Without a well-built transfer case, your 4x4 is far from performing at its intended capability. This fact is further compounded after suspension lifts and big tires come into play. By addressing the internals of your transfer case, you can easily create more Low-gear 'crawling ability while on the trail, but still have a higher ratio in the differentials to keep highway driving a smooth-cruising endeavor. Check out the following to get a few ideas about how to beef up your transfer case and ensure continued happy trail-running, with the least amount of gear envy for those who reach the top of the obstacle before you.
SYE And Tailshaft Conversions
Slip-yoke eliminator (SYE) kits are designed to upgrade the stock slip-yoke assembly at the rear of the NP231 transfer cases to a fixed-yoke output. A slip-yoke is a splined sleeve that surrounds the output shaft for drivetrain-length variation. One of the problems with this design is that the rear driveshaft can't be removed without the fluid draining out. This can be troublesome after suffering driveshaft breakage on the trail that requires limping out in front-wheel drive. The slip-yoke also adds driveline length and typically won't allow use of a CV driveshaft. Slip-yoke eliminator kits are also referred to as tailshaft conversions, as many kits also offer the opportunity to upgrade to a larger or heavy-duty output shaft. Tailshaft conversions are also available for Dana 20 and Dana 300 transfer cases and are great means of strengthening a stock transfer case.
Although it was a factory option on some early model transfer cases, many can still be converted for twin-stick operation. Twin-stick shifters offer the ability to disengage the front drive axle so 2-Lo can be achieved. This is particularly advantageous in vehicles equipped with front lockers, when a tight turn up an obstacle or even into a parking spot may bind the frontend. One shifter is designed to control front axle engagement in Hi, Neutral, and Lo, while the other produces the same function in the rear axle. Twin-stick shifters are available for Dana 300s, Dana 20s, Toyota pickups and 4Runners, and others. Teraflex offers a pseudo-twin-stick option for NP231 transfer cases, with its 2Low231 kit designed to provide 2-Lo or 4-Lo with a mechanical disconnect for front and rear output shafts while in Neutral.
Being that the majority of popular transfer cases suffer from a lack of respectable Low-range gearing, swapping in a set of aftermarket transfer-case gears is a popular option. Most models of transfer cases are afforded this ability through the aftermarket, though some have more variety to choose from. The NP231 and Dana 300 are offered in as much as 4:1 reduction; the Dana 18 and 20 can upgrade to 3.15:1 reduction. Toyota pickup and 4Runner transfer cases can achieve Low-range ratios of 5.0:1. Until recently, Toyota Land Cruiser owners had limited options, but the new Orion transfer case from Advance Adapters now satisfies its need with a low 4.0:1 gear reduction. Suzuki owners have a wide variety of gear selections to choose from, including three low ratios of 4.16:1, 5.14:1, and 6.50:1, available from Calmini Manufacturing.
Replacement T-Cases and Gear-Reduction Options
When you've exhausted every option to achieve more performance from your stock transfer case and still find that you desire more, swapping in an alternate stock 'case with more upgrade offerings or even a purpose-built unit designed for maximum gear reduction, is an excellent means of increasing your vehicle's trailcrawling ability. The aftermarket is well stocked with purpose-built Low-geared 'cases designed as direct replacements for nearly every model of transfer case discussed in the "Guide to Popular Transfer Cases" article in this issue.