Jeep 4.0 Engine Upgrades - Powerplay

A Jeep 4.0L Wrenching Session With Noticeable Results

By Robin Stover

Photography by Robin Stover

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.

1. To generate data that we... read full caption 1. To generate data that we could actually use, we first had to get some baseline dyno numbers. To do this we simply drove down to our local dyno facility and made three runs with the original equipment intact. No aftermarket modifications existed when we generated this data. Peak power was 126.4 hp @ 4,474 rpm and 176 lb-ft of torque @ 2,385 rpm-not bad, considering this Jeep had only 25,000 miles on it and an automatic transmission.	2. This is the JBA Cat4Ward... read full caption 2. This is the JBA Cat4Ward header as it came to us. This product is available in raw stainless as shown here, or if you want to pony up another 200 bucks, you can get it with a ceramic coating to reduce underhood temps. We decided to keep things simple to reduce costs.	3. The JBA header requires... read full caption 3. The JBA header requires that you remove the factory intake manifold to gain access to the exhaust manifold. Here you can see the JBA technician removing the intake manifold from the Jeep.
4. With the factory intake... read full caption 4. With the factory intake manifold removed, the OE cast-iron exhaust manifold could be seen. Notice the rusty color of this part. Rust is the number one enemy to a vehicle's exhaust system; that's why JBA constructs its headers out of nonrusting stainless steel material.

Jeep 40L Engine Rebuild Compare Manifold

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.