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Here's some light reading:)

When the 4.0 first came out, AMC was using two six cylinder engines, a troublesome 2.8 liter V6 from General Motors (with a problematic Ford carburetor) and AMC's own 258 CID straight-six, with 4.2 liters of displacement, which had been launched in 1964. The 4.2 was used in the CJ7, Concord, Spirit, and Eagle after having debuted as the sole Eagle engine and an optional Wrangler motor. A related engine, the 2.5 liter four-cylinder, was based on the same basic architecture, and appeared in 1983. The four-cylinder produced 125 horsepower in its final years, and replaced a 2.5 liter GM engine; to complicate matters, Chrysler made a 2.5 liter four as well, producing a meager 100 hp without a turbocharger. The 2.5 was engineered by AMC to be “part of a two-engine set,” according to Willem Weertman; the other engine would be the 4.0 liter six.

When the 4.0 burst onto the scene, it had a full 180 horsepower in the Wrangler (a German publication listed it as 172 horsepower, and, in the Cherokee, it started out with 177 hp).

In 1990, when the 4.0 still had 177 hp, the GM 4.3L V6 only made 160 hp, the Ford 4.9L I6 (used in the F150 and Econoline) only made 145 hp, the Ford 4.0 V6 made 155 hp, the Chrysler 5.2L made 170 hp, the Jeep/AMC 2bbl 360 V8 made 144 hp, and the Nissan 3.0 V6 (used in the Pathfinder) made 153 hp.

Jeep upped the ante in 1991, when the Cherokee's version made 190 hp.

The YJ didn't get the 4.0 until the 1990s (it was still using the old 258 six). When the YJ did get the 4.0, it only made 181 hp... possibly due to a more restrictive exhaust.

Peter wrote:

In 1987 I wondered why they didn't drop the 4.0 I6 into the YJ, which was still using the 112-117 hp 258 (4.2 liter) six; or the Eagle, Concord and Grand Wagoneer, which came with the four-barrel 360 V8, making just 144 hp. The 4.0 I6 made more power than the Ford 302 V8, Chevy 305, Chrysler 318, and AMC 360, as well as any of the 6 cylinder engines the Japanese were putting in their trucks... and it had comparable or better fuel economy.

An AMC Concord with the 177HP 4.0 I6 might have been a better police pursuit vehicle than the Dodge Diplomats (140-150HP), Chevy Caprices (160-170HP) and Ford Crown Victorias (160-165HP). The Dodge Dakota, Ram, and vans would have been better with this engine rather than the 3.9L V6 or the pre-Magnum 318 V8. Compared with other contemporary engines, the 4.0 was strong up until the end.

Willem Weertman, in his definitive book, wrote that the 4.0 shared many features of the 2.5 liter overhead valve engine, including the crankshaft journal and crankpin diameters, basic architecture, bore, cylinder spacing, and valvetrain parts. Indeed, both had the cylinder heads machined on the same Kenosha lines. The 4.0, to save money on machine tools, used the same cylinder spacing as the 4.2 and 232-cid Typhoon engines, along with some other dimensions. It did have a longer stroke, achieved via a taller cylinder block deck height. Its late arrival meant that it skipped carburetors and single-point fuel injectors entirely, moving straight to mujltiple point injection.

Weertman also wrote that many changes were made between 1996 and 1998 to cut noise, vibration, and harshness from the 4.0 engines, including a new cam lobe profile to cut valve seating velocity, isolated valve cover fasteners, and adding a main bearing brace.

Starting in 1991, the Jeep 4.0 engine used Chrysler engine computers, allowing for easier diagnostics; before that, Crysler had to honor contracts with AMC's ignition system vendor, Renix. There are two yellow rubber covers on the right side of the engine compartment; dealers can use the DRBII and the Jeep adapter to get codes and do certain tests. (Codes might not be stored in the Renix system and would have to be regenerated while the adapter was connected, according to Rob Mayercik.)

AMC - Jeep 4.0 liter engine repairs and such
Bob Sheaves wrote:

The reasons the 4.0L went away [except in the Wrangler, so far] was due mainly to age of the manufacturing tooling, which was worn out. The long stroke made it harder to clean up the emissions of NOx, and NIH ("Not Invented Here"), in my opinion, also reared its head. To completely retool would have cost as much (almost) as the 3.7L v6 did, and the Dodges were going to use the 3.7 as a base engine. Logically, the engine group did what they were told to satisfy the dealers...."Make a modern engine, and junk the old ones." This statement is my opinion, based on conversations at the time.

Specifications (2003)
Overhead valves (two per cylinder), flat-face followers, hydraulic lifters, cast iron block and head
3960 cc (242 cubic inches)
98.4 mm bore x 86.7 mm stroke
8.8:1 Compression ratio
Redline: 5,300 rpm
Maximum power, 2003, Wrangler: Wrangler: 2003 figures USA Europe
Horsepower 190 177
Pound-feet 235 218
Kilowatts 142 130
Newton-meters 319 296
Power peak 4,600 4,600
Torque peak 3,200 3,500

Acceleration. In 2003, in the Wrangler, 0-60 in 9.9 seconds (manual; 10.6 seconds, four-speed automatic); top speed 108 mph - the Wrangler weighed over two tons, so those aren't bad times. Liters per 100 km (EU), 2003 Manual Automatic
Combined cycle 13.2 15.0
Ex-urban cycle 9.0 10.7
Urban cycle 20.5 23.0
Combined CO2 315 g/km 362 g/km
USA (EPA) miles per gallon Manual Automatic
City 15 14
Highway 18 18

Built in Kenosha, Wisconsin (5555 30th Avenue) - the plant also made the 2.7 and 3.5 liter engines at the same time. In 2003, it could make 405,673 4.0 engines per year (as well as 315,638 2.7s and 220,000 3.5s). The plant was built in 1917!