2.5L I4 TBI (fuel injected) - 121 hp @ 5000 rpm, 135 ftlb @ 3500 rpm - used in '87-'90
2.5L I4 MPI (multi-port injection) - 130 hp @ 5250 rpm, 139 ft lb @ 3250 rpm - used in '91-'95
4.2L I6 Carburated AMC 258 - 112 hp @ 3200 rpm, 210 ft lb @ 2000 - used in '87-'90
4.0L I6 MPI AMC 242 "Power Tech HO" (High Output) - 190 hp @ 4750 rpm, 220 ft lb @ 4,000 rpm - used in '91-'95
Please not 2.5 139 ft lb at 2000 4.2 210 ft lb @ 2000
Your funny this is the biggest load of crap I've seen in a good while on here !
A 4.2l being more powerful than a 4.0l
Let's look at the spec's of the 2 engines.
4.0L 190 hp and 225 lb·ft
4.2L 112HP and 210 lb.ft
I'm not the best person at math but I see a 78hp & 15lb.ft difference #'s dont lie
I hate to stray from the OP topic, but I have to chime in about this 4.0 vs. 4.2 thing. Look at the RPM's to reach that torque on the 4.0: 4,000 vs the 4.2's 2,000. This is why people do the 4.0 head swap and keep the 4.2 "lower" block. That way, they get the best of both worlds (the HP of the 4.0 and the torque of the 4.2).
The 4.2's engine is less complicated, hailing from its father, the CJ7. Really all they did was add emissions crap to satisfy the EPA when they made the YJ, which robbed it of a lot of its power. It wasnt until the 4.0 came along that they had refined this process and got better specs, but at much higher RPMs. For those that have altered their rigs, they have seen a much better increase in HP and torque as well. But, comparing a 2.5 to a 4.0/4.2 is apples and oranges.
Rounding back to what the OP asked about, since he has to replace the 2.5 engine in a 1989 model jeep, he could easily:
a) put another 2.5 in. This is not the popular answer, as most of us would agree that putting in a 4.0/4.2 is a better solution. Especially if he was going to dd this jeep, as the 6cyl is the best of both on and off road worlds.
b) choose to put in the 4.0 OR 4.2. This would allow him to get the best of both worlds, but the "which engine is better" debate isnt going to help. The 4.0, while a great engine, has more computer control than the 4.2 does.
For someone who doesnt want to get into a huge project (as the OP has stated), putting a replacement 2.5 is the way to go. It will be a direct swap and he doesnt have to worry about a new wiring harness, computer, bellhousing, tranny, etc. He can just put the 2.5 in and go.