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Advice On Which Way To Go With Drivetrain.


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Ive read up on a lot of different ways to do this but now I just want to hear what others have to say. I am converting my MJ to a Cummins 6AT 3.4. I am going to tune the engine to get the nearest I can on both power and fuel economy. But I can't decide what transfer case to go with. So I would be looking in the neighborhood of 140-175 hp and 250-300 lb-ft. My build thread has more basics about the engine being used. 

 

I like the idea of having AWD but if I am able to shift into a 2x4 mod that would be nice. I'm still figuring out what transmission to use, maybe an NV3500-4500 or a hybrid Jeep/Toyota transmission and tease. The Cummins has a chevy bell housing on it. Also I had thought I would need to covert to a front axle with the pumpkin on the passengers side between the turbo and exhaust are on the drivers side. But if I have the room I might pipe it under the engine like it currently is on the stock motor. Just don't know how a three inch pipe is going to fit. So I could use some advice on what way to go here. 

 

The biggest tire I intend to use would be a 33" but I liked using 235/85R16's and might just stay with those. The cummins fits in the engine bay so I won't need to worry about clearances as you would with a 6BT. 

 

The truck is intended as a daily driver, weekend warrior, lite towing and hauling stuff. 

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If it's a Chebby pattern, I'd make my life easy and install a 4L80E and a Chevy passenger side t-case(NP205 would be awesome). Add a D44HD in the front and a semi-floating 14 bolt in the rear and you get a durable, cheap to maintain drive train. The full width axles might be tricky to adapt to a MJ and would look a bit weird, though.

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If it's a Chebby pattern, I'd make my life easy and install a 4L80E and a Chevy passenger side t-case(NP205 would be awesome). Add a D44HD in the front and a semi-floating 14 bolt in the rear and you get a durable, cheap to maintain drive train. The full width axles might be tricky to adapt to a MJ and would look a bit weird, though.

 If I go the 4L80E route then I need to run either a controller or set it up to run manually. I think a 700R4 would work better. I will look into the axles and the NP205. 

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I would go with a 4L80 over the 700R4/4L60. not a fan of the 700. Stand-alone harnesses are easy to come by these days. And I wouldn't touch a semi-floating 14-bolt. Go full floting corprate 14-bolt. cheap and easy to find. An 8-lug 44 should be plenty strong for what you're building. If you're wanting an on-the-fly 2 to 4wd you'd probably better off with an NP 208 or NP 241

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FF 14 is MUCH stronger, I agree. The SF can handle quite a bit of punishment, though. I only mentioned the SF due to thw tire size he plans on using. I have 37's on my K5 with a healthy 400 SB and the SF14 has no issues with that set up. I think he'd be dragging the hell out of that FF14 with tires that small.

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A 700R4 from 87 onwards is rated at 450ft-lbs. That's assuming it's in brand new condition, pulling small-ish tires, and not seeing any abuse (towing, off roading, racing). Also, keep in mind that auto trannys develop their holding power through fluid pressure. If he's getting 300ft-lbs at 1000 rpm, the 700R4 might not have enough pressure on the clutches to holdbup to the torque. I still say 4L80E. Get a stand aline for the tranny, or, if you want to be bulletfproof and don't need overdrive, a THM400 or SM420 would be the way to go.

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As far strength and size goes, the Dana 20 is a fantastic case.IIRC, Jeep used them in a lot of FSJ's up to and including the J4000 trucks. The low rangr is only 2:1, but if you're not rock crawling, it should'nt be a problem. You may have to mod thr TH400's output shaft to get the two units to mate. I'm not to clear on the details. I had a T18/D20 combo for a long time in a J10 with a 4.2, and it was great. Small, light, and tough.

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As far strength and size goes, the Dana 20 is a fantastic case.IIRC, Jeep used them in a lot of FSJ's up to and including the J4000 trucks. The low rangr is only 2:1, but if you're not rock crawling, it should'nt be a problem. You may have to mod thr TH400's output shaft to get the two units to mate. I'm not to clear on the details. I had a T18/D20 combo for a long time in a J10 with a 4.2, and it was great. Small, light, and tough.

 

I have access to an adapter plate with gear that mates a Chevy TH400 and the Dana 20. 

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Just lept into my mind....

 

When I swapped from the T18/D20 combo in the J10, I threw in a THM400/NP205 combo out of a CUCV. It works well, but I did notice that my rear driveshaft is oh-so-slightly angled to the passanger side from the rear output of the 205. This leads me to believe that the Dana 20 has a slight offset. I have had ZERO issues with tuis set up. No broken joints or binding and I have plenty of travel. I just thought I'd mention it in case your TH400/20 combo causes your driveshaft to angle out.

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Just lept into my mind....

 

When I swapped from the T18/D20 combo in the J10, I threw in a THM400/NP205 combo out of a CUCV. It works well, but I did notice that my rear driveshaft is oh-so-slightly angled to the passanger side from the rear output of the 205. This leads me to believe that the Dana 20 has a slight offset. I have had ZERO issues with tuis set up. No broken joints or binding and I have plenty of travel. I just thought I'd mention it in case your TH400/20 combo causes your driveshaft to angle out.

 

Thanks.... If I had gone with one of my other choices Id be using FZJ80 front and rear axles with factory lockers. Then it would really be offset to the passengers side. :)

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