4.0/4.2 head swap

[backroader]

Anyone here ever do the 4.0 head on a 4.2 block swap?

I rebuilt my 4.2 last summer. Runs good, but it smokes blue after leaving it sit & idle warm for 5 minutes or so. I took the head to a machine shop when I did the rebuild. They magnafluxed it, installed new valve seals, and shaved the head a little, but I'm thinking the guides must be worn. Doesn't use much oil - maybe a quart to a thousand or 1500 miles, but it bugs me to no end, so I'm going to fix it.

So if I'm going to take the head off anyway to have it gone over, maybe I will just buy a rebuilt 4.0 head and put on it... Any input, thoughts, or experience will be considered & appreciated..

[tgreese]

There were a couple of good articles online about the 4.0L HO head swap. Pretty sure the ones I recall are gone. You might read/post in the swapology forum at jeepstrokers.com for first-hand experience.

You used to be able to buy a new 4.0L HO head; maybe not any more. When they were available, about $500. There are good versions of the 4.0L head and bad ones - see the parts article at jeepstrokers.com. The bad ones crack. I presume this problem supported the production of new aftermarket heads.

Supposedly much of the HP boost comes from the higher compression ratio of the 4.0L head, not just the HO head ports. EFI provides a knock sensor, and this allows a higher compression ratio while preventing pre-ignition. Why not swap in a complete 4.0L HO motor with its excellent Mopar/Bosch MPI?

[rocklaurence]

Edelbrock produces a good aluminum replacement head to get the best performance and only costs a lot more than an OEM head but IMO is worth the expense. Like Tgrease said, why not do a 4L swap?

[tgreese]

New 4.0L HO head - https://www.cylinder-heads.com/product/ ... -complete/

There are lots of listings if you search. I believe this is the seller I recall. You might ask for advice re sources on JeepStrokers or NAXJA.

[backroader]

There are actually about 3 versions of the 4.0 head from what I've read, Casting #7120 is apparently the head of choice for someone who is doing this swap. Rebuilt 7120 heads are still available for about $500.

The 4.2 block that is currently in the J10 is in excellent condition now after the rebuild, with the heavy crank and a new RV cam & lifters, so I want to keep the block. Runs over 50# of oil pressure at cruise.

The 4.2 is already actually a stroker, so it has very good torque at lower rpms. I found one article on the Jeep CJ forum with several posters with real life experience with this swap, and most were very happy, claiming more power and also better gas mileage if set up correctly.

I plan to keep the carb, and the stock Duraspark setup seems to be working fine, but I may switch to an HEI ignition at some point.

Mostly just wondering if anyone on this forum has done this swap, and what their experiences were after putting some miles on.

[backroader]

I'm thinking I can get the swap done for about $1000.

I can re-use the stock aluminum intake with a couple of small grinding modifications.

The mount for the power steering pump needs to be altered somewhat to allow fitting a new exhaust manifold. I would also need a new exhaust manifold & down pipe and a new catalytic converter, since it is 2 1/4", rather than the stock 2". The rest of the exhaust currently on the truck is already 2 1/4" from the cat back. Sounds like the valve cover may also need to be replaced, and there are some extra coolant passages on the 4.0 head that need to be filled and sealed with JB weld.

Of course, it would be much easier and less expensive to just buy a stock rebuilt 4.2 head and bolt it on for less than $500.

Many people claim an increase of about 30 to 40 hp over stock with this mod, with more power and better mpg as an end result.

[tgreese]

Mmm. Owning a 258 pickup, I've researched and thought about this a lot. Sorry my only experience is with some basic mods to the original 258.

My rudimentary understanding of how such things work points to the bore/stroke ratio as most important to torque curve characteristics. The 4.0L engines have a shorter stroke and larger bore than the 4.2L engines, and this puts the peak torque at quite a lot higher RPM. Power is torque times RPM. I would guess this change in the torque peak makes the largest difference in the power output, comparing the 4.0L to the 4.2L. Even the early 4.0L makes significantly more power from less displacement than the 258.

