip line up properly. Grab your 15 socket with that clamp and tighten it down. All right, finally, the tip on your driver side. You want to make sure you're sliding it in that isolator first, slide it back as far as you can, and then connect it to your pipe. Perfect. All right, from there, make sure it's aligned, tighten down your clamp. Now, you can just repeat the exact same process on the other side. At this point, make any adjustments that you need to to get everything lined up properly, tighten down all your clamps, and you're good to go. Well, guys, that's gonna wrap up my review and install for the SLP Loudmouth II Catback for the '99 to '04 GT and Mach 1. If you want to pick this one up, you can do so right here at

      Product Information

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


      • Rich, Throaty Exhaust Tone
      • Loudmouth II "Bullet-Type" Race Mufflers 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 famous Loudmouth II "Bullet Style" race mufflers for minimal restriction, increased horsepower and an aggressive tone, that are only second to the Loudmouth "shotgun" race resonators for sound.

      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 bullet mufflers, the Loud Mouth II system is compatible with all SLP mufflers and resonators including the shotgun style Loud Mouth Resonators 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 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 II 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# M31007A

      CA Residents: WARNING: Cancer and Reproductive Harm -


      Customer Reviews (91)

      Write a Review

      Ratings Summary

      4.9 out of 5 overall rating from 91 customers

      Installation Time: Afternoon

      Installation Cost: $100-$250

      • January 10, 2020

        5 2002 GT - Maxey93 Installation Time: Afternoon

        Allsome cat back exhaust system.

        Killer slp2 cat back exhaust system thanks American muscle.

        Helpful (0)
      • May 30, 2019

        5 2003 GT - Laith Installation Time: Afternoon

        SLP Loudmouth II

        Exeeded expectations. The quality is excellent. The install was super easy. I paired this with an off-road h-pipe and haven’t turned my radio on once since the install (three weeks ago). The exhaust note is clean, deep, and LOUD! Very minimal drone inside the cabin but turns heads everywhere.

        Helpful (1)
      • May 03, 2019

        5 2003 GT - Gunman Installation Time: Afternoon

        couldn't be happier

        Installed on my 2003 Gt with the BBK 1-5/8 IN. Long Tube Headers and BBK Competition Series off road x-pipe and it sounds amazing, fit finish and the installation were spot on. between the exhaust the Vortech with their Maxflow Race Blow-Off Valve my Mustang turns everyone's head to see whats coming. 5* all the way

        Helpful (1)
      • April 03, 2019

        5 2004 GT - Charlest Installation Time: Afternoon

        That rumble

        Just had the loudmouth 2 install with the SLP catted x pipe sounds amazing could be a little louder for my taste but that will come when I add headers. Total time for install was a little under a hour and a half with a lift fit was great no complaints

        Helpful (1)
      • March 20, 2019

        5 2003 GT - Shane Installation Time: Shop Install

        Great option

        Sounds very nice, not so loud that it shuld only be on a track but enough to say my car means business. I added cat-less bbk x-pipe so it sounds race ready and complements my other mods.

        Helpful (1)
      • June 09, 2018

        5 2004 GT - TroyC Installation Time: Afternoon

        Great exhaust, read this for torque specs

        This system was easy to install, although if you have access to a lift I highly reccomand you use it. I used ramps, and I could have saved myself a lot of time if I had access to a lift. Really lube the hangers before you try to wrestle them off or just remove the 2 bolts and pull the hanger down, but you’ll still need to lube the hangers to really easily get them off. I found it easier to just remove the hangers and do it standing up rather than on my back under the car. I had to call SLP for torque specs on the band clamps and even the guy I spoke with had to call me back later because he wasn’t sure. 25-30 lbs of torque is what the torque should be on the band clamps. I knew I was going to add an X-pipe at some point, so I went with the LM 2, but if I wasn’t, I would have gone with the LM1. This system is much improved over stock, is much deeper, and has a great growl to it, but doesn’t have much more volume to it over stock which is great if you’re looking for a quiet exhaust. Once I add an X-pipe to it, it will be a little bit louder for sure. I certainly don’t want an offensively loud exhaust, but this one is very quiet even for me just as a catback alone.

