4.0 Head Swap on 4.2L 258 - Page 5 - JeepForum.com

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post #61 of 68 Old 11-04-2013, 07:23 PM
swatson454
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Quote:
Originally Posted by NH2112 View Post
3) How about a cam swap to take advantage of the 4.0l head's better flow characteristics? I'm running stock cast pistons and can't tell you how much was removed from the head & deck, but the machinist said it'll still run on 87 octane. I've been told swapping to a. 4.0l head is a bolt-on 40hp and that with the right cam & jetting I'll see a nice increase in torque off idle as well, I might as well take advantage of it! If I can't get this thing right on the next try it's coming out and I'm swapping a diesel in like I should have done in the first place!
Well, a head's ability to flow air is only relevant if the cam gets the valve off its seat.

My gut tells me that a cam having roughly 260 degrees of advertised duration and cut on a 106 LSA (custom for sure) should pull like a freight truck in a 258.

Aside from that, you've moved beyond what a Weber 32/36 is capable of doing. I'd go to a 38 at a minimum or entertain a manifold swap and stepping up to a Holley 390 or even a 470 T/A.


Shawn


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post #62 of 68 Old 11-04-2013, 07:51 PM
NH2112
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Quote:
Originally Posted by swatson454
Well, a head's ability to flow air is only relevant if the cam gets the valve off its seat. My gut tells me that a cam having roughly 260 degrees of advertised duration and cut on a 106 LSA (custom for sure) should pull like a freight truck in a 258. Aside from that, you've moved beyond what a Weber 32/36 is capable of doing. I'd go to a 38 at a minimum or entertain a manifold swap and stepping up to a Holley 390 or even a 470 T/A. Shawn
Thanks, Shawn. I really know next to nothing about the particulars of these engines. My '83 CJ had the stock BBD and burned so much oil I never bothered with it and my '00 TJ's stock 4.0l did everything I wanted and needed it to do, so I didn't get into performance. I've been told by some that the 32/36 is more than adequate, others say a 38 at a minimum like you do. A 390cfm 4-barrel does sound good, it's not going to overfuel but should supply enough for anything I might want to do short of forced induction. I was looking over my engine earlier and apparently a lot of the preservative oil from the machine shop ran into the pan, so I'm most likely going to pull it out and have the bores, deck, and head freshened up to get rid of the rust. Looks like it's time to really start researching these engines and deciding what exactly I want out of mine. Thanks again.

Phil

'90 YJ w/4.2l & 4.0l head, Weber carb, 5-speed, 4" RC lift, 33x10.5x15 BFG MT KM2s.
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post #63 of 68 Old 04-27-2016, 09:56 AM
blaise44
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Hey guys, this has been super useful for me. I was wondering if anyone can help me out. Currently I have a 4.2 258 I want to use in my cj7 however, I want to the head from an AMC 4.0 because from what I've read the angle is 45 degrees which is better for fuel flow, I think please correct me if I'm wrong. With that being said I also have a webber carb for the 4.2. My question is can I use the carb for the 4.2 with the 4.0 head and will I need the intake with water jackets to mate with the 4.0

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post #64 of 68 Old 04-27-2016, 02:15 PM
Cashcrazy
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Quote:
Originally Posted by blaise44
Hey guys, this has been super useful for me. I was wondering if anyone can help me out. Currently I have a 4.2 258 I want to use in my cj7 however, I want to the head from an AMC 4.0 because from what I've read the angle is 45 degrees which is better for fuel flow, I think please correct me if I'm wrong. With that being said I also have a webber carb for the 4.2. My question is can I use the carb for the 4.2 with the 4.0 head and will I need the intake with water jackets to mate with the 4.0 Sent from my SM-N920V using Tapatalk
You can run the carb with some modifications. I think Mike Romain is running a carbed 4.0. Maybe he will chime in.

