2001 GT X Pipe

It is rumored that Ford engineers spent considerable time and money developing just the right exhaust note for the 99-04 4.6L 2V Mustang. We give them props, it is a pretty respectable rumble in stock form, but as with most Mustangs, the stock exhaust tone does leave something to be desired. The factory of course has to appease the masses, most of whom do not want a alarm-triggering bark off idle and "look-at-me" growl at wide

Hear X pipe sound (or download).
open throttle. Fortunately for those of us that do appreciate the raucousness of un-stifled V8 there is nothing like an off-road X pipe. We recently pulled our stock four-cat pipe off our 2001 Mustang GT and installed in its place a stealth looking black crossover pipe.

It's no news that there are dozens of companies and brands of exhaust components for Mustangs. Some are high end and to their credit use outstanding materials, while others are shoddy and have poor fitting products. We picked up this X-pipe from Pacifica Performance mainly because we were intrigued with it's black coating and resonable $139 price tag. Also appealing to us was the three piece design, making installation without a car lift much easier then wrestling with a full length unit.

While improving the sound of your pony is reason enough to install an off-road X pipe, the gain in power is also a nice side effect. Even though Ford uses a 2.5" diameter exhaust system on the '99 and up 4.6L Mustangs, the factory H-pipe utilizes two pre-catalytic converters in front of the two full converters. The result is a fairly restrictive system, especially once you start putting mods on the breathing end of the motor. Granted the stock exhaust manifolds are the true culprits in the anemic exhaust system, their incredibly tight confinement between the wide 4.6L and shock towers makes them a nightmare to swap out. We'll tackle that in the future, for now we're content with a simply cat-pipe swap out. We left the stock mufflers in place so as not to get the awful drone associated with "cat back" muffler systems. On the dyno the restriction free X-pipe showed a 6-8 horsepower gain from 3000 to 5000 rpm, with a 2-3 HP loss on the low end. Torque was up a max of 10 lb. ft. at 4400 rpm.

X- Pipe Installation

With the car secured on jack stands, the pre-cat O2 sensor harnesses are disconnected and the sensors are removed from the left and right pipes.
Next we disconnect and remove the post-cat O2 sensors. The harnesses will be modified to prevent the PCM from triggering a malfunction light due to the lack of catalytic converters.

Using several extension bars and swivel adapter to snake our socket in the confined space we remove the two flange nuts securing the H-pipe at each header.
Next we remove the two nuts per side which hold the muffler pipes to the rear of the H-pipe.

Finally the stock cat pipe is free to drop down and be pulled clear of the car.
The passenger side header flange on 99-04 Mustangs uses a gasket rather than a ball/socket connection. We taped the supplied gasket into place to keep it in place while installing.

The Pacifica Performance X pipe comes in three segments. Here we install the driver side down tube first.

Next goes in the passenger side down tube. Leave the connections loose until everything is lined up.

With both down pipes aligned the x section is ready to be bolted into place. The front O2 sensors are reinstalled at this time.
Prior to torquing all the nuts down we made sure the mufflers and tail pipes were straight and line up properly.

MIL Eliminators
We won't get into the legal disclaimers here regarding off-road pipes, you know the laws in your state so do as you see fit. We will mention however that removing the catalytic converters will trigger a "check engine" light on 1996 and up Mustangs. This is because the EEC-V engine management system utilizes oxygen sensors both before and after the catalytic converters. The computer expects to see voltage from the aft O2 sensors corresponding to a lean mixture as a result of exhaust gas cleaning by the converters. Removing the converters sends the wrong signal, triggering a check engine light. Fortunately there is a way to eliminate the problem. Several companies sell "MIL Eliminators" (Malfunction Indicator Light). The MIL eliminator is simply a resistor and capacitor circuit placed in the O2 sensor harness that sends the computer the lean signal (300mV) it expects to see. Below we've shown you how to install the MIL Eliminators, or build your own, to ensure check engine light free driving.

The MIL Eliminators contain a circuit which will simulate the expected lean condition at the post cat O2 sensors. These are available from a number of sources, or make your own with Radio Shack parts.
Cut the black and gray leads from the O2 sensor. Splice in the MIL eliminator such that the green lead connects to the cut black lead from the 02 connector. Match up the remaining gray and black ends.


In This Article:
We replace the stock, restrictive h-pipe on our 2001 Mustang with a free-flowing X pipe.

  X Pipe
The X-pipe from Pacifica Performance is an intimidating component with it's black "HTS" corrosion prevention coating. With over 3000 miles on our pipe since installation the coating has shown no signs of flaking or burning off. Pacifica also makes cat back systems with similar coating.

Stock Cat Pipe
The factory H-pipe found on '99-'04 Mustangs is an improvement over previous years, being a true 2.5" diameter pipe. However it is still restrictive to power and sound with its four catalytic converters and sharp bends.


Pacifica Mustang & Cobra
25060 Hancock Ave 103-158
Murrieta CA 92562


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