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NP231 Transfer Case 4:1 Upgrade

Posted in How To: Transmission Drivetrain on May 1, 2012
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The New Process 231 transfer case is one of the most common and durable transfer cases ever produced. Found under Jeep Wranglers, Jeep Cherokees, Dodge Rams, and Chevy S-10 Blazers and pickups, the NP231 has proved itself reliable under some of the most hardcore rigs.

From the factory the NP231 is fitted with a 2.72:1 low range planetary gearset. On most stock applications this low range ratio is very sufficient, as it offers enough low-speed control for crawling while not being so low that you can’t create wheelspin for hillclimbing and mud bogging.

The 231’s cast aluminum housing and slip-yoke rear output have long been its downsides. Fortunately, dozens of slip-yoke eliminator kits are available from the aftermarket and protecting the lightweight aluminum case is usually as simple as building a skidplate.

The new Tera Low 231 HD front case comes with the 4:1 five-gear planetary gearset already installed. Our heavy-duty case was built with a 21-spline input to match our AX-5 transmission, but the more common 23-spline units are also available.

So what else would one want to do with such a well-rounded case?

Well, once you’ve fitted your truck or Jeep with heavy aftermarket bumpers, larger knobby tires, and traction aids like selectable or automatic lockers, you may find that the 2.72:1 low range isn’t enough gear reduction. Sure, a V-8 under the hood could take care of most of your power woes, but that simply isn’t a cheap option. Take our ’00 Jeep Wrangler for instance. Powered by a 2.5L four-cylinder, AX-5 manual transmission, and an NP231, the Jeep works well but is often down on power off-road.

Once your original 231 is torn apart you’ll begin transferring over your stock drive gear parts. Though a two-wheel low range option is available from TeraFlex, we opted to go back with the original 2-Hi, 4-Hi, neutral, and 4-Lo shifter.

Like most rigs, the Jeep was fitted with numerically higher 4.88 differential gears to help compensate for the larger-than-stock, 36-inch-tall tires. Though the lower axle gears help, the Wrangler still heavily lacks in low-end torque. So to provide the Wrangler with more grunt, we opted for a 4:1 Tera Low 231 HD transfer case upgrade. The Tera Low 231HD case from TeraFlex replaces your 231’s original front half with a cast aluminum unit that comes fitted with a heavy-duty five-gear 4:1 planetary gearset.

Increasing the low-range ratio from 2.72:1 to 4:1 effectively works as a torque multiplier, allowing the Jeep to crawl more easily in low range.

Since our original 231 front output bearing was in fine working order we transferred it to the new housing. We found that a hammer and seal driver worked great to install the bearing and new seal.

To perform the upgrade, we dropped the case from the Jeep and hit the work bench for an afternoon of wrenching. It took us roughly half a day from start to finish, and once we shifted into low range we noticed an immediate power difference. Though our 2.5L may not be a powerhouse, the extra gear reduction has given our Jeep the performance it lacked and confidence to take on more challenging terrain.

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Advance Adapters
Paso Robles, CA 93446
Murray, UT 84107

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