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How to Install a Flowmaster American Thunder Catback Exhaust on Your 1999-2004 Mustang Cobra

Installation Time

2 hours

American Muscle
Shop Parts in this Guide



1. Raise the vehicle up on a hoist or rack to working height. If you do not have access to a hoist or rack raise the vehicle and support securely with jack stands.

2. Remove the two nuts at the muffler to catalytic converter connections.

3. Separate the steel hangers from the rubber mounts at the two support points in the middle of the assembly. Disconnect the flange from the cat assembly and lower the front assembly. Slide the rear hanger out of the hanger support at the tailpipe tip. Lower the assembly from under the vehicle and set aside.


1. Before installing anything, prep the clamps supplied in the hardware kit by removing the outstand applying a thick lubricant such as white grease or anti-seize to the threads.

2. Position muffler #952549-139 into the stock location on the right side of the vehicle. Slide the hanger at the rear of the assembly into the rubber mount on the vehicle and connect the flange to the head pipe assembly. Reinstall the two nuts back onto the studs. Tighten the nuts enough to hold in place, but still allow for adjustment.

3. Repeat the above step for the left side assembly#952549-138.

4. Place the right tailpipe #16259 into position and slide the hanger on the stainless tip into the rubber mount at the rear of the vehicle. Slide the front of the tailpipe over the muffler outlet and install the other two hangers on the assembly into their rubber mounts on the vehicle. Place a clamp on the slip-fit connection and tighten enough to hold in place, but still allow for adjustment.

5. Repeat the above stop for the left side tailpipe#16258.

6. Adjust the tailpipes for proper alignment and tighten all connections. Maintain a minimum of3/4" clearance between the Flowmaster system, the drivetrain and chassis components, while also keeping suspension travel and vibration in mind.

7. After the system is secure, slide the 7/16” hanger keepers onto the ends of the hangers located at the rear of the mufflers and tailpipes and slide them up to the rubber mounts. This will prevent the hangers from slipping out of the rubber mounts.

8. For a cleaner appearance and more secure installation, we highly recommend welding all slipfit connections. If you live in a geographical area that has harsh winters or sees a great deal of precipitation, the use of high temperature paint over the welded areas can help to prevent surface rust and premature corrosion.

Packing List

1Right Muffler952549-139
1Left Muffler952549-138
1Tailpipe, Right16259
1Tailpipe, Left16258
1Parts KitPK342
22 1/2" Horseshoe Clamp
4 7/16” Hanger Keepers
1Instruction Sheet

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