What transfer case? 3.9L Diesel Build

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What transfer case? 3.9L Diesel Build

Postby Adventure_Wagon88 » Sat Sep 16, 2017 6:42 am

Tomorrow I am picking up the motor to start my Diesel swap! It's a 1990 Isuzu 3.9L 4BD1T with a GM Bellhousing adapter. I'm super excited to say the least. Since it has the adapter to run a GM trans, and I'm a MT guy all the way, I plan to run a stout 5 or 6 speed behind it, either the NV4500 or NV5600 most likely. It seems everyone runs a different transfer case for different reasons and I'm trying to decide which to run in mine. This is going in an 88 GW, with factory D44's front and rear currently, though I intend to change those for built 1 ton axles in this process. I will be running locked diffs with 37s so I'd rather buy once/cry once and not have to worry about breaking on the trail. I want to settle on a transfer case before axles so I can decide whether to run a driver or passenger side diff front D60. I want to be able to drive this Jeep to Moab someday, wheel without reservation, and drive home. Of course I'd love to eventually have an Atlas under there, but I would rather spend that money on other things as this is not going to be a cheap build.

Side note: I do have a Dana 20 already which needs to be rebuilt, and I am not afraid to rebuild/upgrade anything. Ideal budget would be ~$1500 including all adapters.

So in order of importance, here's what I need in a TC:

Durability/Strength - must hold up to diesel torque! I'm not excessively hard on things but I can't be blowing apart TCs left and right
Lower than 2:1 ratio - even if that means going with a low range upgrade kit
Adaptability (would love some options that will bolt right to a NV4500)
Clocking that doesn't leave the case vulnerable
Twin stick-able :-)

Not a factor for this build:
Full time 4wd/AWD

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Re: What transfer case? 3.9L Diesel Build

Postby rocklaurence » Sat Sep 16, 2017 6:52 am

I would take whatever was bolted to the the trans that you buy. The 4500/5600 are late model trannies and their TC will most likely be on the Drivers side. Those NV cases will have a 2.72:1 low range.

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Re: What transfer case? 3.9L Diesel Build

Postby Stuka » Sat Sep 16, 2017 7:39 pm

Easiest t-case to use would be like an NV241HD. Most likely bolted to those transmissions already.
1981 J10 AMC 258 with HEI and Weber 32/36 DGEV Carb, T176, NP208, D44/M23 with 4.56 gears and Detroit Locker, 6" lift, 33x12.50 Cooper STT's, BJ's Offroad Rear Dozer Bumper.
2017 JKU Rubicon Lifted and stuff
Pevious Jeeps: 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ

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Re: What transfer case? 3.9L Diesel Build

Postby Adventure_Wagon88 » Thu Dec 14, 2017 10:13 am

Just wanted to follow up on this with some information. First the decision, then the explanation. I went Ford ZF-5 and NP203/205 doubler. Do I need a 4:1 crawl ratio? Maybe not, but what's it hurt?

So the engine I bought had a transmission adapter which it turns out would only work with GM automatics and older 4 speed transmissions. So to run a 5 speed I'd need a different adapter altogether, and adapters being the same price for a NV4500 vs Small Block Ford, I shopped for transmissions. I had been shopping for NV4500s for a few months and no luck, but scored a Ford ZF5-42 4x4 trans with SBF bell housing for an incredible deal. That sealed the deal to go SBF adapter. Turns out a heavy duty clutch for this application is also significantly cheaper than the NV4500 clutches I had seen too. The ZF 4x4 trans is basically the same tail as a C6 auto. I believe most trucks with this trans would have had the BW1356 case behind them.

After some research, I found the best T-Case for me was going to be the NP205. Well, the 205 doesn't bolt up to the ZF easily because of shift rail placement on the transfer case. For some dumb reason one of the shift rails on the 205 is literally within the mounting bolt circle. Some have cut the tail of the ZF to work, but it being cast aluminum that's not a compromise I was willing to make. So to run an NP205 the most sensible option was to run a NP203 rage box in front of it. I never liked the idea of a doubler because the ones I had seen involved cutting one of the cases. This one is all done by unbolting the stock cases and bolting them together with an adapter. The only custom parts are the adapter plate and adapter shaft which couples the 203 planetary output to the 205's input.

Basically the 203 + adapter adds about 8" to the front of the 205, and adds a selectable set of planetaries to double the low range. I saved about 3" off the back of my engine by changing adapters. The doubler is surprisingly only 1.5" longer than a NP229, and measuring from the surface of the water pump pulley (where mech fan mounts) to center of the rear output u-joint is only about 3-4" longer than the stock 360/727/229. I bought both cases for under $400 total, and the doubler adapter is going to run me $450-770 depending on which adapter I decide on. The adapter accommodates the 205's shift rail placement and gives an added cross member mounting point should I decide to use it.

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