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Thread: Transfer Case Gear Ratio Change

  1. #1
    Registered german_cargo's Avatar
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    Default Transfer Case Gear Ratio Change

    I haven't see anything about this here, but instead of regearing both front and rear diffs in my jeep is it possible to change it in the TC? I have select trac... NP231 I believe? I assume you can't do it because no one's talked about doing it but I know that there are two gears with a chain around them which I assume makes the transfer case work and I can't help but wonder why you can't get some smaller gears and effectively "re-gear to 4.55" or something close. Someone on here sent me a link to a website that calculates what speed you'll be at and what RPMs in each gear with different diff ratios (its designed to help people figure out which gears to get when they lift and get bigger tires) and there's a spot in the formula for your transfer case in / out ratio so this is what got me thinking.

    Could someone explain to me how and why this would or wouldn't work?
    1998 ZJ 4.0L. Selec Trac. 225K kms. K&N panel filter. Liberty Rims.

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    Registered ryan86jeep's Avatar
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    its the 242 not the 231

    most gear swaps are for low range in TCs
    ive never heard of a high range one
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    Quote Originally Posted by german_cargo View Post
    I know that there are two gears with a chain around them which I assume makes the transfer case work and I can't help but wonder why you can't get some smaller gears and effectively "re-gear to 4.55" or something close. ....
    Could someone explain to me how and why this would or wouldn't work?
    The two gears and the chain are what connects power to the front driveshaft. That is the 'transfer' part of what makes the transfer case work. There is no gear reduction involved in this 'back half' portion of the transfer case. In the 'front half' of the transfer case is a planetary unit that provides either a 1:1 direct (hi range) drive or a lower, say 2.72:1, (lo range) that multiplies the torque delivered to the front and rear axles for 'extreme' conditions.
    Theoretically, it should be possible to change this planetary unit to whatever ratio you might desire, but the downside is that your new 'lo range' might only be 1.20:1 netting an effective axle ratio of ~4.56 while in 4Lo. But thats all you get. And other mods would be needed because 4Lo 'locks' the transfer of power in the 'back half' which means on-pavement operation is sure to equal driveline damage .. can you get a twin-stick kit for a 242 to workaround?don't know..? Needless to say, by the time you get a custom-planetray setup, by or build a twin-stick to get 2Lo option, rebuild your t-case while its out, yadda yadda yadda ...
    not sure if you'd be ahead economically, but the permanent loss of a deep lo range would certainly hinder off-road performance.

    I forget ... do you have the 6 or 5.2???
    There is an option if you have a 5.2 with the 46RH to swap the low gear set in the transmission for more 'launch' without changing the final drive ratio. Swaps the 2.45 1st gear ratio for a 2.77, same as going from 3.73 to 4.27 axle gears. Its old school, popular with RVers as well as 1/4mile cars.
    Tiggers

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    Registered german_cargo's Avatar
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    I have the 6... I'm getting the tranny rebuilt though, would I be able to do that swap while its being rebuilt?
    1998 ZJ 4.0L. Selec Trac. 225K kms. K&N panel filter. Liberty Rims.

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    nope. Your 42RE already has the 2.77 1st gear set. 'Tinkerers' take the set from the 'junior' transmission and do some splicing to make it work in the larger transmission. The reason why the smaller trans has the lower gearset is to make it more liveable to have a large heavy car with a less powerful engine. Most people judge things by the 'seat of the pants accelerometer'.

    I wonder if someone makes an underdrive gearbox that you could install between the transmission and the transfercase to get what you want. Hmmm .. would mean custom length front and rear driveshafts, some custom linkages ... but then you would have Hi (1:1), not so Hi(1.2:1), Lo (2.72:1), and superLo(3.26:1). hmmmmm
    Tiggers

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    Registered german_cargo's Avatar
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    Quote Originally Posted by TiggersBounce View Post
    I wonder if someone makes an underdrive gearbox that you could install between the transmission and the transfercase to get what you want. Hmmm .. would mean custom length front and rear driveshafts, some custom linkages ... but then you would have Hi (1:1), not so Hi(1.2:1), Lo (2.72:1), and superLo(3.26:1). hmmmmm
    Well wonder all you like but I'll tell you what... If you come up with something that costs less than a diff regear you'll be a rich man.
    1998 ZJ 4.0L. Selec Trac. 225K kms. K&N panel filter. Liberty Rims.

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    Registered mendelmax's Avatar
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    german_cargo, you are talking about t-cases like in Suzuki. There, there is a low range gear, and next there is a chain, that transmits the torque to the side of the t-case. From there, both front and rear driveshaft goes to the axle. Indeed, in suzuki you can change these gears and achieve different final gear ratio, avoiding diff regear. Fortunately Jeep has only front driveshaft propelled of the chain- the rear one goes straight from the t-case mainshaft. This excludes the possibility to regear by chaing gears, but gives you big durability and strength benefit. Look how many people here had to change their t-case chains. Now take a look on suzuki forum. Tons of them. Simply because in jeep chain takes at most 50% of the work, and in many cases less (you rarely drive 4x4, usually RWD)
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