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How to install a SLP Loudmouth Catback Exhaust on your 1999-2004 GT Mustang

Installation Time

2 hours

American Muscle
Shop Parts in this Guide



Item #QuantityPart NumberDescription
1.1310304795Driver-side Intermediate Pipe
2.1310304796Passenger-side Intermediate Pipe
3.1310304797Driver-side Over-axle Pipe
4.1310304798Passenger-side Over-axle Pipe
5.2308560782PowerFlo Muffler (M31004)
5.2310013919Loud Mouth Resonator (M31007)
5.2310013818Loud Mouth II Muffler (M31007A)
6.1310304792Driver-side Tailpipe
7.1310304791Passenger-side Tailpipe
8.2310305785ATailpipe Tip
9.2307334788Ball Flange
10.83081443802.5 in. Exhaust Clamp

Warning: SLP recommends allowing the vehicle to cool (not running) for five hours before beginning the installation. SLP recommends wearing safety glasses for the complete installation.


1. Raise the vehicle to allow for a sufficient work area underneath the vehicle. A vehicle lift will make this installation much easier, however, raising the vehicle can be accomplished using ramps or a jack with jack stands.

2. Remove the old exhaust from the vehicle by unbolting the 2 flanges from the H-pipe and removing the exhaust hangers from the rubber insulators.

Note: You may need to cut the old pipes in the vicinity of the rear axle or disconnect the shocks to gain enough suspension clearance to remove them.

3. Place the ball flange of the driver side intermediate pipe (shorter of the two) onto the driver side hpipe outlet, slide the flange into place over the studs, and loosely tighten the nuts.

Note: Tighten all clamps and fasteners just enough to hold them in place. Leave them loose until the end to allow adjustment.

4. Place one of the supplied band style exhaust clamps onto the inlet of one of the resonators (PN M31007) or PowerFlo mufflers (M31004) or Loud Mouth II mufflers (PN M31007A). The inlet side is easily identifiable because it is larger and slotted. Slide the inlet of the resonator or muffler over the end of the driver side intermediate pipe and loosely tighten the clamp. The PowerFlo mufflers are the same for the driver and passenger side but the outlets must be angled away from the center of the vehicle for the system to fit properly. If one of the outlets is angled inward just turn the muffler over.

5. Slide another exhaust clamp onto the inlet of the driver side overaxle pipe, place it over the axle, and slide it onto the resonator or muffler outlet. Now insert the hangers on the overaxle pipe into the rubber insulators on the vehicle underbody. Loosely tighten the clamp.

6. Place another exhaust clamp on the front of the driver side tailpipe and slip it onto the overaxle pipe. Loosely tighten the clamp.

7. Again, slide an exhaust clamp onto the end of one of the tailpipe tips, slide the tip onto the tailpipe, and slide the hanger into the factory tailpipe hanger. Loosely tighten the clamp.

8. Repeat this entire procedure (steps 3 through 7) for the passenger side.

9. Now adjust all the pipes until the tips are located as desired and all the pipes have enough clearance from all suspension, brake lines, fuel tank, driveline, and body parts.

10. Tighten all the clamps and fasteners working from the H-pipe back, while checking and maintaining correct tip location along the way. The H-pipe flange studs/nuts should be tightened to 26 lb/ft.

11. Start the vehicle and inspect for leaks.

12. Double-check all fasteners and clearances after test driving. Adjust as necessary.

Dear SLP Enthusiast,

SLP LoudMouth Exhaust Systems DO include a muffler. While it does not “look” like the traditional muffler, it is indeed a muffler by definition.

The muffler (aka resonator - actually a misnomer or ill-understood word) was designed by SLP to smooth the exhaust gas pulsations by methods of wave rarefaction to make them as inaudible / quiet as possible. This is a type of reflection muffler; Most people are much more familiar with more common 'absorption type mufflers', which typically include internal perforations, sections, and/or absorption insulation.

More simply put, a muffler exists and was designed with intent to reduce exhaust noise.

The Specialty Equipment Manufacturers Association (SEMA – has staff dedicated to representing the aftermarket industry in regulation/legislation pertaining to aftermarket parts. SEMA is an excellent source for further information and/or news and updates regarding the latest laws governing aftermarket parts.

The SLP Loud Mouth exhaust system is 50 state, emissions legal. It will not adversely effect emissions. Every local jurisdiction creates and enforces their own laws regarding ‘noise’. Local jurisdictions have the ability to write more detailed and stringent laws regarding noise - but commonly do not outlaw 'replacement exhaust systems'; Unless they have their own specific design criteria for a 'muffler', which specifically excludes reflection type mufflers, the Loud Mouth would typically be acceptable. Only actual laboratory decibel measurements are a fair measurement of a muffler’s effectiveness to quiet exhaust and should be required and completed to judge an exhaust system. Most jurisdictions do not have the ability to measure noise levels accurately. The testing is very subjective if not properly and accurately completed.

Due to the nearly infinite number of local juridictions, 'related noise regulations/laws', and their high frequency of change, SLP does not participate and/or represent customers, companies, shops, technicians, or sellers in any litigation, traffic or legal matter regarding or related to the legality of an SLP exhaust system and disclaims any responsibility. It is solely the responsibility of the customer to investigate and verify whether or not SLP products are legal for use. SLP recommends checking local legislature for specific laws regarding the legality of aftermarket exhaust systems.

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