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Discussion Starter · #1 ·
I have decided to get a late-model 4.0L H.O. and stroke it out using some of the parts off a '75 4.2L in my friends J-20 (I can pick up the whole truck for cheap.) I have found out that the compression ratio (if you use all stock parts) works out to be 9.43 which is on the limits of pump gas. I take it that I wont be able to supercharge this engine then unless I run airplane fuel?I've heard that going with longer rods and shorter pistons will increase torque yet decrease compression (or at least not increase it anymore) and that putting the pistons in backwards provides a small increase in torque, but more cylinder noise (not good with a knock sensor I've heard.) What do you all think about this, does anyone here have a 4.5L stroker?

My Jeep is just a leaky submarine in disquise
 
G

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You can run more than 9.4 compression on pump gas. You can run like 11:1 as long as you run premium. If you are REALLY worried about it, tkae the pistons to a machine shop, and have them remove some material from the top of the piston. I'd do it.
BJ

"If you chose not to decide, you still have made a Choice" RUSH
 
G

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Actually, if you are considering forced induction do not remove material from the pistons.
You can probably buy lo compression pistons for supercharging it.
Gibby

~Someone's got to keep the bastards on their toes~
-HST
 
G

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good point Gibby
i meant if you already have the pistons.. i believe it's possible to reuse the 4.0L pistons, on the 4.2L rods& crank
New rings and bearings are a given, but is there anything wrong w/ re-using good pistons?? Non forced air, and forced air induction answers wanted

BJ

"If you chose not to decide, you still have made a Choice" RUSH
 

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Discussion Starter · #6 ·
How about using the stock 4.0 pistons, with the 4.2 rods as recomended (aluminum rods would be sooo much better for this application though) and have the 258 crank off-set ground to reduce comperssion? would it reduce it enough? I'm trying to make an everyday driver with good mileage, great torque, that could do 4500 rpms for sustained periods since there is nothing but sand and mud in FLA...

My Jeep is just a leaky submarine in disquise
 
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