1982 CJ7(sold), 1983 CJ7(sold), 1989 YJ(sold), 1983 CJ7 undergoing frame-off.
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post #65 of 68 Old 04-29-2016, 09:53 PM
johnson30
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If you know any of this already, forgive me. Here's a recap in case you're curious or needed clarification:

1) You would need a 4.0L head with the water jackets welded shut. The openings hang out in space if they aren't sealed due to differences in the 4.2L and 4.0L blocks and heads. Take a look at this thread to understand the water jackets. I just so happen to be the author, so hopefully it's clear:

http://www.jeepforum.com/forum/f8/4-...ained-2647586/

2) You'd need to move the dowel pins that locate the aluminum 2bbl 4.2L intake on the new 4.0L head. The intake ports on the 4.0L head are higher on the head than those on the 4.2L head.

3) You'd need a new intake, and a new exhaust manifold.

4) You'd need a set of rocker arm spacers to get the pushrod preload correct.

5) With hindsight being a great instructor, I would argue that the most compelling reason to put the 4.0L head on the 4.2L block is to be able to run the 4.0L EFI- if you could find a donor and pull all that out. Since I definitely did not have that hindsight when I did exactly what you are proposing, I can say with confidence (and it's just my opinion) that the benefits vs the cost with the result being simply a carbed 4.2L block were minimal. You could donor the head and EFI onto your 4.2L block, which is a cool project, but I think the standalone MOPAR MPI kit suggests a higher octane fuel- some say no issues, personally, I would follow the suggestion. At any rate, this is why I would, if I could turn back time, just get a donor 4.0L with EFI before I spent a dime on any other effort to tune up the old engines.

I'm not debating the merits of each engine, because I loved my 4.2L in some situations: it was awesome in the low end and I enjoyed messing with carburetors and gaining some experience there. But I've also owned a Cherokee and the 4.0L is a really great motor for lots of other reasons. And lots of the same reasons.

I would just suggest that it may not have the payoff you are looking for to do the 4.0L head swap and put a carb on it (which I have done, in case anyone missed that), and I would've spent that money on something else fun if I could go back in time. I've slowly become a believer in using engineered systems as they were designed instead of what I would imagine is the fairly common early-in-life impression that my piecemeal modifications could improve the engine that highly paid engineers designed, tested and optimized. I realize there are flaws to the early stuff, but there aren't many band-aids in engineering that don't have second and third order effects. So I'd say to just go pay $600 the engine that was designed to go 500,000 miles, self monitor, and start under all conditions.

I did learn a lot, so if that's your motivation, absolutely go for it. I definitely got my money's worth in that regard.
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post #66 of 68 Old 04-30-2016, 07:14 AM
swatson454
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^^^Great post^^^

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post #67 of 68 Old 07-27-2016, 06:01 PM
Gausswave
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Quote:
Originally Posted by blaise44 View Post
Hey guys, this has been super useful for me. I was wondering if anyone can help me out. Currently I have a 4.2 258 I want to use in my cj7 however, I want to the head from an AMC 4.0 because from what I've read the angle is 45 degrees which is better for fuel flow, I think please correct me if I'm wrong. With that being said I also have a Weber carb for the 4.2. My question is can I use the carb for the 4.2 with the 4.0 head and will I need the intake with water jackets to mate with the 4.0

Sent from my SM-N920V using Tapatalk
I'm running a 4.0 with a Weber carb. I have a Clifford intake for the 4.2 I modded to work on the 4.0 head. It has it's own water jackets. I have run with out the water jacket and haven't noticed much of a difference. OTOH I don't live an area with a huge temp swing. Upper 20's is usually all the colder it gets in my neck of the woods.
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post #68 of 68 Old 07-27-2016, 09:20 PM
Chrisfol82
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i have a Clifford performance 4.0 intake for carburetor and a Clifford performance dual outlet header that fits the 4.0 head i'd be looking to sell if anyone is interested.
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