4.0 Head or V8 conversion?

[ChiefBigB]

What 4.0 Head do I need to upgrade my 258. I want to improve mpg and power. I am open to a V8 upgrade as well. Any insight will be greatly appreciated. I would like to hear both sides pros and cons. Thank you!

[Dmntxn77]

I think anytime someone looks for a mpg improvement on a FSJ, they are just setting themselves up for disappointment. As oppose to engine upgrades, your best first step might be gears though.

[PlasticBoob]

I think anytime someone looks for a mpg improvement on a FSJ, they are just setting themselves up for disappointment. As oppose to engine upgrades, your best first step might be gears though.

I definitely agree...go with gears first.

Are you set on fuel-injecting a 258 with a 4.0 head, system, and custom tune? I assume that's why you posted this in the EFI forum. You'd get more replies in General Tech, though. Oh, and a V8 conversion on an I-6 FSJ is more complicated than you might think - it's not strictly a drop-in deal, even if you want to swap in an AMC 360.

[Mikel]

If you are adventurous, have time and money... And plan on driving the truck a lot: Diesel.

[addicted]

If fuel economy is your biggest goal stick with the 258 and put a '91-'95 4.0 head on it. Upgrade to a 2150 carb with the factory 4.0 manifold/header or an aftermarket 4.0 header with a better flowing exhaust and upgraded ignition. All of that will also give you pretty big power boost. Fuel injecting it is a decent amount more work, money and not much if any improvement over the other upgrades I suggested.

[tgreese]

IMO your best route to maximize fuel economy is a complete '91 or later 4.0L transplant. That will give you a lot more power on the highway, and has the potential for the high end of what these vehicles can do, mpg-wise.

The design of the 258 is not good for efficiency. The stroke is too long compared to the bore (undersquare) and this will always affect the mileage characteristics of the engine. The 4.0L has a much shorter stroke and a larger bore than the 258. Look at some modern engines, and you'll see that they all have an oversquare design, with a short stroke.

The 4.0L also has a much more modern head design, and the Mopar multiport fuel injection (MPI) that comes with the 4.0L is excellent. And there are bunches of them around - buy a complete rusty XJ donor and transplant what you need, then scrap the rest.

<added> As mentioned above, there are quite a few differences between the 360 trucks and the 258 trucks. You may as well go with a MPI Chevy conversion as an AMC 360. The only thing the 360 gives you is the transmission will bolt to the new engine, but everything moves around - the radiator and core support are different, engine mounts are different, driveshaft lengths change, transmission and transfer case move in the chassis ... you may as well go Chevy and get a modern engine design and cheap-and-modern MPI in the bargain.

But nothing needs to move for the 4.0L swap. All your effort will go in to connecting the new systems.

[mdill]

Why mess around with a half step ? go for a 4.5+ stroker IMHO.
(Less farting around with filling head holes .. you have the 258 crank, pick up a 4.0 and put all together)

What 4.0 Head do I need to upgrade my 258. I want to improve mpg and power. I am open to a V8 upgrade as well. Any insight will be greatly appreciated. I would like to hear both sides pros and cons. Thank you!

[CJ the noisemaker]

Keep that inline six. I say this because of two things:
One, you have very low end torque that rivals that of the Ford 300 six.
And Two, there are too many AMC 360s on the road, a 258 is better

With that said, a 4.0 head swap sounds very great with benefits that make it better than swapping in a 360. Unless you want to build an OMFGSUPERFAST1000MPH engine, inline six is your engine.

[Mr Carts]

I was thinking of going the other way 360 to 258. What are the difference in the mounting of the drive train?

[tgreese]

I was thinking of going the other way 360 to 258. What are the difference in the mounting of the drive train?

The frame towers are in basically the same location (though different), and the inline six is longer. So everything that the engine length impacts changes - transmission location, drive shaft lengths, etc. The core support is different, and the radiator is a different size (though you may be able to make the V8 radiator work). There are lots of other little differences, like the fuel lines cross over to the passenger side on a six cylinder, cabling, etc. Not difficult, but the only thing the AMC heritage gives you is the engine bell pattern. Otherwise it may as well be a Chevy or Ford engine.

