Four-wheelers are a diverse bunch. There are rockcrawlers, mud runners, trail hounds, and dune bashers. There are big-truck fans and small-SUV fans. Ford guys, Chevy guys, and Jeepers. Even a few Scout lovers.
But as diverse as we are, there is one thing that we have in common: If it weren't for the invention of the transfer case, we'd have other hobbies, like needlepoint. Or, if the call of the wild were just too strong, we'd get stuck an awful lot.
So we owe a debt of gratitude to that big hunk of metal slung under the cab. But sadly, to some 'wheelers, their transfer cases are just that: a big metal case that somehow, magically, drives the front wheels and lowers gears for climbing rocks. If that's as far as your T-case knowledge goes, give us a few minutes and we'll fill you in on what that hunk of metal is doing. Or, if your T-case expertise only goes as far as the full-time box under your old Blazer, allow us to bring you up to speed on newer transfer case developments.
The Basics
No matter how simple or complex your transfer case is, this is what it does: It splits the engine torque coming from the transmission and delivers it to output shafts that connect to driveshafts leading to the front and rear axles. Most transfer cases designed for off-road use are able to mechanically lock the front and rear driveshafts together, so each end of the vehicle gets an equal amount of engine torque.
Depending on the age and design of your transfer case, torque is delivered to the front output shaft either through a set of gears or via a chain. Most present-day T-cases are chain-driven, as the chains have proven to be lighter and quieter than gears. Hard-core 'wheelers tend to prefer the old-school gear-driven cases, however, because they can withstand greater amounts of torque without breaking. And gears will never stretch like a chain might when it's under extreme stress.
The age of your transfer case also has a lot to do with the material used to construct its housing. Older T-cases, like the NP203, NP205, and the Dana 300, were built with cast-iron housings. They're heavy, but bullet-proof. Since the 1980s, the quest for fuel efficiency has caused manufacturers to turn to lighter materials, like aluminum and magnesium, for T-case housings. These lightweight materials don't necessarily mean the cases are weaker, though. For example, Ford's Super Duty pickups carry NVG271 transfer cases with magnesium housings, and even the venerable Hummer has an aluminum housing around its NVG242 HD. But certain aluminum housings-like the NP208 and NP231-have earned reputations for weakness and cracking under severe-duty conditions. So if you're modifying your 4x4 with big tires or a torquey motor, make sure your stock T-case is up to the load, or consider a swap.
One Speed Or Two?
For most of us, our transfer cases share another common factor: They're two-speed units, which means a set of gears in the case-almost like an auxiliary transmission-can be engaged to lower the gear ratios coming from the transmission. This action, which occurs when you engage low-range, multiplies the torque going to the driven wheels.
For effective climbing and pulling power, you want as low a low-range ratio (as high a number) as you can get. Most current factory T-cases have low-range ratios of between 2.50:1 and 2.70:1, though you can go far lower for specialized applications. (See the Ultra Low sidebar for more on these transfer cases.)
Single-speed transfer cases, like those found in many cute-utes and all-wheel-drive cars, are designed to split engine power fore and aft without a direct mechanical link between the two ends of the vehicle, and without any provision for low-range gear reduction. They work fine on graded dirt roads and on wet pavement, but when the going gets tough, their abilities are limited.
 If you're junkyard shopping...  If you're junkyard shopping for an NP205, it's sometimes tough to tell it apart from the chain-driven full-time NP203 case. The quickest way to identify a '205 is by looking for this rear three-bolt cover on the idler shaft. |  Here's another view of the...  Here's another view of the venerable NP205. Six different versions of this case were made during its long life, with front outputs on both the left and right (shown here) side of the case. One downside to the '205: Its low-range ratio is only 1.96:1. |  A gear-driven transfer case,...  A gear-driven transfer case, like the NP205, uses separate sets of gears to achieve the high- and low-range ratios. The NP205 is one of the most common old-school cast-iron cases, having been installed on 4x4s from each of the Big Three truck makers. They were used as recently as 1991 on GM trucks. |
Another popular Jeep transfer...
Another popular Jeep transfer case is the gear-driven Dana 300, used aboard CJs in the 1980s. Its helical-cut gears are stout enough to handle a V-8's torque and it has a healthy 2.61:1 low-range ratio, but it's plagued with a small, 26-spline output shaft. Advance Adapters sells a beefier, 32-spline shaft that'll solve the problem.
Part-Time Or Full-Time?
It used to be that almost all transfer cases fell into one of two categories: part-time or full-time. A part-time transfer case is one that can shift between two- and four-wheel drive, allowing the driver to select the mode that best suits road conditions. Wear on the front-drive components is reduced with a part-time system, since they're not always engaged, and fuel economy is generally better for the same reason.
A full-time case is permanently in four-wheel drive, so no action on the driver's part is required to engage the system. In order to use a full-time transfer case on hard-packed surfaces without binding the driveline, the case must be built to allow driveline slip between the front and rear wheels. Some (like the NP203) contain differential gears inside the case, while others use some sort of clutch or viscous coupling between the T-case and the front driveshaft. In both cases, there is typically a means to lock the slipping mechanism for a true 50/50 torque split when conditions warrant.
We say these "used to be" the only two classifications because 4x4 makers have, over the past decade, developed all sorts of part-/full-time hybrid transfer cases that have broadened the driver's choices. Jeep was among the first with its NVG242 Quadra-Trac system, introduced on the Grand Cherokee, that enabled the vehicle to operate in two-wheel drive, full-time four-wheel drive (with the center diff open) and part-time four-wheel drive (with the center diff locked) in both high- and low-ranges. In 1999, Jeep further refined the system by mating a new NVG249 T-case with sophisticated locking diffs in the Dana 30 and 44 axles to create the Quadra-Drive system. General Motors also got into the hybrid T-case game in 1998, when it introduced the NVG246 Autotrac case, with a computer-controlled all-wheel-drive mode as well as the traditional part-time 4-High and 4-Low modes.
