There seems to be some confusion on the engines used in Jeep vehicles over the last 68 years. A lot of it springs from the fact that different manufacturers produced engines of identical displacement over the years, such as the 327 Rambler V-8 and the 327 Chevy V-8, or the 304 AMC V-8 and the 304 IH V-8.
Back in the days when real Jeeps were manufactured by Willys Motors, they used a flathead four-cylinder that was actually designed in the '20s and then extensively updated by Willys Chief Engineer Barney Roots in the late '30s, so that by 1940 it was by all accounts the best four-cylinder engine in America. Displacing 134 cubic inches, the "Go Devil" produced about 60 hp. This motor remained basically the same from the World War II military years to the early 1950s. Then it was modified to accept a new cylinder head, with the intake valves in the head and the exhaust valves in the block that produced about 72 hp. It was now labeled the "Hurricane."
This engine was only available in the pickup/station wagon lineup until about 1951 when it went into the all-new body style military M38-A1, and in 1953 in the CJ-3B (commonly referred to as a "high hood" due to the raised cowl, hood, and grille to handle the additional height of the taller cylinder head). This engine was referred to as an F-head-not to be confused with a "flathead," where both intake and exhaust valves are in the block. Underpowered as it was, this motor became the standard engine in CJs and was also used in both the station wagon and truck line until the early '60s. In 1954, the pickup/station wagon line got a new flathead six-cylinder, called the "Super Hurricane," displacing 226 cubic inches, which put out a huge increase of power to 115 hp.
The CJ had an optional four-cylinder Perkins diesel available in 1961 that produced 63 hp. It never proved to be very popular and it was dropped from the lineup by 1969.
Henry Kaiser, after building Jeep vehicles for a few years, decided (rightfully so) that the trucks and wagons needed more power. A new 230ci inline six in 1963 came with, of all things, an overhead-cam cylinder head that upped the horsepower to 140. While it was an impressive sight to see with its large cast-aluminum valve cover, it just didn't do the job. Plagued with oil leaks past the valve cover and by the valve guides, it only stayed in production until 1965 and was used in the newly designed Wagoneer.
The next engines to be used, starting in 1965, were a 232ci overhead-valve six-cylinder engine borrowed from the Rambler, which made 145 hp and 16 more lb-ft of torque than the "cammer motor" which still remained in the lineup. Jeep also saw its first factory-installed V-8, a 327ci Rambler block (which had none of the characteristics of the famed Chevy 327) rated at 250 hp. The next year, there was a four-barrel option that raised the horsepower another 20. However, these engines again never saw use in the CJ series.
During the late '50s and '60s, engine swaps of all types started taking place. Chevy V-8s became the popular swap, as was (believe it or not!) the Studebaker V-8. While this engine wasn't as light or compact as the Chevy, its one main advantage was that-like Jeep at the time-it also used a T-90 transmission, therefore requiring no adapter.
Another popular engine swap was the Buick V-6. Jeep must have thought it was a pretty good deal because in 1965 Jeep bought the engine rights from GM and used the engine until 1971 when Kaiser Jeep was acquired by AMC. Let's not forget the 350 Buick V-8 (again, not to be confused with the Chevy V-8 of the same displacement), which was used from 1968 to 1971 in the Wagoneer and Gladiator trucks.
American Motors had its own line of engines. For 1972, the CJ finally got a 304 V-8 or the 258 I-6, which gained a great reputation as a low-rpm torque producer. (International must have thought so too, as they purchased quite a few of these engines to use in their light vehicle line of Scouts and Metro delivery vans. By the way, the 304 IH V-8 came from the IH medium-duty truck line and is not the same as the AMC 304). Wagoneers and Gladiator trucks used the same basic 304 V-8 in displacements of 360 and 401 cubic inches. Even after the Chrysler buyout and until its demise, the Grand Wagoneer still used the AMC V-8.
In the 1980's gas crunch, Jeep decided it needed an economical engine, and GM was again tapped as the source for the four-cylinder Pontiac "Iron Duke," displacing 151 cubic inches. (In reality, this engine was nothing more than the old Chevy II engine with a few revisions.) By 1983, the V-8 option was no longer available in the CJs and the 258 six stayed around until 1991.
Soon, Renault entered the picture with a large-percentage buy-in to AMC, and an entirely new motor in 1984 was destined to fit into the downscaled new Cherokee and Wrangler: It was an all-new four-cylinder, carbureted (and later updated with EFI), and displacing 2.5 liters. It has no common ancestry with the GM four-banger.
In 1985, Jeep Cherokees got a new engine, for the second time in history a diesel, this time with a turbo, displacing 2.1 liters and reported to get up to 36 mpg. It didn't gain a record of dependability, and few were sold, so by 1988 it was dropped.
To keep in the horsepower race, Jeep again went to GM for the 2.8L V-6, which in realistic terms wasn't all that much better performance-wise. With tightening emissions regulations, the add-on modifications had left the old Rambler six wheezing.
Probably the best engine to ever wear the Jeep marque is the 4.0L fuel-injected six that was developed for the Cherokee and finally put into the Wrangler. It has a common heritage with the old 4.2L (258c) AMC six. With Chrysler now at the helm of Jeep, they put the 318 V-8 in the Grand Cherokee as well as the 4.0 motor.
The latest offering from Chrysler in the Jeep lineup is the 2.4 Power Tech four-cylinder producing 150 hp, and the 3.8 Power Tech V-6 that has 210 questionable horsepower that arrived in 2002 for the Liberty and in 2003 for the Wrangler, and 4.7L with 265 hp for the Grand Cherokee. We got to keep the great 4.0 in the YJ until the JK came out in 2007 and went to the questionable 3.8L V-6.
OK, there you have it. Most likely, I made a few mistakes, but I tried my darnedest to be as accurate as possible, so I am open to any updates or corrections you may want to contribute.