[RubiconMike]

FWIW - I have an '82 Cherokee with the inline 6/5 speed combo and was looking at upgrade options. The biggest requirement to me is that it is a modern, fuel injected design. (and since I live the the lovely state of California, it had to pass smog). I considered a few options:

4.0 head and fuel injection on my current engine. Since my current engine has over 250k miles on it, I need to include the cost of rebuilding it. Figure $1,500 ~ $2,000 by the time I buy the head, fuel injection, computer, rebuild kit, and necessary machine work. I'll end up with a torquey six that should produce around 175 ~ 180 hp. (up from the stock 110). If your engine is in good shape you could probably do this for a lot less money.

4.0 HO engine with my 258 crank for a 4.5 stroker. From what I've seen other do on the 'net, this should cost around $2,000 ~ $2500 and produce around 225 hp. I've heard that the T5 tranny I have is marginal and may not like doubling the hp.

AMC 360 swap. I said "modern fuel injected engine", remember?

Chevy 5.3 LS engine/4L60E overdrive automatic. Chevy made a zillion of these and you can pick them up for $1,000 ~ $1,500 on Craigslist. Novak and others make engine mounts and adaptors. I think I can make it happen for $2,500 ~ $3,000 (at least that's what I'm telling the wife) and will get 285 hp (or more with some easy mods), and as good or better gas mileage than any other combo along with Chevy reliability. To top it off, there is a couple of great writeups on this forum from others that have done this, and they love the result.

So that's my current plan (which could change, but I think this is the one I will do) - drop in the Chevy combo.

Just my $ .02, but I had the same questions, and this is what I came up with.

[austinaubinoe]

Keep that inline six. I say this because of two things:
One, you have very low end torque that rivals that of the Ford 300 six.
And Two, there are too many AMC 360s on the road, a 258 is better

With that said, a 4.0 head swap sounds very great with benefits that make it better than swapping in a 360. Unless you want to build an OMFGSUPERFAST1000MPH engine, inline six is your engine.

I agree. I love V8's, but there just aint alot of I6 rigs left. They were not that popular when gas was cheap, and then they went away in the early 80's. Hop up that 6 banger!

[brielly]

I did the 4.0 head on my old jeep with mpfi- no noticeable mileage gain but lots of power. I just recently added mopar mpfi to my 258 without the head swap. Same power but again no mileage gain over the stock carb. With both setups i got 14 mpg city and 17 hwy. Thats with 3.31 gears and a t176 and t5 respectively. I think your best bet would be a complete 4.0 or better yet a stroker and of course the efi to go with it. This will get you the most power and possibly a gain in mileage but i wouldn't expect much. The six just has to work too hard to move our rigs.

[backroadin']

What 4.0 Head do I need to upgrade my 258.

Any 87-99 will work. 91-99 was the HO head, a little better flowing. If you found a free 87-90 head I'd take it, you won't notice much difference. They do flow a bit less than the HO heads, but not enough to justify paying alot more for IMO and you'll only notice that small difference at high rpms.
The 96-99 heads are basically the same as 91-95 heads (they flow the same) with a few minor differences - and I think the exhaust manifolds need to correspond.

The 4.0 head swap can be done cheap and you can modify your original carb manifold to work with the 4.0 head, making it even cheaper.

For mileage, a v8 swap wouldn't be cost effective IMO, even with a v8 that could get 20mpg, it would take you a very long time to make your money back. Power, on the other hand, knows no spending limit!!

With all that said, what gears do you have - because as said earlier, that migh be a better first step.

[CJ the noisemaker]

...even with a v8 that could get 20mpg, it would take you a very long time to make your money back.

Why do people state this? Nobody will ever get their money back from a swap, it should be thought of as this: How far can you go now with that same amount of gas?