4.2 to 4.0 head...is it worth it? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 04-12-2002, 12:45 AM
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4.2 to 4.0 head...is it worth it?

I have a 1988 Wrangler 6 cyl. 258. I just purchased the Mopar fuel injection kit and was in the process of installing it when I saw the 4.2 to 4.0 head conversion on Kollerparts.com. The head sells for $600.00 with a core return of a used 4.0 head. My questions are, is the 4.0 cylinder head conversion really going to add 40hp? and if so, what is a better choice, Borla header or exhaust manifold? or do I leave the 4.2 head and install a Borla header or exhaust manifold on it?
Any help...
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post #2 of (permalink) Old 04-12-2002, 01:49 AM
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Re: 4.2 to 4.0 head...is it worth it?

Not sure about the 40hp, but it'll DEFINETLY get you some extra ponies. I don't see paying $600 for the head though. You could probably get a junkyard one with low miles and do just fine. As for the exhaust, I see NO point at all installing a header, ESPECIALLY on a 4.2 that isn't going to see much above 3500rpm's. I don't believe however that the mopar FI kit will work on the 4.0 since the intake manifold is different.
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post #3 of (permalink) Old 04-12-2002, 02:39 AM
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Re: 4.2 to 4.0 head...is it worth it?

Jamgood, I think you've got it the other way around. You need a different header (oem or aftermarket) for the 4.0HO then the 4.2, the 4.2 manifold just won't fit on the 4.0. But I'm 99% sure that the intake manifold for the MPI is the same as for the 4.0 HO, in fact I believe Koller has a link to a write up of a 4.2 with 4.0HO head and Mopar MPFI. ;-)
Like Jamgood says, buy a 4.0 head from a junkyard and pay a machine shop $100-$150 to go through it.

Ben
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post #4 of (permalink) Old 04-12-2002, 02:48 AM
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Re: 4.2 to 4.0 head...is it worth it?

To really get the full benefit of the mpi kit, get a 4.0 head. The difference will blow you away. You get all the reliability of the MPI, plus additional power from the 4.0 head. I got a head for $100 at a junkyard and spent another $150 getting a 3 angle valve job and hardened exhaust valve seats put in. Do the research. You should fill the water jackets. I used epoxy with no problem while others have had freeze plugs or just the head gasket.

You've spent the big money on the kit, now if you get the head, you'll love it. You will need a 4.0 exhaust manifold with the 4.0 head which usually will require a new tube from the header to the cat.
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post #5 of (permalink) Old 04-12-2002, 06:42 AM
 
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Re: 4.2 to 4.0 head...is it worth it?

Get the 4.0 head and the Borla header for it. You will be quite impressed by your power gains. Take a look at Jaffer's site <a target="_blank" href=http://monsterslayer.com/jeep/Default.htm>http://monsterslayer.com/jeep/Default.htm</a>. He has pretty much the best write-up out there for what you are considering.

As long as you have the head off, you may consider swapping to a slightly warmer cam as well. Stay small if you want bottom end torque: Crane 260, Clifford 264, etc...

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post #6 of (permalink) Old 04-12-2002, 08:35 AM
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Re: 4.2 to 4.0 head...is it worth it?

Be careful on the cam swap... if you're thinking of doing one with the MPI. That computer dosen't cotton too much to swaps...and a different cam may throw out the MPI's spec. window.
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post #7 of (permalink) Old 04-12-2002, 09:28 AM
 
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Re: 4.2 to 4.0 head...is it worth it?

Good point, LEVE. Sorry 'bout that. I have a carburetor on mine.
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post #8 of (permalink) Old 04-12-2002, 09:40 AM
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Re: 4.2 to 4.0 head...is it worth it?

YES, do it!!
It is worth it. You will be amazed at the difference in HP and reliability. I used a '97 TJ head and MPI (with '95 throttle body and sensors)to do mine. The Mopar MPI will work with the 4.0L head, but I recommend a head in the '92-'95 time frame for use with the Mopar MPI. The head I used came with a header so I did not need to purchase one. Make sure the header doesn't have any leaks or cracks. You can do the swap with the bottom end (block) remaining in the jeep. Do a compression check (dry and wet), to help you determine the condition of the block. Once you have the 4.2L head off, you can look at the cylinder walls and check for wear (ridges) in the cylinder walls. Hesco sells all of the conversion parts and they have great customer service. I was even able to use the stock YJ air box with a little trial and error and a BFH.
The head swap is one of the best mods you can do to your jeep. Once the search is back up, you will find lots of info on how to do it.
Lots more power and 18.4 MPG city is a big benefit.
Good luck and keep us posted.
post #9 of (permalink) Old 04-12-2002, 11:12 AM
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Re: 4.2 to 4.0 head...is it worth it?

Actually, Mopar has some very good cams available that work well with the MPI. It has been 4 yrs for me, I don't remember the specs, I believe I went with a little more lift, with about the same duration. check it out.
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post #10 of (permalink) Old 04-12-2002, 02:25 PM
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Re: 4.2 to 4.0 head...is it worth it?

Well this is great news for me then. Cause I've always toyed with the idea of building my 4.2 block up once my 4.0 bites the dust, slapping my 4.0 head and FI on it and away I go. Think this would work? Also, there's got to be some other ways around running a different cam in it. Or will a torquey came completely mess up the computer?
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