4.2L Performance rebuild Help - JeepForum.com
 
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post #1 of 6 Old 12-23-2013, 01:22 PM Thread Starter
jeffjones35
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1981 J-Series Truck 
 
Join Date: Dec 2013
Location: austin
Posts: 5
4.2L Performance rebuild Help

I am aware that this post probably exists elsewhere but I am tired of getting bloated search results or results that do not help me. I have recently pulled the motor out of my J-10 it is a 1981 258 so as most of yall know it only has just over 100 hp and that just wont do.

I was planning on putting a 4.0 head on there but have decided that i would like to stick with a little more original as it was my grandfathers. Also the head that I have is a 2686 casting and it ended up needing new valves. The motor that I have was rebuilt around 10,000 miles ago so internally it should be great, it ran fine when the transmission went, other than the silly weber electronic carb bs. Here is a list of parts that have for it

Summit distributor
MSD Blaster Coil
Comp Cams Cam Kit http://www.summitracing.com/parts/CCA-K68-239-4
Header off of cherokee 4.0 will be modified to fit
2150 carb new built for 4.2
Roller Rockers

My question is what else can i do to really get some more power out of this motor. I do not care about emissions or fuel efficiancy. I just really want the power as i plan on running 37s with a 6 inch lift and regearing the dif to 4.88 to compensate. I was going to mainly do the head swap for added compression.

The motor is already completely out of the truck and i will be rerouting the belt so i do not have to run compressor or smog pump.

So If i can get some help with:
1. More power/ should i machine off block for compression
2. does anyone have a template for rerouting belt.

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post #2 of 6 Old 12-23-2013, 06:20 PM
uberxj92
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1992 XJ Cherokee 
 
Join Date: Oct 2012
Location: umatilla
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The 4.0 head conversion is best bet. Basically a low stroker then. The efi will be the biggest improvement.
but, you can order true zero timing gears. A moderate roller cam. Port and polish everything. Use electric fan instead of clutch. Decent exhaust helps. The roller rockers will be a decent upgrade. Forward facing snorkel that's at least the same diameter of the Carb intake mouth. K& N.
That is short if punching her out and stroking her. Don't deck too much or shave too much from the block and head. You want the displacement for the big tires. 5 angle valve job.
It all depends on your wallet.

https://www.jeepforum.com/forum/f177/92-xj-sadness-2075657/index2.html
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post #3 of 6 Old 12-24-2013, 05:33 AM
vadslram
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1990 YJ Wrangler 
 
Join Date: Jan 2006
Location: VA Beach
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If you are up for a challenge then a supercharger will pack on some more ponies.
A little bit of creativity and you should be able to get something like this in there
http://www.jegs.com/i/Vortech/933/4F...ductId=1137093
They are basically bolt on air pumps that give boost immediately, no turbo lag, no need to repipe the exhaust.
A mild boost should give at least an extra 25 - 35 hp without risking the block.
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post #4 of 6 Old 12-24-2013, 08:49 AM Thread Starter
jeffjones35
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1981 J-Series Truck 
 
Join Date: Dec 2013
Location: austin
Posts: 5
Although it would be great to have it supercharged it is not nearly as effective to put on a push through set up with a carb. Also for the 25 hp at 3000 dollars i would be better off to put in an efi kit. I want to keep the motor fully mechanical no computer. I am more looking for am i better to go with new pistons or machine the block, for more compression to really get the power out of the cam package. I do not care if i have to run 93 octane with octane boost in it. If pistons is the way to go and that's the way i am leaning what pistons are best for my set up. I do not want to spend more than another 1000 as i am already 1500 into it.
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post #5 of 6 Old 12-24-2013, 09:09 AM Thread Starter
jeffjones35
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1981 J-Series Truck 
 
Join Date: Dec 2013
Location: austin
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or should i deck the block and get different pistons
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post #6 of 6 Old 12-24-2013, 09:36 AM
uberxj92
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1992 XJ Cherokee 
 
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Location: umatilla
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Decking the block will cause you too spool up faster and take away from the power and torque. It does this by taking away stroke and displacement.
you can bore the block and get gains. I would go about .040 to .050 overbore. With overview pistons.
A good cam will help too.
But this route is costly.

https://www.jeepforum.com/forum/f177/92-xj-sadness-2075657/index2.html
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4.2 liter , 4.2l , performance , rebuild

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