1991
This model year reveals a better-flowing Power-Tech Six High Output version of the 4.0L. Changes are made to the cylinder-head design, the camshaft profile, and the block castings. The YJ makes 180hp and 220lb-ft of torque while the Cherokee's mill has 190hp and 232lb-ft of torque (variation in numbers is due to differences in design of the exhaust and air cleaner).
1996
The 4.0L gets some upgrades, including lighter pistons, new cylinder-head casting (to improve the exhaust flow), a new two-piece thin-wall cast exhaust manifold (that replaced the one-piece, tubular style), and steel valve covers (again).
1997
The block is tweaked so that the oil-filter mounting can be relocated, meaning the Grand Cherokee no longer needs an adapter.
1999
This is the last model year for improvements to the 4.0L, which include new direct-mount accessory drive bosses, a thrust washer added to the camshaft, a new chain oiling system, and larger-diameter casting between lobes (to stiffen the camshaft in "bending" mode). Also, ribs are added to the rocker pedestals and holes are tapped for the new coil rail system in the cylinder head.
2006
The 4.0L ends production. The TJ is the last application.
Jeeps and 4.0Ls
* '87 Cherokees and Comanches were the first to get the 4.0L.
* '91 YJ was next.
* '93 is when the Grand Cherokee got it.
* '97 was the first for the TJ.
Inline Six-Cylinder Facts
*AMC Advanced Engineering out of Detroit came up with the AMC/Jeep six-cylinder's design concept, but it was AMC Kenosha Engineering that handled final design and development.
*The Nash 196ci flathead engine was the inline-six in the AMC family prior to the 232ci. It continued to be in the Rambler American until 1965 because it was shorter and had A/C. The 199ci was too long to mount the A/C drive -- that is, until the 1966 model year, when the Rambler American gained 3.8 inches under the hood to accommodate the 199ci.
*The 232ci featured seven main bearings (solid as a rock, compared to the OHV 196's four) and hydraulic tappets, was sub-assembly balanced (crank, vibration damper, and flexplate/flywheel), and had shaft-mounted rocker arms.
*The 232ci engine was painted red, the 199ci in blue. In 1983, all engines became black (long live the French).
*Think the 4.0L was based off the 4.2L? Wrong. Its design actually stemmed from the 2.5L four-cylinder introduced in 1984, the first four-cylinder built and designed by AMC/Jeep (GM was responsible for the 2.8L V-6). Sure, it had the same valvetrain as the 258 (minus cylinders two and five), but it was then modified for performance (the design team took advantage of the new block, head, and crank).
*Speaking of making the switch from 2.8L V-6 to 4.0L six-cylinder, no one wanted to change the Cherokee's looks, so they had to make the new engine shorter. They eliminated the normal fan mounted to the water pump, which allowed the water pump to be shortened. A single electric fan was attached to the radiator along with a smaller mechanical fan for a total of two cooling fans.
*When the 4.0L Cherokee was being finalized, management had planned for the volume split to be 60 percent 2.5L and 40 percent 4.0L, but after dealers got behind the wheel of the peppy Cherokee it was switched to 20 percent and 80 percent, respectively.
Some of the Aftermarket Mods
Flip through the pages of Jp and you'll find plenty of companies offering upgrades for the inline-six. Just because they are not mentioned here doesn't mean they aren't worth checking out. We've simply included some of the most-talked-about upgrades over the years.
* 258ci cranks from the 1972 to 1980 model years are often considered the best to use in 4.0L stroker motors because they are stronger and smoother.
* The 4.2L launched with a reputation of being torquey and for also having a weak cylinder head that didn't flow with an unlikable carburetor. Extensive porting has been a solution for some, but a more common modification is to convert the 4.0L cylinder head onto the 4.2L block. Hesco has used the 4.0L cylinder head and Mopar fuel injection (which it invented) combo to get 200 horses. Another way to get more power is to run a Hesco aluminum head, which can help the engine crank out up to 300hp. Additionally, Clifford's Performance makes a carbureted intake manifold. A popular fix for the leaky, stock carb has been to switch to a two-barrel Holley or Weber.
* Chad Golen of Golen Engine Service says the 4.0L's computer is pretty flexible. You can add a larger camshaft and modify the cylinder heads without having to do much to the base computer. Also, Golen makes a 4.6L stroker long-block from the 4.0L (covered in Jp's August '05 story "The Insane Inline, Part 1").