From Jeeptech and other sources -
Wagoneer 258: 110HP@3500, 195lb-ft@2000
'87 XJ 4.0L: 173@4750, 220@4000 (no RPMs for these numbers, but I presume they are the same as the HO)
'91 4.0L HO: 185@4750, 220@4000

In terms of HP, the 258 will always be handicapped by its long stroke and small bore. The 4.0L short stroke puts the torque peak at twice the speed of the 258, it seems.

I would point out that the Renix (Renault-Bendix) fuel injection of the '87ish 4.0L uses a knock sensor; the Mopar MPI lacks one, relying on the more sophisticated programming and improved cam profile of the HO.

The '87 4.0L has 63 more HP than the 258 (up 57%) and the 4.0L HO has '75 more (up 68%). The main difference in the two versions is the head design with higher intake ports, and the more advanced Mopar MPI. This gives a gain of slightly less than 7% from the Renix engine to the HO. Seems likely you'd see similar gains by putting the 4.0L head on a 258; 7% of 110 is roughly 8 HP. Leaving the rest of the 258 unchanged, a gain of 30 or 40 HP seems wildly unrealistic for this change.

Not meaning to rain on your parade, but ... geez, 30-40 HP? Plus 30 is 27%; that's like a boost from 258 to 328 cid. Only in their dreams. The head swap just does not seem worth it on its own. Especially when any 4.0L engine will give twice that boost.

[backroader]

There was a guy on the CJ forum who actually had his engine on a dyno before and after the swap, and the results were pretty dramatic.

I'll see if I can find it again. If I can, I'll post it.

To really get an accurate apples to apples comparison, you would need to look at the torque curves at the various rpms. I know my truck will never turn 4000 rpm. 2000 would be a lot closer for regular travel on the highway.

I'm generally pretty satisfied with the power of the present setup for most of my driving, but it is underpowered for passing and going up long grades at highway speeds. A little more power would be nice. I think my truck is also over-geared, so that doesn't help either. I have the 727 tranny with lock-up torque converter, and I'm running 31x10.5 tires on Chevy 6 hole rally wheels. Rear end is the higher 3 ? something option for the year.. Some lower profile tires would probably help on the highway, but I'm not sure they are even available in a 15" size for light trucks. Low profile tires probably wouldn't help the ride much either. Most of the roads around here are beat-up and rough 2 lanes.

[tgreese]

Year, model, equipment? I recall there was some discussion with you previously of different vintages of 258s, plus different transmissions? The context matters.

Reminds me of some discussions on the CJ board about the 304. Someone asked what's the most-cost effective modification to make more power from the 304? The consensus was - replace with a 360.

Here's the 4.0L HO torque curve: http://www.jeep4.0performance.4mg.com/stroker.html

Looking at this and comparing to the above shows 212 ft-lb at both 4750 and 2000. At 2000, that's 42% of the engine speed, which is 78 hp. Not as strong as the 258 at that speed; if you drop into passing gear, that's about 3000 and 136 HP. More HP at a lower speed than the 258. Dunno... seems quite unlikely you will ever get that much power from a stock-ish 258, considering you would be picking parts from the 4.0L that's combined with a lot of other improvements to make that power.

[backroader]

"Done and Done! It took a little longer than I thought. But the results are well worth it #7120 head-I had some time on my hands so I polished the ports then did the swap. We ran it on the dyno 3 times I got this long piece of paper for b4's and then peak afters. Here are my results- b4 was 142@ 3200 rpm peak(with headers and 4bbl truck avenger carb on offey intake) afters peak 178@ 3200 rpm. thats 36 HP gain! I have the torque and and a whole bunch of other :dung: written down, but I'm not gonna type it all right now. The real test was going up the hill to red mountain by my house it always bogged down-no faster than 55-60 on that road. remarkable difference now easily 75+ in the same spot. and thats on 37" tires with 4.56 gear ratio. Thats all I have for now will have step by step pics later!"

https://jeep-cj.com/community/threads/2 ... lts.40216/

[tgreese]

Huh. Went all through that thread and did not find the post you are quoting.

