Originally Posted by colt44

I've always been interested in how certain things came to be what they are now and my XJ certainly qualifies. I went on internet and started reading about the 4.0 and found some really interesting fact. AMC/Nash designed this motor at the end of the '50's to replace the flathead 6's they were using. The first relative of the 4.0 was 199 cubic inch 6 that was released in 1964 with about 125 hp. They put it into Ramblers and eventually expanded it to 232 and 258. Chrysler began the buyout in 1987 after AMC's 4.0 was already being used.When Chrysler bought AMC, they didn't want to continue using the 2.8 GM engine with a Ford carb. ,Never a Ford carb on the 2.8. It was purchased complete from GM. so they took the 258 AMC had been using and modified the XJ chassis to accept it. Have you ever wonder about that concave area on fire-wall behind they engine? Chrysler put it there so the straight 6 would fit. AMC did that in 1987. They also had to modify the radiator support area so the straight six would fit. They did this in 1991. Actually 1987.Before that, AMC had used 4 bangers and GM V6's so they would fit in engine compartment. Chrysler also added fuel inj. Nope. Renault and Bendix did a joint venture and had Renix fuel injection as early as 1986. and redid the under-bore with larger pistons, and less stroke. I read that there is such a family resemblance between the old AMC and the 4.0 that things like cranks and heads are all interchangeable between years. You could probably use a head from a non-HO engine and put it on the original 199 from 1964 and it would work. Except for GM small blocks, how many engines have such a long and varied history?

Originally Posted by cruiser54

Backwards my friend. The 242 aka 4.0 was developed BEFORE the 2.46. Just wasn't ready as quickly.

Engineer buddy of mine at Jeep told me the story of the 2.46. They took a 4.0 engine, sawed out the middle 2 cylinders, had a crankshaft etc made, and then did a test mule run to see how well it would work.