Pull up a chair, son, 'cause have we got a story for you. If you're sick of the wimpy stock gearing in your Jeep, believe us when we say you don't have to have a 100-inch wheelbase to run Advance Adapter's slick dual low range 4 Speed Atlas T-case. We added a 10.34:1 unit to a '97 Wrangler and actually gained an inch of driveshaft length. But we should back up a bit.
A couple years ago Advance Adapters took its exceptionally durable and versatile Atlas II, machined the case a bit, and added a low range planetary gearbox with a 2.72:1 gear reduction to the front. Behind this planetary, Advance Adapters stuffs the compact Atlas II T-case with either 2.0:1 or 3.8:1 low range gearing. That means you can have 1:1, 2.0:1, 2.72:1, and 5.44:1; or 1:1 2.72:2, 3.8:1, and 10.34:1. The result is brute strength, three shifters, and a gear for any occasion. And it's all in a package that's almost an inch shorter than a standard NP231 T-case equipped with a slip yoke eliminator.
For the installation, we leaned on TAG Motorsports in Escondido, California, because the shop is no stranger to modifying Jeeps and 4x4s. Shop foreman Jay Miller served up his '97 TJ as test fodder. Since Advance Adapters includes detailed, step-by-step instructions we'll only skim the highlights. But be sure to check out our sidebar, "Crawling Time" to see just what these crazy-low crawl numbers mean in the real-world.
Crawling Time
You can talk about super-low range gearing all day long, citing its increased benefits in control, finesse, and capability in certain off-road situations. But what does that equate to in the real world? To illustrate, we set up cones 100-feet apart and recorded the time it took to drive the course at 2,000rpm with the Jeep's AX-15 in First gear (3.83:1). The Jeep runs 4.88 axle gears for a final Low range of 193.25:1.
| T-case Gearing | Time (sec) |
| 1.00:1 | 7.50 |
| 2.72:1 | 17.60 |
| 3.8:1 | 22.20 |
| 10.34:1 | 60.01 |
Here's the Jeep's original...
Here's the Jeep's original NP231 equipped with a Teraflex slip yoke eliminator kit shown next to the 4 Speed Atlas. We measured the Atlas at 15 3/8 inches and the NP231 at 16 1/2 inches from the tranny mounting surface to the rear driveshaft flange. If you need even more rear driveshaft length Advance Adapters offers the Atlas with a short tailhousing option with no speedometer provision which adds an additional 2 inches of rear driveshaft length.
Advance Adapters includes...
Advance Adapters includes a -inch spacer you can install between the T-case and tranny to prevent the output shaft splines from bottoming. If you're going for maximum rear driveshaft length, you can opt to cut -to-1-inch off of the tranny output shaft. Either way, you should verify the shaft won't bottom in the case and/or there is sufficient spline engagement before cutting or modifying your transmission output shaft.
Since Miller's Jeep runs a...
Since Miller's Jeep runs a Teraflex BellyUp skidplate and no body lift, the shifter bracket for the 2.72:1 planetary gearbox contacted the floor next to the stock T-case shift cutout. Miller notched the floor slightly for clearance. The resulting bump out doesn't interfere with the factory plastic center console.
Here's the setup before, with...
Here's the setup before, with the NP231 sporting its slip yoke eliminator and 1310-CV driveshaft. The rear shaft measured 17 1/4 inches from the center of the rear driveshaft U-joint to the center of the U-joint at the CV. For the curious, Miller was able to retain his front driveshaft, as the before-and-after swap measurements remained the same at 42 inches between U-joint centers.
Miller also had to create...
Miller also had to create a bit of clearance in the BellyUp for the case of the Atlas. The Atlas offers several clocking options, so this step may not be necessary in Jeeps with a body lift that would allow the front output to be clocked upwards or Jeeps running the stock T-case skidplate.
The new rear driveshaft for...
The new rear driveshaft for the 4 Speed Atlas features a 1350 CV with 13 degrees more angularity than the 1310 CV it replaces. Since the driveshaft flange required for the 1350 CV eats up some length, the new rear shaft actually shares the same U-joint-to-U-joint measurement as the old driveshaft. However, had Miller chose to retain a 1310 CV driveshaft in the rear with a CV yoke the U-joint-to-U-joint measurement would have increased from 17 1/4 inches to 18 3/8 inches.
Advance Adapters has always...
Advance Adapters has always done a great job of making its shifters work within the confines of a stock Jeep interior and this 4 Speed Atlas application is no different. The twin sticks in the stock T-case shifter location operate Low, High, and Neutral rages for the front and rear outputs, respectively. The shifter next to the cup holders operates the 2.72:1 planetary box yet still allows full function of the factory E-brake lever.