Teraflex Transfer Case Upgrade - Gearing DownPosted in How To: Transmission Drivetrain on May 12, 2014
Back in the bad old days, going down a steep hill in a horse-drawn wagon or stagecoach called for desperate measures. The horses or mules couldn’t hold the heavy coach back, and it could even run them down. The drivers or muleskinners would tie a log or spare single-tree to the rear wheels to lock them up. The drivers would then have the horses drag the coach down the hill. This kept the horses and passengers alive, but the coach’s steering control was all but lost. The steering would have been vastly improved if the drivers could have figured out a way to hold back the coach and still allow its wheels to rotate.
Sliding down a steep, loose trail with both feet on the brakes can give a Jeep driver the same butterflies in the belly as the stagecoach driver. With the brakes locked up, steering becomes very iffy—the tires are sliding rather than gripping—so you’ll want the tires rolling. If they’re rolling, you’re under control; if they’re sliding, you’re on the edge of control.
Low range axle gears can provide the hill holding solution, but that also uses more fuel while on the highway. However, TeraFlex has a better solution to the butterfly problem—and it won’t use any more gas on the way to work. It’s a 4:1 low range kit that is installed in the transfer case. With our ’93 Jeep Wrangler YJ the transfer case is an NP231, and TeraFlex offers 4:1 kits for other models too. Only the low range is affected, therefore highway gearing is not affected and neither is highway mileage.
We also added another product to the NP231 transfer case for safety’s sake and longevity, a slip yoke eliminator kit. This also necessitated a different rear driveshaft. (If the slip yoke is eliminated, a driveshaft with the older style of U-joint is required.) A slip yoke eliminator kit is highly recommended. The nice part is we did it all ourselves at home—two people and a weekend can get things rolling.
Why Lower Ratios?
An OEM NP231 comes from Toledo with 2.72:1 low range and 1.00:1 high range. While this works great in most situations, when a Jeeper wants to become more adventurous with higher suspensions, taller tires, and more technical trails, lower off-road gears are needed. Lower gears allow the Jeep’s engine compression to slow the wheels down so that they rotate more slowly without using the brakes, holding the Jeep back while still rotating and keeping the Jeep’s steering under control. The lowered gearing also aids in traction control while climbing steep hills. Through gear multiplication, the Jeep’s torque can be multiplied for easier climbing. The ’93 Wrangler’s three-speed TorqueFlite automatic’s gear ratios are 2.74:1 in First, 1.54:1 in Second, and 1.00:1 (straight through) in Third. Since the YJ’s differentials are equipped with 3.07:1 gears.
Running the Numbers
Gear multiplication can work even better with a manual transmission because an automatic goes to First gear automatically, whereas a manual tranny can be shifted to Second or even Third to start off with slightly higher gears, lessening the chance of spinning the wheels during takeoff.
Differential gear: 3.07
Transfer case OEM low range: 2.72
TorqueFlite First gear: 2.74
These numbers mean that to understand by what number the engine’s torque is multiplied, you have an equation such as this:
3.07 x 2.72 x 2.74 = 22.88
With the TeraFlex 4:1 low range in place, the equation would read:
3.07 x 4.00 x 2.74 = 33.65
That it is an increase of 33 percent to the engine’s power and compression holdback. And that’s without changing the differentials’ gearing!