A Systems Approach to
|Jason Bunch –A die Hard stick man, one of the few leading ARCA competitors to use a stick. Jason Runs the stock Jeep AX4 with a Klune underdrive into an Atlas. (that's me, with the Green Bronco, following Jason - we're headed through Surprise Canyon - 700R4 Auto into Klune into Np205F Tcase)|
Automatics provide less compression braking on steep down hills. (Not true if you get the rest of the gearing equation right)
Automatic transmissions make heat, cause overheating problems.
Automatics are less reliable.
Humbug: If you prefer the driving characteristics of an automatic, don’t be scared off by these concerns: These problems have technical solutions. Some of the top “big-dog” and competitive drivers use automatics.
This NP435 transmission has a 6.35 to one granny 1st gear. This tranny could be used to replace a stock close ratio tranny for extended low gears.
What I am calling "granny trannies" are 4 and 5 speed transmissions that are extremely wide ratio, and have a very low “granny gear” 1st gear. Popular granny 4 speed units are the SM420, SM465, NP435, T18 and the T19. Granny trannies can get your crawl ratio down into the 70 to one range still keeping sensible axle ratios.
The “granny” first gear is really too low to be generally useful on the street. Typically, you would drive them like a three speed using gears 2-3-4 around town, and the granny gear only when you needed that extra low gear.
|NV4500 5spd is an extremely popular swap|
Granny-Gear 5 speeds: The most popular is the NV4500, which can be adapted to a wide variety of applications. Others are available. A fifth gear is a wonderful bonus. Allowing a lower axle ratio with excellent highway gears.
Most transfer cases come from the factory with low range gearing between 1.96 to one and 2.72 to one. With a low geared transfer case, high range stays the same for the street, and you are geared lower for the trail. The main drawback is that while you get the lower gears, you lose the original low range gearing, which is usually best for sand and easy trails, and sometimes better for mud or hill climbing.
A popular compromise is to replace the gears in the factory transfer case with lower gears . Replacement gear sets are available for various T-cases with between 3.15 and 4.88 ratios. 4:1 replacement gear-sets are available for most Jeep transfer cases from 1980 on. The nice thing about this modification is its relative low cost (around $750-1100, plus a bit more for installation) and that once you put the new gears in the stock case, it all bolts back together without any other changes.
|Atlas Transfer Case on rear of NV4500 5speed|
The Atlas II Transfer Case is an extremely important contribution to this category. Developed and manufactured by Advance Adapters, the Atlas is a complete, purpose built fully gear driven transfer case available as a bolt in replacement for a wide variety of applications. It is available ina conventional 2spd case in 2.0, 3.0, 3.8, 4.3, 5.0 and 6.0 low range ratios It is designed as a fully synchronized unit: This helps make up for the fact of losing the in-between range because you can go between high and low range while driving without stopping. This unit is also a length improvement over the chain drive cases used in most 4x4s since the late 80s. Drawbacks are relatively high cost (Over $2k for the unit), plus you will most likely need to modify at least one of the driveshafts.
April 2006 - the New Atlas 4 speed begins to ship. This unit qualifies as an "Ultimate" system as described below
This type of system has by
far the greatest flexibility and versatility in gearing options.
Properly done, it is the Cadillac option: Awesome. In
this manner you can accomplish enough gearing flexibility to always find
the right gear from highway all the way to extreme crawl ratios. Cons:
Tends to make a longer drivetrain, and is a more complicated
Klune-V Extreme Underdrive shown bolted in between 700R4 Automatic Overdrive Transmission and Dana 300 Transfer Case. This 4:1 ratio unit, coupled to the 2.72 ratio NP241, allows driver selection of 1:1, 2.72:1, 4:1 and 10.88:1 effective Tcase ratios
An "Extreme Underdrive" is an underdrive gearbox that bolts in place between the transmission and transfer case. We call it "extreme," because the underdrive multiplies your existing gear reduction by up to 4 times, rather than a usual underdrive of 1.2 times, or so. It is intended for extreme four-wheel-drive applications, rather than towing or gear splitting in a big truck. It works equally well with both manual and automatic transmissions. Pros: Because the unit can be purchased with a 4 to one ratio, when used with a conventional transfer case typically between about 2 to one and 2.7 to one) up to four different transfer case ranges can be selected: Both boxes in “High”, you get a 1:1 highway range. Put the Tcase in “low”, leaving the underdrive in “High”, you get a 2 to 2.7 to one transfer case range. Put the Underdrive in “low, leave the Tcase in “high”, you now have a 4:1 low range. Put them both in “Low” you get an extreme crawl range, equivalent to transfer case gears of between 8 to 12 to one.
(Dealing with the short wrangler driveshaft issue)
(Direct Replacement for Wrangler NP231 Tcase, Has 3" shorter overall length than stock NP231 Tcase)
A Dual Transfer
Case is a method of doing a similar thing. A transfer case is essentially two parts: A two
speed gear box (High and Low ranges) and the a system of gears or a
chain to send power to both the front axle and the rear axle. What is
done in a dual transfer case is the reduction aspect is used, discarding
the front-axle drive portion. A systems of adapters, shafts and custom
shifters bolt the second transfer case’s reduction portion between the
main transfer case and the transmission. This system accomplishes much
the same thing as the Extreme Underdrive, and is sometimes a less costly
alternative due to the lower cost of the recycled parts from the
“donor” transfer case. Drawbacks to this design is that they are
frequently heavier or clunkier than the purpose designed “Extreme
Underdrive”, and won't usually give four gear ranges discussed
The "Doubler" consists of the gear reduction portion of an NP203 Tcase coupled to the main transfer case. In the example shown the Doubler is mounted to an NP205 transfer case, allowing possible effective transfer case ratios of approximately 1:1, 2:1 and 4:1.
This unit is functionally a Klune-V Goliath bolted to either a 3.8 or 2.0 Atlas. (Klune-V / Atlas Combo - Article) There is an advantage to this Advance Adapters version: By purpose-engineering the combo, Advance Adapters has done a nice job of integrating the assembly, shortening the overall combo by 3.5". Another benefit of this system is that the Atlas 4-Speed as a unit costs less than buying a Klune and buying an Atlas 2 speed and combining them
As you can plainly see, there are a vast number of options to consider. Read through these pages. Then give us a call, we are happy to work with you to sort out the optimum gearing solution for your rig.
Then, if useful, we can research out all of the details to make your system a reality, and provide you with a complete kit.