        Helpful (6)
      • April 29, 2018

        5 2003 Mach 1 - StephenY Installation Time: Afternoon

        Really nice cat system

        Quality cat system that has great tone. Not obnoxiously loud and looks great installed.

        Helpful (4)
      • April 25, 2018

        5 2002 GT - MattM Installation Time: Afternoon

        Best exhaust on the market

        The exhaust is by far the best on the market! I was running a magnaflow catback system and wanted something different. The install was super easy, I did it in my garage in about 2 hours! Everything lined up well and fit perfectly! Not one thing bad to say! The sound was the best part, being paired with an O/R h pipe; it rumbles at idle and sounds amazing throughout the RPM range! Highly recommend, and thanks again American Muscle!

        Helpful (4)
      • March 26, 2018

        5 1999 GT - ChuckA

        Nice system

        I bought this system for my '99 GT convertible and was a little worried about it being to loud with the top down. I will say this system sounds great and is not overly loud has a nice throaty sound. I installed the system at my shop and took my time just over an hour or so. Very straight forward bolt up. If you are looking for a nice sound but not extremely loud this is the right system even great in a convertible. Great purchase very satisfied!

        Helpful (4)
      • March 07, 2018

        5 2000 GT - DarrenF Installation Time: Less than 1 Hour

        Sounds great and throaty

        I have no cats and at one point no muffler either, the car was too loud so i looked around for a aggressive exhaust that will also muffle the sound some but stay loud and good sounding. So i landed on this and it sounds great after about a week the exhaust broke in and sounded great!!. Would definitely recommend for cars with no cats. If you have cats i think it will be too quiet. Btw i had a lift for my install.

        Helpful (5)

      Questions & Answers (12 answers)

      5 Top Rated Questions

      Would I need to purchase an X pipe separately?

      Are these good with stock mh pipe

      • Yes, this cat-back will bolt right up to the factory mid-pipe.

      Helpful (0)

      I have a pypes catted x pipe will this bolt right up or no?

      • This would bolt up to the Pypes catted X pipe as both have 2.5" collectors. 

      Helpful (0)

      What makes this model different the the first version? Thanks.

      • The difference between this exhaust and the first version would be the muffler style. This exhaust would have a bullet style muffler rather than the shotgun resonators that the first loudmouth exhaust has. This exhaust would be louder as well as have a deeper throatier tone to it when compared to the first loudmouth exhaust.

      Helpful (0)

      Will this exhaust fit if have the cats taken off?

      • The SLP Loudmouth II Cat-Back Exhaust will work with an off road mid-pipe. If you have any other questions or concerns feel free to call or chat in.

      Helpful (0)
      Post Your Question
      7 More Questions
      Does anyone know if these pass smog in California w/ catted x pipe? 1 answer
      I have pypes violator catback how does this compare in tone, aggression and loudness? 1 answer
      Would this system bolt up to bbk headers and xpipe? 1 answer
      will the lm2 for 99 gt fit my 84 gt can it be made to fit 1 answer
      So I like the outside sound of these but I was wondering what's the drone like on the freeway? My friend has the borla ataks w/catted Xpipe and bbk short headers. They sound really nice but after we put on the header and Xpipe they started to drone so bad. I will be daily driving this car and would like a nice sound with as little drone as possible. 1 answer
      any body have these with o/r h pipe ? i have lm1s with a mac o/r h pipe and was lookin for a better sound not just loud . how do the lm2s sound ? or whats a better resonator? i like it loud tho 3 answers
      Can this product fit under a v6? I have looked at the dual conversion kits for v6 on here and they aren't true dual conversions as far as I could tell. They use a y pipe.. 3 answers

      Installation & What's in the Box

      Installation Info

      What's in the Box

      Hardware included:
      • (8) exhaust system band clamps

      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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