Engine?

I'm going to have to do something about an engine for the CJ at some point due to the old one is burning oil and just generally worn out. I have another 304 I can rebuild. I'm also considering swapping a 4.0 in. If I stay with the 304 I'm going to run GM TBI. I don't want to go through all the trouble to swap in the 4.0 if its not going to be enough motor. I'll be running a T176, d300, 4.56 gears, and 33 in tires. I may eventually go to a 35. Do you guys feel like your 4.0 has enough power? Its not normally a daily driver but we do drive it back and forth to trails so I'd like it have enough power to climb hills on the interstate/hwy. It appears a 4.0 may be cheaper to do. I keep finding 30-80K mi engines for $500-750. I'm going to have around 2K in the 304 and FI. I'm figuring the 4.0 will get better fuel mileage as well. What do you guys think?

John

 

Which engine questions can be difficult to answer when both engines are pretty decent choices. I mean...if one of them was no good, then the choice would be made more clear. The 4.0 has good power and a nice flat torque curve and is a great engine. The 304 has proven itself to be a sound little engine as well. Neither of those answers help you a bit...so I am going to have to go with a "if it were me" scenario.
If I were going to choose, I would go with the 304 because it's already in my Jeep and will require less fiddling to reinstall. But you mention going to fuel injection if you keep the 304, so less fiddling is out. Back to the "if it were me" thing. I would just stay with the 2100. When set-up right, a jeep will about flop over before it sputters with a 2100.
Mileage? I suppose that the 4.0 will be better here, but your driving a Jeep with 35 inch rubber and 4.56 gears, so it may be negligible.
Sorry I am not helping out a lot here, the problem is, you're not making a mistake if you go either direction.
Now...if you REALLY want to know what I would do, it does not involve either of your engine choices. I would go with a 360, an hei ign, a small cam and go from there. A 360 is pretty much a bolt in deal. A 401 in close to bolt in, if it's internally balanced then it is a bolt in.
But back on track.....I personally feel that you will not be unhappy with either engine. Sounds as if you're leaning toward the 4.0 if it has enough power (it does).

 

Personally, I would not bother with the 4.0L unless I was interested in getting the complete Mopar MPI (multiport fuel injection) along with it. Without the MPI, it's just another inline 6, and I doubt you'd do any better on fuel than with a 304.

If I was staying with a carburetor, then I'd certainly keep the 304. Even if I wanted fuel injection, I'd likely still go with the 304 and GM TBI with spark control and a knock sensor (planned for my 304).

If you found a MPI 4.0L parts car for $500, then you might be money ahead if you did not have to work on the engine at all. You'l still need to change the frame towers to mount the inline 6, and likely change radiators. If the Jeep already had a 258, then the 4.0L would be a no brainer. The conversion cost will be higher than you expect - it always turns out that way. Certainly you will need the crank mounted CPS conversion (HESCO), or else use the 4.0L CPS and have your bellhousing modified to accept it. You'll need a flywheel, or else a donor car with a manual transmission (less common certainly). All these costs add up.

You can control the costs of a rebuild so that it will be less than $2000. You won't really know what's needed until you take the block apart and measure the bores. The major expense is new pistons - if you can get by with the old pistons, your rebuild will be down in the vicinity of the money you'd spend for the 4.0L. But you have to measure - measure, measure, measure. You won't know which of the two blocks is better for your use until you measure them. I'd go ahead and tear down the extra 304 and measure it. Then you can decide whether you want to use that block or not.

<edit> Also, compare the numbers between the 304 and 242. The 4.0L make a little more power at a higher RPM, but the 304 makes more torque and at a much lower RPM. I'm sure that if you flattened out the torque curve of the 304 that it would make much more power than the 242. Just FYI.

 

Or....I have seen quite a number of 304s for reasonable on Craigslist and such. Maybe go that route so your Jeep is not down for very long.

 

I'd put my old 304 up against a 4.0 anyday. As for a 360 or 401, they both are direct, no hassel bolt-in engines. All AMC V8s are. The only difference between the V8s is the flywheel. Make sure you have the correct flywheel for the displacement used, and bolt it in. Everything externally is the same. I would go with a 360 or 401. I have already switched out my 304 for a 401 and could not be happier. You can find good running 360s in old rusted out waggoneers and such for a dime a dozen. grab a 360 flywheel, bolt on your old 304 ex. manifolds and engine mounts and drop it in. You will be money well ahead and have much more power to boot.

 

The 401 is a great engine - forged steel crank, forged steel rods, special block alloy... but the word is out and it may be tough to find an inexpensive one. They also have thin cylinder walls, and can't be bored out much; 40 is the safe limit, though some blocks can go somewhat more if core shift is minimal. Core shift is measured by sonic testing.

The 304, 360 and 401 all share the same external block dimensions, but as noted, they are balanced differently. Thus they must use their own balancers and flywheels/flexplates. The 360 uses the same crank as the 304, but it has a larger bore and different heads (bigger valves). The 360 was used in "Grand" Wagoneers right up to the end of the run in 1991, so there are still quite a few of them out there, and lots of them have found their way into other Jeeps.

Neither the 360 or 401 will give you any better economy though!

 

I guess I should have mentioned my CJ had a 258 originally. I put the 304 in it myself. I have the 6 cylinder frame engine mounts. I'm kinda leaning toward rebuilding the other 304 if its decent. I'm going fuel injection either way. I'm tired of dealing with carbs. I think it would be kinda neat to keep the 304.

John

 

If you're happy with the power that your 304 makes (many are) then there is no reason not to keep it. Just be sure to keep in mind some of the suggestions given about the 360. You may have really close to the same amount in a 360 as you would a 304 and the power difference between a 360 and 304 is HUGE. As far as mileage goes. I have a CJ5 with a 360 and a CJ7 with it's stock 304. The five weighs a bit less...but they get the same MPG if I don't have my foot buried in it.