1987-2006 Jeep 4.0L Inline Six - Inside The Inline

If any engine can give a grizzled Jeeper the warm fuzzies, it's the '87-'06 Jeep 4.0L inline six. Designed by AMC and refined by Chrysler, its basic architecture was based on the '64-and-up AMC 199-, 232-, and 258ci OHV sixes. The fuel-injected 4.0L ruled supreme in Jeeps until forceably dethroned in 2006. We would like to say, "The King is dead, long live the King," but, thus far, nothing has emerged in diehard Jeepers' eyes worthy of wearing the crown. Before the late King's body cools too much, let's walk through tweaking this legendary engine from the inside out.

The old saying goes, "There's no replacement for displacement." Darn few single mods you can make to the 4.0L will give you more than a handful of ponies or pound-feet. One of them is to increase the displacement via an increase in stroke. Several companies sell high-quality stroked Four-O's as complete engines or kits, most notably Hesco, Golen Engine Service, and Custom Design Performance. Engine gurus have commented that, while stroking an engine is not to be taken lightly, it ain't brain surgery on the 4.0L. We wondered if the Average Joe, capable of an engine overhaul on his own, could do the job.To find out, we enlisted the aid of the University of Northwestern Ohio (UNO) and its High Performance Motorsports program to provide the facility and manpower for the job, as well as companies known for providing great products and services to enhance the Jeep 4.0L. In this article, we'll walk you through the job of stroking the engine; we'll cover other performance mods and finish up with dyno tests next month.

Like most modern engines, the 4.0L (bore and stroke: 3.88 x 3.44 inches) is oversquare, meaning the bore dimension is larger than the stroke. The 258ci (4.2L) Jeep six (3.75 x 3.89-inch) used before the 4.0L was undersquare, meaning the bore dimension is smaller than the stroke. Generally speaking, the oversquare engine is decent at low rpm, most efficient in the middle rpm range and has good high-rpm performance. The undersquare, long-stroke engine produces the best low-end torque, has a good midrange, but is so-so on the upper end.

Increasing displacement can also come via a larger bore, but a 0.060-inch maximum overbore on the 4.0L results in an increase of only 8 cubic inches. If you install the long-stroke 258 crank into the big-bore 4.0L block, it gains 32 cubic inches and becomes almost square, with the bore and stroke the same dimension (3.875 x 3.895 inches). The usual overbore, 0.030 inch, makes it slightly oversquare at 3.905 x 3.895 inches, and it gains 34 cubes. An 0.060-inch overbore gives a 41-inch increase.

There are two roads to building the 4.0L stroker. The budget method uses a 258 crank, 258 rods (5.875-inch long), and pistons with the stock 4.0L piston pin height (around 1.60 inches.). This is called the "short-rod" engine. The more expensive "long-rod" method uses the 258 crank, the longer 4.0L rods (6.125-inch) and a shorter piston pin height dimension (slightly variable but around 1.38 inches).

While the differences in a long- and short-rod motor can be huge in a high-revving', mega-power V-8 engine, the performance differences between the long- and short-rod Jeep engines is small. That's partly the inherent design limitations of an inline-six with a long stroke, and partly the way the engines will be used. The long-rod Jeep strokers offer a somewhat broader rpm range and, in theory, the short-rod setup is subject to more wear. Frankly, we doubt the wear differences will amount to much for most of you. The rod ratio (the ratio of rod length and stroke length) of the short-rod engine is identical to the stock 258, and they were known for a long life. Because we wanted to make this an "every man's" budget stroker, offering mainly a big boost in torque in the ranges where most 'wheelers work, we stuck with the short-rod option and off-the shelf parts.