I see my last post in this thread was back in November. At the end of January, on the way home from the last day of duck season, my head gasket gave up the ghost. As long as I had to pull the head, I decided to go ahead and do the 4.0 upgrade. Rather than pulling a head from a donor truck and having it redone, I bought a new one (i.e., remanufactured) from AutoZone, along with new pushrods (the YJ ones are a tad longer, so don't use the CJ rods) and pivots/bridges. I did go to the U-pull-it lot for the valve cover, a stainless exhaust manifold - the one with the accordion sections to prevent cracking - and an exhaust pipe. I also pulled some vacuum plumbing which simplified hooking all that stuff up to the new valve cover, which has different size holes so the old CJ PCV valve and such won't fit. All that cost about $48 and the new parts about $400. Amazingly, everything pretty much bolted up, except for the power steering pump mount, which required a little modification. Oh, and the CJ head bolts are slightly smaller in diameter than the YJ ones, so you have to stay with the CJ bolts. Mine were in good shape so I reused them. The new head weighs 75 pounds bare, which was too heavy for me to manhandle into place, so I braced the hood with a couple of 2x4s and used a block and tackle to lower it right on. My wife handled the rope and I guided it into place. I have posted more details and a photo elsewhere in this forum so search for that if you're interested.
But back to performance. I don't know about the 40 horses claimed by some, but my jeep now starts immediately, runs much stronger and accelerates more briskly than before. There is still plenty of torque down low, but the main increase in power seems to have moved much more into the mid-range. Long story short, I'm very glad that I did this. It was much easier than I expected, and the results are absolutely worth it.
I'd already added MPFI awhile back, and while that brought much better response and reliability, not to mention much easier starting, adding the 4.0 head made a noticeable difference. I was already using a '91 intake manifold, and replacing the CJ exhaust with the YJ one made everything fit properly. (Use 4.0 gaskets for everything.) The YJ stock airbox won't fit, so I used the curved intake tube from a Cherokee with a K&N-style filter at the open end. This not only works well, but looks like it was intended for the application.
"We are all of us in the gutter...
Some of us are looking at the stars."
~ Oscar Wilde via Chrissie Hynde
Last edited by Colt44; 04-04-2011 at 12:46 AM.
Reason: more detail