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NP231 doubler / crawlbox

LincolnJr

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Jan 30, 2021
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Harrisburg, PA
Looking for info/experience about building a t-case doubler.

06 LJ with 6spd/231/3.73 on 33s. I’d like to have lower gearing options for better off-road control that won’t effect on-road behavior.

  • as far as I know, my NP231 can’t be regeared to 4:1 or anything like that (Teralow 4:1 is no longer a thing, and probably for good reason since they apparently blew up a lot)
  • swapping t-cases for a NP241 might be an option, but they’re expen$$ive and it’s not a straight plug in
  • regearing the diffs is also expensive and/or time consuming, and I don’t think going to 4.10 would be enough change to justify it, and going to 4.56 would change highway driving (higher RPM)
  • Rubicrawlers can only be mated to 42RLEs

So that leaves me with the intriguing option of building a doubler… but also some questions:
  • most search results for doublers/crawl boxes/etc are from 10 years ago or so, which seemed like a red flag – is there a catastrophic reason not many people go this route? Or just that most people don’t have the extra wheelbase to extend the drivetrain and/or are satisfied with regearing or other options?
  • I’ve found a few options:
    • NWF has their BlackBox, which is as complete of a setup as you can buy off the shelf and the most expensive (but still cheaper than buying an Atlas or even most used 241s!) ~$1700
    • NWF also has their EcoBox, which is several hundred $ cheaper, you just have to supply all of the guts from a donor 231 and have access to a press to install the ring gear. ~$1000
    • Froehlich Suspension makes an adapter plate, which requires cutting up a donor 231 (removing the front driveshaft yoke) and welding the adapter in place of the back half. $475 + donor 231 + probably paying a shop to do the welding

All doubler options would require fabrication of a mount to help support the longer drivetrain, SYE and new driveshafts (longer front, shorter rear), and finagling double t-case shifters. Also, I would upgrade my NP231 with the 6-pinion planetary gear and wider chain.

So… what else can/should I consider? Has anyone built a doubler with an adapter, or know where such a build is documented?
 
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LincolnJr

LincolnJr

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Changing your highway driving rpm higher is a good thing. Better fuel mileage, better throttle response, less downshifting.
Better gas mileage? I understand the other two benefits, but that part is counter intuitive to me because the rpm would be higher. But if it will be better, great!
 
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LincolnJr

LincolnJr

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We are running a NWF Ecobox and a flipped Dana 300. Info in my build thread.
It is great off-road but will not help you on road. Gearing your axles properly should be done first.
Doing everything myself the above setup ran me about $2500.
Good to know. Both the total cost and that regearing should be done first. Thanks.
 

Gollywomper

TJ Addict
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Feb 9, 2019
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Corning CA
No additional mount was added. After research and talking to advanced adapters the standard transmission mount is adequate. Adding another mount will only cause more potential issues.
 
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TJ Starting

Jeep or sleep
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Jan 22, 2019
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SE TX
Better gas mileage? I understand the other two benefits, but that part is counter intuitive to me because the rpm would be higher. But if it will be better, great!
This explains it somewhat..
Screenshot_20220111-173358_Chrome.jpg

Basically running at higher rpms at the same speed will require less foot in throttle, thus less fuel then lower rpms requiring more foot in throttle using more fuel. The idea is to regear to a level where the engine is as efficient as is practical. 3.73 with 33s is a little ways below below that line for the 4.0.
 

SkylinesSuck

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Northern VA
That is just somebody's reasoning on the subject, not fact. You aren't going to improve mpg with steeper gears, a stroker, a supercharger, or anything of the sort. That being said, I think 3.73's are undergeared with 33's on the road with a 4.0 and a manual transmission.

All that aside, I'm interested in the initial subject as my gearing is fine for the street but not enough for the trail.
 

SkylinesSuck

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And i just noticed the longer drivetrain part of the equation which makes the above options tough for a TJ. LJs would probably be fine though.
 

