Throughout much of the '80s and '90s, four-wheelers were engaged in a one-upmanship game of "how low can you go?" Hard-core enthusiasts sought endlessly for lower gearing and greater strength in their transfer cases. While the game continues to a certain extent, it has been pretty much a one-horse race since Advance Adapters entered the market with the Atlas II transfer case. Featuring massive geardriven strength, a reasonably compact size, and an available 4.3:1 reduction in the original offering, the Atlas quickly became the 'case of choice for most extreme rockcrawling and competition applications. That has held true to this day, with the revered Atlas the ubiquitous choice for enthusiasts seeking heavy-duty lower gearing than their stock transfer cases can provide.
With a dominant position in the market, was Advance Adapters content to sit still and rest on its laurels? No way! As the years have passed, the team from Paso Robles has added additional reduction ratios (now stretching all the way from 2:1 to 6:1!) that allow a better match between particular applications and the gear reduction available and significantly increased the strength of the front output shaft by making a 32-spline offering available. Want more?
Advance Adapters delivers again. The company has significantly upped the ante this time by offering the Atlas 4SP transfer case. Yes, that's right: a four-speed Atlas transfer case! Starting with the venerable Atlas, the company has integrated a planetary reduction gearset into the front of the 'case, providing two additional reduction options in a single unit.
The six-gear planetary set utilized is the same assembly that was used in the NP241-HD transfer case. With a torque rating of 5,555 lb-ft, the strength of the planetary is the perfect complement to the well-established beef of the Atlas. The additional reduction housing only stretches the Atlas 4SP about 3.25 inches and adds a mere 15 pounds. The total length of an Atlas 4SP is only 17 inches, significantly shorter than the stock NP231, only slightly longer than the NP241 Rock Trac found in Jeep Rubicons, and definitely shorter than any of the other current options to add an aftermarket planetary gear reduction in front of a transfer case. Advance also offers a divorced version of the Atlas 4SP for those applications that require a separately mounted transfer case.
The planetary selector function is a model of simplicity. A small, easily thrown lever on the top of the reduction housing selects the mode. A nonsynchronized design, the planetary does require the vehicle to be completely stopped to make the gear change (the transfer case section retains its synchronized shift-on-the-roll capability). The selector lever is designed to be actuated by either a cable or electric shift option. The Atlas 4SP is currently available in two ratio combinations. The first offers 2.00:1, 2.72:1, and 5.44:1 reductions. The second offers 2.72:1, 3.80:1, and the astounding 10.34:1 options.
The 4SP includes as standard equipment the stronger 32-spline front output shaft, a redesigned shaft and bearing arrangement on the rear output, and slightly different machining on the aluminum case to accommodate the new configuration. Other than these changes, the transfer case portion of the 4SP is the same tried, used, abused, and proven Atlas on which we all depend. The new Atlas 4SP retains availability of both lefthand- and righthand-drop models, the multiple clocking options with four sets of mounting holes, numerous yoke choices, and most of the various input shaft spline counts that are offered on the original Atlas II.
I have an Atlas II in two of my trail Jeeps and absolutely love the deep Low range and abuse-proof attributes they offer. The only drawback I could find was the 4.3:1 reduction that I preferred for rockcrawling was too low and slow for moderate trail work. While great for tackling challenging boulder fields, the 4.3:1 was not so well suited for a simple dirt two-track into the mountains to find those boulder fields.
The Atlas 4SP solves that problem. With the 2.7 reduction ratio in the planetary set engaged and the transfer case in High range, the new 4SP offers a "stock" transfer case-like reduction for those long sojourns in the mountains or desert. If we run across a trail section that requires a little more control for elegant rock dancing, popping the planetary back to 1:1 and dropping the Atlas into its 3.8:1 position creates the deep gear reduction required to carefully crawl through the biggest rocks. Engaging the deeper gearing on both the transfer case and planetary? It yields a "snap-a-chalk-line-to-verify-if-it-is-moving" 10.34:1 reduction. In the Scrambler with its Chevy NV4500 and 4.88 gears, the total crawl ratio is now 324:1! That is a might too low to be of much practical use in my particular application, but an automatic transmission and higher axle ratio would utilize this "doubled-down" configuration quite nicely.
We hope you enjoyed stepping into the shop as we dropped the "old" Atlas II in our trail-worn '82 CJ-8 and replaced it with a new Atlas 4SP. We also used this opportunity to try out a slick new possibility from Off Again Auto that eliminates the common but bulky output flange to accommodate a 1350 CV driveshaft.