A transfer case is a part of the drivetrain of four-wheel-drive, all-wheel-drive, and other multiple powered axle vehicles. The transfer case transfers power from the transmission to the front and rear axles by means of drive shafts. It also synchronizes the difference between the rotation of the front and rear wheels, and may contain one or more sets of low range gears for off-road use.
Vehicles are equipped with at least one differential and some also have a transfer case. Each need to work efficiently in order transfer torque from the transmission to the wheels. A four-wheel drive (4WD) and an all-wheel drive (AWD) vehicle, however, needs a transfer case in addition to differentials on each axle.
Much like the rest of the powertrain under your 4×4, the transfer case has seen a steady evolution over the years. However, that’s not to say that all aspects of a modern T-case are better than those of yesteryear. Part-time, full-time, two-speed, four-speed, gear-driven, chain-driven—there are plenty of differences from one T-case to the next.
From the Toyota 4Runner to the Chevy Suburban, you can find the option of a full-time 4×4 transfer case. This means the T-case is always sending power to the front and rear axles. In most full-time T-cases, you’ll find some sort of limited-slip or open differential. These prevent the drivetrain from binding on the road and moderate the power split between the drive axles. Over the years, we’ve seen full-time systems become more advanced and refined. This has helped increase drivability and reliability, but you still are going to lose a bit on the efficiency front as there are more components to turn (and wear) with a full-time 4×4. To get an even power split needed for off-road driving conditions, you’ll often find a transfer case differential lock button, and/or you’ll need to shift the system into low range. Since full-time transfer cases are in many cases paired with high-horsepower vehicles, strength is rarely a concern. Depending on the vehicle, there are ways to convert to a more traditional part-time case, but don’t expect it to be an inexpensive conversion.
While full-time T-cases are something seen frequently in the SUV world, part-time transfer cases are the mainstay in trucks. A part-time T-case will give you the option of sending power to the rear wheels only. This means there are fewer components to wear out and will often equate to increased power and fuel efficiency. Shifting into four-wheel drive (4-Hi as it’s represented in most cases) will engage the front driveline, creating an even split of power between both drive axles. Like a full-time T-case, most part-time T-cases are two-speed units (more on that in a minute). This means there is a four-wheel-drive low option (4-Lo). The big difference between the two cases is that no additional action is required to achieve the 50/50 split in low range with the part-time case.
Crossovers, and even some pickup trucks such as the Honda Ridgeline, are generally paired with all-wheel-drive traction control systems. While some of these are surprisingly effective off-road, the lack of a two-speed transfer case keeps them out of flagship competitions such as Four Wheeler’s SUV and Pickup Truck of the Year. Two-speed simply means you can engage a single different gear ratio in the transfer case. Take the NVG241OR that’s found in the Jeep Wrangler Rubicon, for example. When the Jeep’s part-time case is in either 4-Hi or two-wheel drive, the ratio is 1:1. This means for every rotation of the transmission’s output shaft, the T-case output spins one revolution. By shifting into low range, you are changing from the 1:1 ratio to a much lower (numerically higher) 4:1. This means for every four times the transmission output shaft spins, the transfer case output spins only once.
The reason for having a two-speed transfer case is to use the torque multiplication the second gear ratio provides. For wheelers looking for even more gear reduction, there are a few options. One of the most common is to pair the 4×4’s stock two-speed T-case with an underdrive unit like the Magnum Box from Offroad Design. By adding a gearbox in front of your T-case, you have the option of multiplying the stock low-range ratio in the two-speed case by engaging the low-range planetary gears in the underdrive. For example, if your truck was fit with an NP205 and you mated it to a Magnum Box, you would now have four transfer case speed options. Since both the Magnum Box and NP205 have a 1:1 option, you can forgo any gear reduction for 4-Hi and 2-Hi driving. When you want a lower gear you can now choose from the NP205’s 1.96:1 or the Magnum’s 2.72:1. When both cases are placed into low range, the reduction gets multiplied, providing you with an even lower 5.33:1 ratio. Companies such as Advance Adapters also offer complete four-speed transfer cases, which can be had in a variety of low-range gear ratio options.
Gear-driven transfer cases remain the go-to in the world of competitive and serious recreation off-roading. The cases work by using a series of gears to split power to the drive axles. Gear-driven cases are known to hold up to high horsepower and extreme wheeling scenarios better than chain-driven T-cases but can be a little louder and heavier depending on the type. If you’re looking to upgrade your chain-driven case to a new gear-driven unit, you’ll need to look to aftermarket companies.
The most commonly found part-time transfer case under a given truck or SUV is chain driven. Unlike a heavier gear-driven unit, the chain-driven setup uses a heavy-duty chain to drive the front output when engaged. While the most common failure point is due to the chain stretching, modern variants of the chain-driven T-case are very durable. We’ve seen 1,000hp-plus diesel trucks abuse stock chain-driven cases with four-wheel-drive launches again and again without issue. In off-road applications, a more common failure point is rarely the chain, but the actual cast-aluminum case. So long as you can fit it with a good skidplate, a chain-driven case can serve you well.
Until 2006, nearly every Jeep Wrangler lifted over 3 inches needed a slip-yoke eliminator (SYE) or a transfer-case drop. What this does is change the output on the NP231 from a slip-yoke to a fixed yoke. In a stock configuration, the rear driveline slips in and out of the T-case as the suspension cycles. With an SYE kit installed, the slip element is moved to the driveshaft. This not only reduces the chance of the transfer case being damaged, but allows you to pair the Jeep with a double-cardan joint, which is better suited for the increased operating angle that happens when you lift the short-wheelbase Wrangler.
During the heyday of gear-driven transfer cases, most T-cases were housed in a cast-iron shell. These were incredibly heavy, but very durable. Nearly all transfer cases today are housed in aluminum at the OE and aftermarket level. Aluminum weighs less and dissipates heat better than its cast-iron counterpart. While the surface strength might not be as high, adding a skidplate can help take care of that shortcoming.
Depending on what type of lift you have or how neatly tucked you want your drivetrain to be, you may need to clock your transfer case. This simply means you are rotating the position of the case. This is done in lifted applications to decrease the front driveline angle. In more dedicated trail rigs, you’ll often find the transfer case clocked almost perfectly horizontal to achieve a flat bellypan. There are few methods of clocking your T-case, with clocking rings being the most common. There are numerous aftermarket companies that offer clocking rings.
For dedicated off-road rigs, you may find quick-change transfer case gearboxes such as the ones from Profab Machine are helpful. These are often divorced cases, meaning they are no longer married to the tailhousing of the transmission. You can find these styles of cases under diesel pullers, monster trucks, and mud dragsters at the pro level. As the name implies, you can quickly change the gearset out to modify your gear reduction for a given situation.
If you’re trying to determine what transfer case and low-range ratio your truck or SUV has, chances are it’s stamped on a tag on the back of the case. Even older cast-iron cases will often have the case ID casted on the outside of the housing. In the case of the New Process Gear case shown here, the top number (231) represents the model of case, with the J after it signifying that it’s from a Jeep. In a Chevy S-10, you might find a similar 231 with C stamped after the model designation. At the bottom of the badge you’ll see 272 stamped above ratio. This means it has a 2.72:1 low-range gearset.
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