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Discussion Starter · #1 ·
I dunno if its all 4.2's or just the one in my truck but they are tuned way to generally so its time to make mine a little bit more specific and go for the down low grunt. if I back my timing off from the factory recomended 15degrees down to say 8-10 will I gain a bit of power in the bottum end? I dont care if I lose the top end, it pull past up to 4500 rpm, and probally farther if I let it, as well as hitting 6000rpm a couple times when I missed a couple of fast shifts. somehow I dont think it really needs to be able to do that :D
if it maxed out at 4000rpm id be happy as long as I had more bottum end.

anybody have experience with building up a 4.2? any mild camshafts(such as the all to often heard term "RV" cam) that will give me bottum?

I currently turn just over 2000rpm on the freeway in 5th with 30x9.5 tires and 3.32 gears. id like most if not all power below 3000rpm. I tend to shift in town below 2000rpm for 1 through 3rd(4th needs a bit more otherwise it lugs, can cruise and accelrate at 50km/h in 4th though).

so yah, im looking for bottum end, top end I could care less about. I may or may not route my exhaust to the back bumper again instead of the current side exit to gain back pressure again, might just put turn down on the end though and see if that gain much backpressure(low backpressure=not much bottum end)
 

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My CJ has a recommended setting of 10 degrees BTDC at 1600 RPM, but I don't do that. I use a vacuum gauge at idle, hooked to manifold vacuum, (brake booster supply is good). Slowly advance the timing until you get the highest reading you can, usually about twenty inches of vacuum, then back it off until the gauge drops one inch. Lock down the distributor, hook the booster vacuum back up, and test drive.
My CJ pulls like a train.

Frank
 

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You want more bottom end? I have the same set up except i run 31 inch tires. Shifts are all below the 2g mark except 5th. I think this is great bottom end. But to get more i think the cam thing should work well.

Comp cams has a toll free help line. I don't know the number but you can find.

Anyway let me because i have the same setup.
 

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Discussion Starter · #4 ·
frank: what rpm do you do this at? at idle? or do you just keep going for the highest reading and keep adjustin rpm if it changes?


greg82:the bottum end is fine when going slower(ie: not on the freeway) but it dosn't like being at 2100rpm in 5th at 100km/h. not quite enough grunt to hold speed never mind accelerate.
 
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