Exhaust Performance: Doug Thorley Headers And Heatshield Products


In the quest to get more power from our 2001 Jeep Cherokee, Project XtremeJ, we wanted to squeeze everything we could out of the exhaust. To do this, we would be installing new headers from Doug Thorley Headers to the 4.0-liter inline-six powerplant.

Jeep Cherokees have a problem with heat soak and with the factory manifold breaking. Our Cherokee has not broke its manifold yet, but it does suffer from heat soak during the warm Southern California summers. We look at solving both of these issues with installing new headers and heat wrap.

The steel shortie headers are made from T304 stainless steel and have a silver ceramic coating. To keep the heat at bay, we installed Heatshield Products header armor and fuel rail shield kit (PN 177009).



Getting Dirty

We pulled our Jeep into the shop, popped the hood, and got started tearing into the engine. The airbox and power steering pump would need to be removed to clear room to remove the intake manifold. Any connections and hoses that connected to the fuel rail, injectors, and intake had to be disconnected.

Everything was disconnected and removed.

Installation Tips And Tricks

  • Take your time
  • Make sure hoses are disconnected
  • Remove all electrical connecters
  • Disconnect the O2 sensor
  • Dowel pins hold header on after bolts are removed
  • Remove all old gasket material
Removing everything was not a quick process. We took our time to make sure nothing was stripped or broken, and were careful to make sure everything was disconnected before we pulled parts.

The first part to come off the Jeep was the intake manifold and the connected fuel rail. Before we could pull the exhaust manifold off, we had to unscrew the O2 sensor. Once it was off, the manifold came off easily along with the gasket.

To get the motor ready for new parts, we scraped the area with a razor blade and used brake cleaner to make sure the surface was clean. To ensure the new gasket would seal properly, we sprayed it with a copper coat gasket compound.

We wrapped the armor on the header tubes and installed the new gasket.

We measured and cut the Heatshield Products armor to fit the new header tubes while they were off the Jeep. Before the manifold went back on, we took the time to pull the injectors and install the fuel rail kit on each injector and the fuel rail.

While the intake manifold was off the Jeep, we installed the fuel rail kit.

Everything was going back together smoothly when we ran into a problem. As the intake manifold was put back on the Jeep, we noticed a clearancing issue. The thickness of the armor was too large, and needed to be removed on most of the tubes. Once that problem was fixed, the rest of the parts went back together and we buttoned up the Jeep.


Reassembling everything so that we could test out the products.

The Technical Side

We reached out to Steve Heye of Heatshield Products, and Troy Mastell at Doug Thorley Headers. We wanted to learn more and find out exactly how these products would improve our XJ.


Steve Heye told us, “Header Armor features our BioCool technology, which contains a bio-soluble silica. This means it doesn’t smoke up when exposed to heat, and is 100 percent safe to work with. We always recommend using gloves, safety goggles, and a mask, but users will see little to no irritation when it comes in contact with your skin.”

He continued, “This is the next evolution for us. Perhaps its greatest quality is its ruggedness. The armor layer won’t snag like a wrap, so when you are out wheeling, you don’t have to worry about brush or off-road hazards tearing your exhaust insulation apart.”


The Heatshield Products header armor that we would be putting on our Jeep.

Heatshield Products saw a problem with the Cherokee and looked to solve it. “We wanted to further expand on our Header Armor line. We have had numerous inquiries through the years regarding the 4.0-liter and its heat soak issues in the engine compartment. We launched a Header Armor kit for V6 Jeep applications, so it seemed natural to do the inline 4.0-liter at the same time.”

“This isn’t just a Jeep problem. The problem with most factory metal heat shields is that they are metal and are mounted and bolted to hot metal. Most metals conduct heat very efficiently.”

By covering one side of the manifold or header, we allow it to breathe. – Steve Heye, Heatshield Products

“Even though it’s called a heat shield, eventually, the heat soaks the metal so much that it just radiates the heat like an exhaust pipe or manifold would. The Header Armor insulates directly on the manifold or pipe preventing heat from escaping.”

“In addition, by covering one side of the manifold or header, we allow it to breathe. It doesn’t get metal-to-metal conductive heat like a factory shield. If you trim back the edges of the insulation and fold over the aluminum, you give it a nice finished look and something that won’t unravel if you ever get your engine compartment steam cleaned,” Heye continued.


Injector wrap will help with heat soak.

We understood why it was important to add armor to the headers, but we wanted to know why were we adding it to the fuel rails as well. Hey explained, “Jeep owners dealing with heat soak problems know what a pain it can be. It’s better to go a little overboard on preventing the cavitation from occurring. It’s not exactly fun when you’re way out on the trail and your engine doesn’t run properly.”

Steve continued, “It’s bad enough on the street, but a real pain in the middle of nowhere. The Header Armor can take 700 degrees F exhaust manifold-heat down to the 150 degrees F range, so we finish protecting the fuel from the 150 degrees F heat with the injector insulation just to be extra sure.”


Now that we understand why it is imperative to add the armor, it was time to find out more about what we are adding the armor to; our header. To take a deeper look at the header that was going on the XJ, we talked to Troy Mastell at Doug Thorley Headers.

Mastell let us know that this product did more than just add performance numbers, “The problem with the factory manifold is they break. It’s an inevitable issue. There are added horsepower gains when installing them, but more importantly these don’t break, and come with a lifetime warranty.”


The header is ceramic coated inside and out.

“Our headers are made out of T304 stainless steel, which is a stronger material over T409. They come with a ceramic coating inside and out. The kit is 100 percent bolt-on and comes complete with everything, including all gaskets.”

Troy continued, “We do in-house testing and fitment on all of our headers. This product went through our same testing routine, which includes dyno runs.” Now that we have some insight on the products we’d installed it was time to turn the key and see what we felt in our seat.


Final Thoughts

One easy way to see how a product performs once it is installed is to talk to the vehicle owner. We talked to Kevin McIntosh and got his feedback after the install, “I saw an immediate performance increase from the install with better throttle response. The headers were a great addition to the exhaust we already had on the Jeep.”

Kevin did let us know that, “I have experienced heat soak during the warm Southern California summer,” and is looking forward to seeing how the modifications help during that time of the year. Stay tuned for an update as our local temperatures raise to 100 degrees or higher, to see how Project XtremeJ is handling the weather change.

dynorunShare-1 copy

We did have a chance to dyno the Jeep before and after the install. The number correlate to the feeling we are getting from behind the wheel.

With this dyno chart we can see that we made power and torque throughout the entire range. With torque gains peaking in the lower range of 36.1 lb-ft at approximately 2,750 RPM and horsepower gains also more prevalent in the lower range. They peaked at about 20.7 hp at 2,800 RPM, but maintained gains all the way through our max RPM range. 

Over all a great torque and horsepower increase especially in the low range where we want it for the rock crawling duty this rig will be seeing. This installation not only helped the performance of the Jeep, but also will help during the warm summer months.

Article Sources

About the author

Steven Olsewski

Steven Olsewski grew up with a true passion for anything with a motor. He loves his wife and kids, and during the year can be found enjoying quality time together. They are a huge part of his life and their passion for God.
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