4.2 rebuild with intake question - JeepForum.com

 
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post #1 of 9 Old 03-02-2013, 07:41 PM Thread Starter
bck4more
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4.2 rebuild with intake question

So ive got my motor pulled and going to be dropping off at machine shop very soon. My question is is there anything i can do to the intake like bore or polish

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post #2 of 9 Old 03-02-2013, 08:23 PM
roadyrob
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maybe pressure clean it real good. you can port match the head and intake, theres a vid on you tube on how to do it. also if you look inside the intake ports on the head you will see casting ridges, you can remove them with a die grinder. no need to polish the 4.2 doesn't rev high enough to benefit from it.

Do everything with a positive attitude, it won't always work but you'll piss enough people off to make it worthwhile.

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post #3 of 9 Old 03-05-2013, 09:52 AM
Flajeeper1
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Quote:
Originally Posted by roadyrob View Post
maybe pressure clean it real good. you can port match the head and intake, theres a vid on you tube on how to do it. also if you look inside the intake ports on the head you will see casting ridges, you can remove them with a die grinder. no need to polish the 4.2 doesn't rev high enough to benefit from it.
Roadyrob is correct. The 258 is known for it's low end torque, it's not a high revving engine and you won't see the performance gain as a result.
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post #4 of 9 Old 03-05-2013, 01:07 PM
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These guys are right. If you want to up the performance of the 4.2 swap a 4.0 head on and get some headers. That will give you a noticeable jump in performance as the 4.0 head breathes much better than the 4.2.

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post #5 of 9 Old 03-05-2013, 01:53 PM
roadyrob
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actually if your staying with a carb I'd keep the 4.2 head, mill it 10 thousands, port match and clean up the casting. if your going fuel injection the 4.0 head works great, especially if you get a FI cam.

Do everything with a positive attitude, it won't always work but you'll piss enough people off to make it worthwhile.

the voices aren't real but they sure have good ideas.
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post #6 of 9 Old 03-05-2013, 01:59 PM
vadslram
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You might find miniscule gains by polishing if your intake has a lot of sharp turns and big mold sprues but that would only be from keeping the fuel from dropping out in the eddy swirls.
Where you will find huge gains (again possibly) is from port matching. But you need both the head and manifold for that. The last engine I did was my 351c on a mustang. I used playdoh to figure out where the opening in the head and the opening in the manifold runners didn't match up. One was so bad it reduced the opening by about 15%.
At any engine speed you should have a smooth constant path for the air and fuel to travel.
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post #7 of 9 Old 03-05-2013, 02:09 PM
roadyrob
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when I port matched my 4.2 I used the gasket, like I posted earlier, theres a vid on you tube that shows how to do it. the vid just shows doing the head but I did the manifold the same way. if you don't have a die grinder a dremel will work.

Do everything with a positive attitude, it won't always work but you'll piss enough people off to make it worthwhile.

the voices aren't real but they sure have good ideas.
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post #8 of 9 Old 03-05-2013, 07:04 PM
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Quote:
Originally Posted by roadyrob View Post
actually if your staying with a carb I'd keep the 4.2 head, mill it 10 thousands, port match and clean up the casting. if your going fuel injection the 4.0 head works great, especially if you get a FI cam.
Look into a hotter cam. The stock cam is extremely mild and that will make more difference (I believe) than head mods.

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post #9 of 9 Old 03-05-2013, 08:12 PM Thread Starter
bck4more
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Thanks for all the input guys lots of help
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