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Unread 10-02-2001, 06:46 PM   #1
Brendan
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Why is the 4.2 so weak?

I have a pre fuel injected YJ. Needless to say I suffer from a severe lack of power. I don't understand how such a large engine (4.2 freakin liters) could have so little power. Is it the head design? the fuel system. I mean if there is such a big combustion chamber why do I have barely 120 horses. It's embaressing to know that the 4 bangers have almost as much power as me. What can I do to get more power?!

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Unread 10-06-2001, 12:44 PM   #2
87yj350
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The heads on the 4.2's don't flow well, you can put a 4.0 head on which is much better and get more power. You can also change the intake to put a 4bbl or better yet go efi. You should check out cliffordperformance.com they specilize in 6 cylinders.
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Unread 10-06-2001, 05:07 PM   #3
FLHT_FXRS_YJ
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If your power has been very poor, or it's really hard to get any rpm's, check the cat. converter. If it gets plugged up, it will really rob the engine power.
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Unread 10-14-2001, 06:01 PM   #4
Brendan
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My cat is a hinderance, but its performing like it should If I swapped in the 4.0 L heads, could I still swap in the holley carb, and intake manifold, or are the bolting patterns different? Also, what are the different kinds of ignition, and what the benifits and drawbacks? Thanks alot.
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Unread 10-24-2001, 06:18 PM   #5
JL
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I'm researching for the rebuild and build-up I'm about to do on my 4.2. You'll retain the original intake and carb (or whatever aftermarket 4.2 parts)with the 4.0 head swap, but you'll need an exhaust manifold/header from a 4.0. I've heard estimates of between 25-40 hp gains, as well as a gas mileage increase. I'll probably port the head too before I put it on. Might as well while I'm tweaking...

By the way, the kit for this costs somewhere in the neighborhood of $1000 if I'm not mistaken, but all the parts involved can be had for CONSIDERABLY less. This is the most thorough site I've found.
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Unread 10-25-2001, 02:04 AM   #6
CJ junky
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Guys,go here. http:/monsterslayer.com/jeep/40Head/40Head.htm This is the most complete site I've found about doing a 4.0 head swap.
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Unread 10-25-2001, 03:21 PM   #7
Brendan
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thanks you guys, what does porting the heads mean? Sorry for the dumb *** question. Can I swap any 4.0 head in. If I ask a junk yard for a 4.0 Head, what should I look for, one from a Wrangler, and how will I know if the valves sit well? Thanks for all the help
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Unread 10-25-2001, 05:59 PM   #8
JL
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Now Brendan, the only stupid question is the one you don't ask. Seriously, I didn't know any of this stuff either until I started reading about it. Porting the head involves grinding the inside of the head out to let more air flow through. Machine shops do it, and you can buy kits to do it yourself. (I'll probably opt for the latter)

As far as what year, most sources I've seen say '91 and up. These are supposed to be the high output (HO) 4.0 engines. The casting # for these heads is on the site I mentioned.

Don't know about the valves, but let me know what you find out.
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