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Jeep 4.0 Engine Upgrades - Powerplay

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Robin Stover | Writer
Posted February 1, 2009

A Jeep 4.0L Wrenching Session With Noticeable Results

Upon adding 33-inch-tall tires, a flexy long-arm suspension system, and enough camping essentials to forget home while on the trail, our donor's '05 Jeep Rubicon Unlimited felt a bit on the sluggish side. The 4.0L powerplant struggled with even slight hills at highway speed. We knew his stock axles were in dire need of a regear, but like many readers in this scenario, we thought we could skate around the costly ring-and-pinion swap by simply bolting on a few known power adders. At the same time we figured it would be a great opportunity to quantify some real-world results via a chassis dyno.

First, we added a JBA Cat4Ward header to promote exhaust flow. Next, we bolted on an Airaid throttle-body spacer to achieve optimal fuel-to-air mixing. Finally, we added a cold-air intake setup, also by Airaid, to help aid the engine in restriction-free breathing. Follow along as we highlight the process of powering up the trusty Jeep 4.0L.


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5. This photo shows the factory exhaust manifold next to the new JBA header. It's easy to see how the JBA header is able to improve performance over the factory manifold; the gases from each cylinder combine together at the same point, allowing the increased size at the collector to promote more even flow. The factory part dumps gases from one cylinder into another prior to the collector; this causes spikes in exhaust pressure and temperature. Notice the area of the factory manifold that didn't seem to rust. This is because that particular area runs consistently hotter than the rest of the manifold; this uneven heat distribution causes warping, cracking, and eventually failure.

6. Check out the 1/2-inch-thick laser-cut head flange. Not only does this protect the head interface from leaks, but it also ensures that the individual pipes have plenty of weld surface to help the unit stay intact for years to come. During construction, after each pipe is welded in place, a technician goes through and ports each hole with a grinder to make sure each opening meets a minimum flow dimension. Markings show exactly where weld material was removed from this particular port during construction. This critical attention to detail is one of the many reasons we like JBA headers.


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