Rebuild 258 or 4.0 HO? -
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post #1 of 7 Old 06-11-2010, 06:35 PM Thread Starter
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1979 CJ7 
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Location: Modesto
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Rebuild 258 or 4.0 HO?

I have been battling between rebuilding my 258 or buying a 4.0 HO from a newer wrangler for my 79 CJ7. What are the positives/negatives of each option? If i decide to rebuild the 258, should I go with the howell or mopar FI kit? Does anyone know about how much it will cost for each option? Also, if you have rebuilt your 258, what parts did you use and would you use them again?

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post #2 of 7 Old 06-11-2010, 08:06 PM
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I pretty much did a rebuild on my 258. It's a great engine. I've been dealing with almost every iteration of Inline 6 Jeep motors since they started building them. Absolutely nothing wrong with it. Now if you want to upgrade your setup that is another story. You could keep the 258, throw on a 4.0 head and then it allows you to run the fuel injection setup. Or you just keep the carbureted intake configuration on the 4.0 head, it all bolts right on. You may have some MINIMAL clearance issues with the stock 4.2 carb intake and 4.0 exhaust manifold. It is easily dealt with. But you do have to use the 4.0 exhaust manifold. Keeping the engine you have and modifying it is the cheapest and easiest solution. Hope that helps
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post #3 of 7 Old 09-29-2011, 09:35 AM
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The carbed 4.2 and the FI 4.0 are like night & day. 4.0 has way more power. Or like stated put a 4.0 head and FI setup on your 4.2. I saw just last weekend a complete running 4.0 go for $150 at a swap meet. The dude gave it away..
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post #4 of 7 Old 09-29-2011, 09:39 AM
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The 4.0's are a dime a dozen here too. If you do the 4.0 head on the 4.2 does the FI still run okay? The tuning is alright?

Wheel - Wreck - Wrench - Repeat!

The VIN says its a 79 CJ
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post #5 of 7 Old 09-29-2011, 08:45 PM
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The 4.0L engine is great. I run mine carbed but running it as with its native FI is great as well.

Given the choice between rebuilding the 4.2L and a 4.0L, I'd rebuild and install the 4.0L.

Vibration? Bump steer? Wandering? Read the article (sticky) on Steering, suspension, and driveline.
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post #6 of 7 Old 09-29-2011, 09:14 PM
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You are probably looking at around 100hp from your current engine in old and worn state as opposed to 160-170hp on a used 4.0 HO. I know they could new generate 190+ but you wil not get this unless you spend bucks on a rebuild.

That is one helluva step up in power.

Downside is that you will need the whole engine, fuel injection, wiring and computers to make that swap. the most difficult part for a home mechanic is the wiring, it is a mass of spaghetti and unless you have an affinity for that sort of stuff it could ruin your build. Some do and make it look easy.

Another route is a 4.0 head on carbs on a 4.2 block. Opinions vary but it could add 30-40hp on a 4.2. If you rebuild the block as well with new rings etc. you could be seeing 150hp+, maybe 160hp. It would not have the complexity of fuel injection and hence is fairly straightforward for a home mechanic.

If a 4.0 head with FI on a 4.2 block you could exceed the power levels of a 4.0HO. I say "could" as that will take some degree of skill but it is quite possible to use the 4.0 injection on a 4.2.

BagusJeep lives in Bali.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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post #7 of 7 Old 09-29-2011, 11:04 PM
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1986 CJ7 
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i have this set-up in my 4.0

2001 block
1995 head h.o.
intake 4.2
carb carter dodge + jeep
exhaust 1995
dana 300 without adapter
distribuitor motorcraft update tfi
accel super coil
autolite xp gap .055

its a monster in rpm. i use the 4 shift every time

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