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Unread 06-12-2013, 07:48 PM   #1
war6191
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1979 CJ7 
 
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AMC 20 question/opinions needed please

79 CJ7 with AMC 20 3.73 gears. I'm building a medium duty trail rig but still a daily driver. The axles are next phase. My initial thought for this AMC 20 is installing a good one piece axle shaft', keeping the 3:73 gears with new Detroit Truetrac LSD instead of current open gears ( in the AMC 20.) I would follow a similar plan with the D30 in the front.
I don't really think I will run more than 33 in tires on this jeep.
The question is this ....Should look for another axle such as D44 or something or is this a reasonable move? It was suggested I will want more more axle than a 20 down the line. Then again, the AMC 20 is run on a lot of CJ's..
What do you Jeepers think? I would appreciate your input!

79cj7. 258 TBI. T176. D300 a survivor Jeep

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Unread 06-12-2013, 08:09 PM   #2
RedCJ83
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You will get a variety of opinions on this. The way you've laid out your plan with light off road duty and 33's the AMC 20 is fine. Do not disregard the advice to truss the rear end and weld the tubes though.
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Unread 06-12-2013, 08:10 PM   #3
brownbagg
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weld the tubes, add a truss and it last a life time
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Unread 06-12-2013, 08:10 PM   #4
holliewood61
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The amc 20 can be made fairly stout. A good 1 piece kit (i prefer moser). And a truss if you plan on beating on it. The problem lies in finding a stronger front to match the width of the rear. You could do a custom width 44 out of a scout.
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Unread 06-12-2013, 08:11 PM   #5
WindKnot
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Personally, I think the AMC20 is just fine ESPECIALLY after the one piece axles. You could improve it further by welding the axle tubes to the pumpkin, and to make it even more bullet proof for stuff you don't plan on doing by adding a truss. Do all that and I think you may have something better than a D44. Others will disagree in just about...

I take my Jeep EVERYWHERE across Arizona on primitive trails and my AMC20 does just fine. I travel over rocks and have changed out my diff covers to cast steel, but I don't do boulders unless I have to. My Jeep isn't a racer, either.
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Unread 06-12-2013, 08:24 PM   #6
BagusJeep
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Nothing wrong with an AMC20, it has been allowing your Jeep to move for 33 years already.

The upgrades mentioned above are all reasonable and will not break the bank.

However your gears will be a tad low for those 33" tyres. At highway speeds you will be pulling 2000rpm on that 258 and it will not be happy. Your profile says you currently have 31" tyres so you would be pulling 2200rpm at the same speed, which is marginal but doable.

I fear that when you have spent all the money on the axle and Truetrac and tyres that you will find it sluggish and thirsty. When you have then changed the gears you will think "I could have got a D44 for this much".

I have a LSD and 3.73s and 31" are as large as I want to go. It is no rocket ship but I know 33" tyres will require a regear, probably to 4.10s.
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Unread 06-12-2013, 08:48 PM   #7
war6191
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What is a good RPM for highway cruise with the 258? I need her to be able to run around decently. I have 31x10.5 on it with 3:73 gears

Thanks for the replies..
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Unread 06-12-2013, 08:53 PM   #8
BagusJeep
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This is a diagram of tyre size versus engine speed at 60mph for various gearings.

The theory is that there is a sweet spot on the engine speed where you can get enough power and back off the accelerator, maximising inlet manifold vacuum and hence reducing fuel flows. In other words a sweet spot of economy and power that maximises flexibility and driveability and economy.

That is supposed to be 2400rpm for a 258 but may vary according to what you have on it. 2000rpm is too low, you will have your foot down to increase fuel flows and it will drink fuel and lug.
gear4.gif  
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BagusJeep lives in Bali, the Land of Temples.
With a Jeep every prayer counts.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1995 Cherokee 4.0 - CHEROKEE
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Unread 06-12-2013, 08:54 PM   #9
war6191
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Also, would you recommend going full lockers or the LSD from Detroit. They get great reviews
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Unread 06-12-2013, 09:06 PM   #10
BagusJeep
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It depends on what you want, where you will take her and how DEEEP your pockets are.

Full air operated lockers are the best you can buy and priced accordingly. It requires an air supply and wiring etc so it is not a lunchtime install but if you are a really serious of roader you will probably aspire to something like this on both diffs.

An Aussie locker is at the other end of the price range but is a simple install and requires no wiring, compressors etc. Grip per buck it is one of the best.

The only cheaper locker is a lump of steel acting as a spool but it will lock your diff permanently.

A LSD is between the two, it will allow some slippage before the second wheel grips but will get you out of most situations in mild off roading and trails. If you do mild off roading and trails the Detroit is fine and it will have nice road manners.
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BagusJeep lives in Bali, the Land of Temples.
With a Jeep every prayer counts.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1995 Cherokee 4.0 - CHEROKEE
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Unread 06-12-2013, 09:23 PM   #11
LumpyGrits
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I have DL's, ft&rr.
If you live in snow country or where it rains a bunch. An LSD is a good choice.
Check out power-loc also.
LG
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Unread 06-13-2013, 02:22 PM   #12
agear
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Everybody says the 258 would be happiest at no more than 2500 to 2600 rpm on the ínterstate. If you have the money and or ability then the ox locker is great!. The con being a cable dangling but there is a con with the arb s too.
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1984 jeep cj7, 258 I6, rebuilt T176,rebuilt Dana 300, amc 20 , dana 30, 4.10's Trac-lok in rear , 4.10's detroit soft locker in dana 30, 2.5 inch bds suspension lift, 1 inch polyurethan body mount lift. Line ex. weber carburetor. factory tach. factory clock ,oem replacement speedomter cluster, and oil pressure gauge, factory volt gauge.
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