 This schematic illustration...  This schematic illustration shows the basics of transfer case operation. Power from the transmission runs into the case and is split to output shafts that are connected to front and rear driveshafts. This particular example is a chain-driven case, which is commonly found on most 4x4s made since the 1980s. It has a planetary gearset that reduces the transmission's gear ratios when the case is shifted into 4-Low. |  Current transfer cases typically...  Current transfer cases typically put the rear output in line with the transmission input and then offset the front output to one side of the engine or the other. That wasn't always the case, though. The arrows and lines on the Dana Spicer 20 (left) and Dana Spicer 18 show a couple of different power-flow paths. These gear-driven cases were used in Jeeps from the WWII era up through the '70s. The 20s were also used on first-generation Broncos. |  The latest developments in...  The latest developments in transfer case technology include hybrids that combine full-time and part-time functions in one unit. Shown here is GM's NVG246, otherwise known as the Autotrac system. It uses a number of sensors and its own computer to direct torque when in full-time mode, plus it has the traditional part-time 4-High and 4-Low modes as well. The Autotrac system is found on fullsize GM trucks and has made its way to the midsize TrailBlazer and Envoy SUVs. |
Swapping T-Cases
If you're planning to make serious modifications to your 4x4's wheel and tire size, or if you're going to put a lot more horsepower through the driveline than the original maker intended, you should consider a transfer case upgrade as part of your vehicle's buildup. As with any other component, T-cases are designed to handle a certain amount of torque loading and driveline stress. Chains will stretch or break and cases can crack if you exceed the T-case's limits.
There are a lot of factors to consider when planning a T-case swap. You may lust after one of the big gear-driven cases, but are you sure it will fit under your truck? If it's an older case from a salvage yard, is it going to need rebuilding? You also have to make sure the case will be compatible with your transmission, or that adapter kits are available, as mounting plates and output shafts vary in size and diameter. And because the transfer cases themselves vary in length, you may have to shorten or lengthen the front and/or rear driveshafts to get them to fit. Also, some T-cases are a mixed bag of good and not-so-good features. Your "ideal" swap candidate may have a weak output shaft, for example, or a case prone to cracking. Be sure to check with your local 4x4 shop or driveline expert while you're doing your swap homework.
As with every other vehicle component, transfer case technology is advancing at a rapid clip. Computers, gerotor pumps, viscous couplings, and clutch plates are taking over the work that used to be done by square-cut gears, twin-stick levers, and the driver's common sense. We'd guess this story, done 10 years from now, would be very different. But we'd also guess that, even 10 years from now, there'll still be 'wheelers out there with a fondness for cast-iron cases with gears inside.
Ultra Low
As rockcrawling (and other forms of 'wheeling) have gotten more extreme over the years, truck builders have realized a need for low-range reductions far beyond what's available from a factory T-case. So in the last few years the aftermarket has stepped in to fill the void. Several 4x4 specialty houses have developed 4:1 gearsets for the Dana 300 and several New Process transfer cases. Or, you could buy an entire ultra-low-range case, like the Atlas from Advance Adapters. It's a gear-driven unit, housed in a heat-treated aluminum alloy case, available with low-range ratios from 3.00:1 (for the Highlander model) to 4.30:1 (for the Atlas II Extreme unit). Another interesting feature on the Atlas is its twin-stick shifting mechanism, which allows engagement of the front and rear axles separately in low-range.
What T-Case Do I Have?
Unfortunately, we don't have the space to run an all-inclusive application chart for every 4x4 and transfer case combination. But just a little research will help you find out what's under your truck. You could start by climbing under and looking for a tag, which is usually located on the back of the case. If it's not there, or you can't find it, transfer case specialists like Advance Adapters or Motive Gear can tell you exactly what you have if you give them your 4x4's year, make, and model (provided your rig wasn't modified with a swapped-in case). Drivetrain Specialists of Las Vegas has a pretty comprehensive list of T-case applications on its Web site (www.drivetrain.com). Or, if you feel like doing some reading, pick up a copy of the Standard Catalog of 4x4s by Robert C. Ackerman, from Krause Publications. You'll learn a lot about your vehicle's history, as well as detailed driveline specs.
Crawl Ratio
Your transfer case's low-range ratio indicates its ability to multiply torque, but that is just one of three pieces in the complete torque multiplication equation. To really figure out how well your 4x4 can effectively use its engine torque on the trail, you should calculate its crawl ratio.
Multiply the three reduction ratios found in every four-wheel-drive vehicle: First gear ratio times the transfer case low-range ratio times the axle ratio. The final number is the crawl ratio, or the total amount of gear reduction and torque multiplication to the tires.
Let's use a Ford Super Duty to show you how this works. All Super Dutys equipped with the six-speed manual transmission have a First gear ratio of 5.79:1. And all Super Dutys have a transfer case with a 2.72:1 low-range ratio. Multiply those figures by 4.30, the lowest axle ratio available on the truck, and you get an awesome crawl ratio (for a stock truck) of 68. However, change the manual to an automatic trans, with a First gear ratio of 2.71:1 and, all else being equal, your crawl ratio is cut by more than half to 32. So when you're shopping for stump-pulling, wall-climbing power, keep all your gear ratios as low as you can go.