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Unread 09-09-2009, 07:23 AM   #1
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4.2 high output,,build input needed

i figured i'd ask you cj guys as you will know more about the 258's.....when i replace my yj motor (92 242) in the future..i dont want to go stroker..so would it be better to just do a usual 4.0,,or get a 258, and put the 4.0 or better cam,and 4.0 head/intake etc on a 258 short block.

So,,,,i know a lot of you have done the 4.0 head swap,,but a cam,efi also,,and if so,,is the longer strok and few more cubic inches noticable over a stock 4.0 h.o............thanks

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Unread 09-09-2009, 08:45 AM   #2
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1983 CJ7 
Join Date: May 2009
Location: Dripping Springs, TX (soon)
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Cubic inches are always better as long as you can feed them. Heads are almost always the bottle neck and unfortunately are usually left alone. If you know how or can sub it to a competent machine shop I'd definitely go with the 4.0 head and have the valve bowls blended paying attention to the short-turn radius (where the port floor rolls down into the valve seat) and the long side radius along the intake valve for improved swirl. Depending on fit, you may want to match the ports to the intake.

If you can then find a cam with a fairly short duration but as much lift as possible and hopefully ground on something tighter than a 110 LCA, pair that up with a correctly done exhaust, re-curve the centrifugal advance on the dizzy and use a CDI ignition you'll probably be a pretty happy Jeeper. Does anyone make high-ratio rocker arms for the 4.0 head?

You weren't specific about your intended use or power range for the new engine, though.
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