SkylinesSuck

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Lots of slipping the clutch and less control until the clutch is all the way out (but then you are going "fast"). I had 6.13 gears or something crazy like that in my Jimny with 31's and a 660cc 3 banger. I was amazed at what I could do by very very slowly creeping over stuff instead of pointing and shooting like I have to do with the TJ. It's a huge advantage that the slushbox driver's enjoy.
 
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LincolnJr

LincolnJr

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That is just somebody's reasoning on the subject, not fact. You aren't going to improve mpg with steeper gears, a stroker, a supercharger, or anything of the sort. That being said, I think 3.73's are undergeared with 33's on the road with a 4.0 and a manual transmission.

All that aside, I'm interested in the initial subject as my gearing is fine for the street but not enough for the trail.
Ok, so even if it doesn't improve the mpg, what you're saying about 3.73s being undergeared agrees with the points about the engine needing to be pushed harder even if its running at lower rpm. I had been thinking I might be able to get away without regearing, but if I do go that route I will apparently appreciate it on the highway more than I thought, so that's nice.

Have you seen any videos of Tacomas with their Taco Boxes and Marlin Crawlers? Seriously impressive! And I would love to have those kinds of gearing options.
 
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LincolnJr

LincolnJr

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Lots of slipping the clutch and less control until the clutch is all the way out (but then you are going "fast"). I had 6.13 gears or something crazy like that in my Jimny with 31's and a 660cc 3 banger. I was amazed at what I could do by very very slowly creeping over stuff instead of pointing and shooting like I have to do with the TJ. It's a huge advantage that the slushbox driver's enjoy.
Last time I went offroading I realized I need to learn better throttle control. I've driven manuals my whole life, but always with low-torque, higher revving 4cyls. My habit is to rev the engine to keep from stalling while engaging the clutch, but that's not great for rock crawling situations and I end up with tons of wheel spin and bouncing around, even in 4lo. Since then I've been practicing engaging 1st gear with as little throttle input as possible so next time I'm out on the trails I will be more gentle. Again, watching these Marlin Crawler dudes creeping uphill at an idle (or even just with the starter!) made me wish for some double low option.
 
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SkylinesSuck

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It's not a matter of controlling the throttle per se. It's balancing the throttle and the clutch while trying to move the Jeep slower than what 1st gear at idle with the clutch completely engaged does. Going slow lets you choose your lines way more precisely and slip less. If you are going slower, you can modulate wheel speed with just throttle instead of having to balance it with the clutch as well.
 
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LincolnJr

LincolnJr

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I can see the hand throttle being helpful for ascents. For descents, I think learning to ease the brake while feathering the clutch is going to be the key, but that's so counter intuitive to how we typically drive manuals. Normally we're afraid of stalling out so its either rev up or clutch in, but either way going downhill will be too fast. I've gone down entire descents just in neutral and riding the brake, but that's not ideal for control or for the brakes. I need to learn new footwork for those situations. And again, having a super super low gearing option would be amazing.
 

jesseshoots

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Feb 20, 2019
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Charlotte, NC
Descents or ascents? Because starting on an uphill would be where a hand throttle could shine.
I've been driving manual long enough that I'm able to get onto the throttle and manage the clutch quickly enough while going up. Going downhill gives me problems for some reason. I typically let the clutch out while braking until it starts driving itself but with lower gearing I feel like I could avoid that awkward phase.

I can see the hand throttle being helpful for ascents. For descents, I think learning to ease the brake while feathering the clutch is going to be the key, but that's so counter intuitive to how we typically drive manuals. Normally we're afraid of stalling out so its either rev up or clutch in, but either way going downhill will be too fast. I've gone down entire descents just in neutral and riding the brake, but that's not ideal for control or for the brakes. I need to learn new footwork for those situations. And again, having a super super low gearing option would be amazing.
I learned early on by getting myself in a super sketchy situation to always power down hills. I was engine braking on a loose dirt descent and the back end started sliding. I could have sworn I was about to lay it on the side.