The Holy Grail
The Atlas II Transfer Case has come to be the "Holy Grail" of performance transfer cases for recreational, and competition four-wheelers. The gear-driven Atlas II boasts superior strength and either 3.0:1, 3.8:1 or 4.33:1 gear reduction. Twin stick operation allows individual control of front and rear axles on the trail. These features, and more, help make the Atlas II Transfer Case a favorite among serious four wheel drive enthusiasts.

We chose the Atlas II for our application because it offers exceptional strength, improved gear reduction, and individual axle control. While other transfer case upgrade options may be more cost-effective, the Atlas packs in a much more comprehensive set of features in a new and ready-to-install package.

The case itself is a rock-solid, one piece design, machined from 356-T6 Heat Treated Aluminum Alloy. The Atlas II is 2.5 inches shorter than an NP231 with a short shaft conversion kit. This will allow us to run a longer rear driveshaft, and further reduce the potential for driveline vibrations. Our transfer case is going to be equipped with a 32 spline front output shaft and a stronger yoke, which is common for 1/2 ton and 3/4 ton trucks. This feature can be identified by a blue anodized front output seal retainer.

Equipment
Our Atlas II is going to be mated to an AX-15 manual transmission in a 1999 Jeep Wrangler, thus, it will be configured with a left-hand drop, 4.33:1 reduction, a 23 spline input, and a CV-1310 rear output. This setup typically retails for around $2,400.00. In addition, we will be adding a 32 spline front output with a CV-1310 yoke, which adds another $300 to the total.

The Atlas II will be replacing an NP231 Transfer Case equipped with a Tom Wood Short Shaft Conversion. The NP231 is a cast aluminum, two piece design, that has a low range ratio of 2.72:1, and is chain driven as opposed to gear driven. While the NP231 has proven dependable, it is by no means bulletproof, and does not offer nearly as much peace of mind as the Atlas II.

The recipient TJ is equipped with approximately 2.75" of suspension lift, 32" tires, and original 3.73 gears housed in Dana 30 and Dana 35c axles. The Atlas II will be providing more than enough strength for this application and will help provide a competitive crawl ratio. Perhaps overkill for this vehicle configuration, the Atlas II offers strength, performance, and room to grow. If the hefty price tag fits within your budget, you will likely be purchasing the last transfer case you will ever need.

Gear Reduction
The primary feature that drew us to the Atlas II Transfer Case is the 4.33:1 low range ratio. Our moderately-equipped Jeep Wrangler is a relatively comfortable daily driver with great high range gearing for all pavement conditions. We do not plan on performing a ring and pinion swap as long as the vehicle is equipped with 32" tires. The Atlas II will allow us to keep the good street gearing, but it will help provide a much-improved crawl ratio for technical terrain.

Currently, the NP231 with our transmission, and differential gear ratio provides an ultimate crawl ratio of 39.86:1. Enter oversize tires into the equation and this ratio can quickly become inadequate. In terrain like that pictured on the right, being able to effectively control your slow speed is almost mandatory.

Not being able to go slowly and carefully can result in catastrophic damage. On the other hand, too much use of the clutch in such a situation might result in temporary, or permanent clutch failure. One needs to be able to keep the speed to an absolute minimum with little, or preferably no use of the clutch.

With proper low gearing for extreme terrain, the driver can pay more attention to steering and watching the spotter, and pay less attention to speed control and keeping the engine alive. Our Atlas II will provide us with an ultimate crawl ratio of 62:1, which brings us closer to blissful rockcrawling. While 62:1 isn't "extreme granny" by today's standards, it is very good, and considerably better than stock.

Photo: Michael McCready

Strength
Overall strength is the other feature that makes the Atlas II transfer case so appealing.

The cast aluminum split case design is no doubt the weakest characteristic of the NP231 transfer case. A common problem that many older NP231 transfer cases experience is a cracked housing, which can mainly be attributed to fatigue. These transfer cases can also easily be destroyed by the front axle being rammed with exceptional force. As a result, the front driveshaft over-compresses and breaks through the front output section of the transfer case. General internal failure, although uncommon, can also cause an NP231 to be blown apart, as shown in the picture on the left.

The rock solid Atlas II transfer case weighs in at 110 lbs (dry), which is approximately 40 lbs heavier than the NP231. The case isn't just strong on the outside, though. The internal workings of the Atlas II are complimented by large helical-cut gears, and an assortment of heavy duty components. Most chains, in chain-driven transfer cases, are prone to stretching out over time, but this gear-driven alternative will likely last forever.

The Atlas II is built to be used and can easily cope with the rigors of serious four-wheeling.

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