One odd little fact, the 4.0L I6 is not a direct reworking of the 4.2L.

The 4.2L was shortened two cylinders and updated in the early 80's to make the AMC 2.5L (150 CID) I4 (AMC had been using the GM "Iron Duke" I4 before this.)

When AMC decided that the GM V6 wasn't good enough for the Cherokee (in the mid 80's), they decided to add two cylinders back to the I4 design, making the 4.0L I6.

So it's a decendant but not a child of, more like a grandchild of, the 4.2L I6.

-Howard

 

AND.....

For a little more history.

The 4.2L (258) engine has been the same since the early 70s right up until 1990 which was the last year. The only real differences was the smog add ons only increased with the years.

Around 87ish the Cherokee went to the 4.0l (242) from the buick V-6 which was a large improvement and the new YJ that year stayed with the 4.2l (258)

Come 1991 was the first year of the 4.0 HO(high output...you remember the stickers) which was an improvement from the regular 4.0 of the previous years. That engine was put in both the YJ and XJ from then on. Incidently 91 was the last year the YJ came in the old style roll bar in the back also.

Thats my story and Im sticking too it......

 

You can bolt on fuel injection from a newer (4.0L) motor and it will be almost as good as the 4.0. Bolt on a 4.0 head, and you've got a great motor.

Ron

 

Ahh.... I see. Thanks. That's pretty late for a carb'd engine to still be around. It must've been a solid platform in its day. I was thinking factory stroker or something. :roll: :roll:

It is a very solid platform, just a pain to keep running. The carb was the ultimate killer for the old 4.2.

You can swap the Crank and rods from the 4.2 to a 4.0 and make the 4.7 stroker (if you punch the cylinders 30 over).

 

actually you can turn your 4.0 litre into a 4.7 stroker...using some 4.2 litre parts and such...

Look up Hesco stroker

or talk to james at jeepers and creepers

 

The 4.2 was gone in 91-92...somewhere around there anyways.

91 was the last year for the 4.2L. The 4.2L motors are used to make the stroker 4.7, I was thinking about doing this a few years ago but went with just the basic overhaual. The 4.2L motors are the same as the 258 6cyl that has been put into jeeps since about 1972 (I think)

 

The 4.2 was gone in 91-92...somewhere around there anyways.

 

Ahh.... I see. Thanks. That's pretty late for a carb'd engine to still be around. It must've been a solid platform in its day. I was thinking factory stroker or something. :roll: :roll:

 

Because the 4.0 is an updated version of the 4.2. The 4.0 is thus a much better motor. Bigger doesn't mean better, kinda like the Ford 5.4 is better than the old 351 (5.8), the GMC 5.3 is better than the 5.7.

 

The 4.2L is a carb'd engine. Most the power is on the lower end of the rpm field. the 4.0 is FI'd and likes the higher rpms. The 4.0 is a superior engine.