Most manufacturers install the cheapest exhaust systems that they can on their vehicles in an effort to reduce costs. This usually means that factory exhaust components aren't even aluminized, so they turn bright orange with rust very quickly. Most cheaper aftermarket exhaust components are made of aluminized steel, or steel that has a thin layer of aluminum on the outside to resist rust. This is better than the factory setup, but not by much. The more expensive aftermarket components are made of solid stainless steel, which is far more resistant to rust than aluminized steel. Exhaust work is not pleasant to do, so the longer you can make your exhaust system last without needing work, the happier your life will be.

The first exhaust component on my '95 YJ to go south was the catalytic converter. Regardless of how long your primary vehicle warranty lasts, the EPA mandates that catalytic converters be warrantied for at least 5 years or 50K miles (or more -- I've seen conflicting sources). My YJ was 7 years & 49K miles when mine started dying. A dying cat is pretty easy to diagnose. First there's the screeching, followed by the tufts of bloody fur floating through the air... no wait, wrong kind of cat. The most common ailment for a cat is the honeycomb inside the cat breaking loose from the outer shell and starting to rattling around. Once that happens, it'll start to plug up, your power & gas mileage will decrease, and it'll get really noisy. Mine started with a noticeable rattle whenever the exhaust would shake severely, such as during engine startup and shutdown. Over a couple months, it got to the point where it rattled badly any time the engine was running. It sounded a lot like somebody shaking a can of marbles. My exhaust wasn't touching anything under the vehicle, so the noise when I shook the cat by hand was obviously coming from inside. Time for a replacement.

I could have saved some pennies and bought a factory replacement cat for under $100, but see the last sentence in the first paragraph. I instead opted for a new high-flow cat from Random Technology. I got it from Quadratec for just over $200 including shipping & a pair of stainless exhaust clamps. This cat has numerous advantages over the stock one. Most notably, it's stainless steel, so it'll take much longer for it to get devoured by rust (and shiny exhaust systems look better). It also allows more air flow than the factory cat. Since the cat is usually the most restrictive component on most exhausts, increasing the flow here will likely improve your top-end power. The Random cat is also much smaller and lighter than the factory cat (5 vs 15 lbs), which is convenient for some people, but didn't matter for my application.

Now, before you run down to your local 4x4 shop to have them replace your factory cat with a Random cat like mine, you should know that it's illegal (thanks to the EPA again) to remove, modify, or replace a properly-functioning cat. If a shop is caught doing this, they can be fined something on the order of $25K, so most won't take the chance. If your cat is still good, you'll have to replace it yourself. Of course, if some anonymous vandal were to bludgeon your cat with a large hammer or stab it repeatedly with a large screwdriver (what a shame!), then the shop would have no problem replacing it for you. I don't condone such vandals, of course.

Here's how to replace it yourself:

If you don't have 6" of lift like I do, you'll probably want to put the Jeep on ramps. The extra clearance will be most helpful. First, support the transmission with a jack, probably just in front of the skid plate. Remove the two nuts on the tranny mount right below the center of the tranny and the one on the torque arm bolt off to the side a ways. For the torque arm, you'll need a 9/16" wrench on the top of that bolt (above the skid plate) in order to remove the nut from the bottom. If yours are like mine, the threaded bolts protruding below the nuts have been scraped by rocks, so removing the nuts may prove difficult.

If your Jeep is more than a few years old, you'll want to start a day or two ahead of time by spraying the skid plate bolts (both to the frame & to the tranny) with PB Blaster once a day to let it soak through the corrosion. If you can see the exhaust joints and clamps, spray them, too. (PB Blaster is like Liquid Wrench or WD-40 on steroids. NAPA and Advance Auto both carry it where I live. It's in a can with a yellow cap & a very "busy" design.) I've seen people snap the heads off skid plate bolts because they were so corroded, so don't put too much muscle into removing them. I certainly wouldn't use an impact wrench here. If they give too much resistance, get out a propane (or mapp gas + oxygen, in my case) torch and heat up the surrounding metal to break up the corrosion and to expand the "nut" area that's holding the bolt. BE MINDFUL OF WHERE YOUR FUEL LINES ARE! They're only a few inches from the skid plate on my YJ. Gasoline and intense heat can be an exciting combination. Then squirt with PB again (it'll probably boil off quickly) and crank on the wrench some more. After 7 years with regular washing, only one of my skid plate bolts required a torch. Of course it was the side by the fuel lines.

Next, remove the clamps that connect the cat to the front exhaust pipe and the muffler. Those bolts only came off with liberal PB Blaster, the torch (again), and my 600 ft-lb impact wrench.