From what you quoted above, I'd guess the poster has something like another $2000 of aftermarket speed parts already installed, plus custom work (port, polish) on the new head. Still not believing it - I would want to see the dyno slip.

[backroader]

Yes, he stated he had an Offy intake with a 4 barrel carb before & after the swap. He would not have been able to use the header. Most everyone who does this swap install a later model stock 4.0 exhaust manifold. Sounds like nothing else was changed from before to after.
Don't think the port polishing would make much difference at those rpms.

Post number 48 page 3. He was also going to post the results...I believe I saw the dyno printouts once, but don't know how to find them again.

[tgreese]

I appreciate your enthusiasm, and I hope you are happy with the new head, should you choose to go that way.

My problem with this - if this change were so profoundly beneficial, you would have expected similar improvements when the industry converted to this improved design. Also, it would have been widely adopted with similar results. If the 258 benefits so much from this new head design, why didn't AMC simply update the 4.2L with this new head, rather than the more complete upgrade of the 4.0L design? Again, the 4.0L only saw a 7% horsepower increase when this new head was added, along with much more sophisticated EFI and other improvements, including a better cam.

Maybe it's possible in limited circumstances, but I sincerely doubt you will see improvement like that without a lot of supporting changes, if at all. You had to go back to 2011 - thirteen years ago - to find a post to support this miraculous result. People claim a lot of things online.

The car manufacturers spend millions of dollars and copious man hours to develop and improve their products. They want power, durability, reliability, economy and affordability, all in one package. How likely you can combine similar parts from contemporary offerings and come up such a vastly superior result? Not very, IMO.

[backroader]

Browsing the Jeep Stroker forum, the 4.0 head swap on the 4.2 block with factory fuel injection is still a very popular choice today..

They don't like the 70's - 80's cam over there, referring to it as a "smog cam"... They recommend using a stock 4.0 cam to anyone doing the swap.

I myself would like to see more actual dyno tests on the swap, but there isn't much out there in the way of hard data - mostly just claims from people who have done it.

[Srdayflyer]

well gents, i have a cheaper OEM high compression head
i picke this up at a bine yard out of a postal DJ series jeep , the research i was able to find was the p.o. wanted more power out if the 232/258 6's and this was jeeps fix, i installed it in my cj5 and there was a noticeable difference in off road low end attached are the DJ (closed chamber head)
and the CJ (open Chamber head)

20210312_112250 (1).jpg

20210312_112316.jpg

so you dont necessarily need to buy new when OEM will do , thought id share this info

[backroader]

Hmm, interesting...wasn't aware of that one..

Can you burn regular 87 octane gas with the closed chamber head? Does it use 1/2" head bolts?


Did some searching, and it looks like 1977-80 casting #3227258.

[Srdayflyer]

yes ,i use 87 octane , no knock or ping , my 75 cj calls for 7 btdc im running 10 because i go up i the mtns of az and dont want to reset timing, thinking about going to holly sniper and hyperspark like in my fsj cherokee, its such a hoot to adjust fuel and timing from your drivers seat, and yes 1/2 bolts

[backroader]

I took some time yesterday to dial in the advance on my Motorcraft distributor. Set it so the power band comes on pretty strong right before the torque converter locks up - about 40 mph. I don't need to worry about changes in elevation here in northern MN, so that's not a concern for me.

Happy with the results, and I'm finally feeling like the cost of the RV cam was worth the cost.

Definitely interested in the closed chamber head, and Jeg's has them available for about $375 with a core return. My engine is a 1978 model, so it would bolt right on.

Thanks for the info...

[Srdayflyer]

glad i was able to pass that on
i have a holly sniper /hyper on my360 powered fsj its amazing' theres a you tube holly video with them installing a system on an amc 6, just fyi, and as far as the motocraft ignition, i found it to be highly failure prone especially when i needed it mostly off road, i swapped it out for a g.m. style old school H.E.I. dist. and havent had a problem ever since can send you pics of the install, picked it up (new) on ebay think i paid 40-50 bucks , but if you want to stay with a moto,ign. i think i have the parts you can have for shipping costs

[Srdayflyer]

obtw, keep me posted on the head swap should you decide to do it ,