Now comes the hard part. When those exhaust clamps get tightened down, they compress the metal so that the two components are no longer joined by a simple slip fit. In order to separate them without destroying them (which was required for me, since only the cat was getting replaced), you need to expand the outer pipe enough that the inner pipe can slide through the compressed area. The factory service manual recommends doing this by heating the joint area up cherry red with an oxyacetylene torch and then, while it's still cherry red, beating the two components apart with a large hammer and a block of wood. Oh joy.

Disconnecting the front exhaust pipe from the cat was easiest if I first disconnected it from the exhaust manifold (don't forget about the O2 sensor). Using PB plus the torch and beating with a 3-lb dead blow hammer on the exhaust hanger by the tranny mount, I was able to remove the pipe from the cat. This required several heat/hammer cycles. This will go better if you can heat only the outer section of pipe and not the inner one. The inner pipe extends about 2" inside the outer pipe, so you've got a ways to go.

Unfortunately, the back side of the cat didn't have a good place to beat on with a hammer, so separating it from the muffler would have been virtually impossible while still installed in the vehicle. I opted to remove the rest of the exhaust assembly from the Jeep so I could heat & beat on it out in the open. At this point, since you'll already have the entire exhaust system removed anyway, you start to wonder if maybe it's worth the extra $300 to replace your perfectly good muffler & tailpipe with a new Borla stainless steel muffler & tailpipe so you don't have to bother separating the old muffler from the cat. I didn't go this route, mostly because I'm stubborn and stupid. Hindsight...

So, to remove the old exhaust: Again, use plenty of PB on the bolt by the rear-most exhaust hanger. Mine snapped off, so I drilled it out & replaced it with a stainless steel bolt & nut. The exhaust hanger above the rear of the muffler is a bugger to remove. There's a 3/8" sheet metal screw that connects to a heat shield under the body tub, and there are two 7/16" bolts/nuts that connect the hanger bracket to the tub. Of course these are above the muffler heat shield, so they're not easy to get to. I had already removed my rear driveshaft for another repair that I was doing at the same time, and I found the added clearance to be most beneficial. You think getting those bracket bolts out is hard? Wait till you put them back in... With everything disconnected, I was able to snake the exhaust out of the Jeep. I've got a 6" spring-over-axle lift with no rear track bar, so this may be harder to do on your Jeep than it was on mine.

To separate the old cat from the muffler, I heated it till it was glowing (as instructed), then stuck a 2x4 & a 36" pry bar between them and beat on the end of the pry bar. This took what felt like a couple hours, and I used an entire oxygen bottle (the little 12" tall one) doing this, but eventually they came free. I stopped for a cool, refreshing beverage and wondered again why I didn't just scrap it all & order the Borla setup. It couldn't help but flow better than the small, crimped pipe that the factory gave me.

While you've got the skid plate off, check the condition of your tranny mount and torque arm bushings. If you're hard on your drivetrain, the factory rubber bushings can get pretty torn up. My tranny mount was starting to rip and my torque arm bushings were torn up pretty bad, so I ordered new Daystar polyurethane bushings (P/N KJ01001 and KG01008) from 4WheelersSupply.com. If your tranny bushings are bad, better check your motor mounts, too. Mine seemed OK.

Remember how badly corroded all the bolts were that you removed? Before you reassemble anything, clean all the bolts with a wire brush and then chase all the threads with a tap & die set. This is also a great chance to clean all the crud off the top of your skid plate or the bottom of your body tub. I wire brushed as much loose rust as I could from the skid plate and frame. When you put the bolts back in, use plenty of antiseize on them so they're not as hard to remove next time. I also coated the inside of the exhaust pipe joints with antiseize. Time will tell whether it'll do any good, or whether it'll just get baked into oblivion, but it probably made reassembly easier. It smells horrible when the exhaust first heats up, so perhaps I should have used the high-temp copper antiseize instead of the normal silver stuff.

I attached the new cat to the muffler out on the garage floor (I had to heat the muffler joint with a torch again to insert the cat's output pipe) and then snaked the whole assembly back under the Jeep for installation. The factory service manual recommends 40 ft-lbs for the exhaust clamp nuts, but my new clamps started bending at 25 ft-lbs, so I stopped there. Those joints are plenty tight anyway. Reattaching everything I'd disconnected was straightforward and completed the job.

After my first highway trip, I could notice a slight increase in power thanks to the new cat. It was enough that I could actually use overdrive on level or downhill sections for the first time in years. The added track width provided by my new wheels greatly helped the stability at these speeds.

Someday I'll no doubt replace my muffler & tailpipe with a Borla unit, and when that day comes, I'll document it here.

This is only one of many things you can do to add more horsepower to a little 2.5L engine.

Photos might be posted someday. Email me if you want to see something specific & I'll send you one. Here are links to pages containing most of the photos (among other things):

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