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How Transfer Cases Work

Today, there’s so much diversity amongst four-wheelers. Whether you’re a dune basher, mud runner, rockcrawler, trail hound, big truck, jacked up SUV, small SUV, Jeep, Chevy, Ford, or even Scout kinda guy or gal, there’s room for everyone at this party. The dust is bringing a tear to the old eye in all this comradeship, right? Seriously, though, while we all have our own four-wheeling preferences, we also have one common thread, or should I say part, that unites us from the word go. Can you guess what it is?

If you said the transfer case, or T-case, you’d be right. Without the transfer case, all us four-wheeling hobbyists and enthusiasts would either be sitting around bird-watching and playing shuffleboard or on a first name basis with a lot of tow truck drivers.

Thankfully, that’s not the case thanks to the transfer case invention. So, everyone give a big round of applause to that hunky chunk of metal under the cab of your four-wheel.

The T-case really doesn’t get the appreciation it deserves. To many drivers, it’s just that nifty device that lowers gears for climbing and drives the front wheels. If that’s you, let’s get you up to speed on what exactly the T-case is doing. We can also help those of you with full-time boxes get up to speed on some newer models.

T-Case 101

• How Does A T-Case Work?

All transfer cases, no matter how complex, work in the same technical nutshell way. They take engine torque from your transmission, split it, and deliver parts of that torque to your output shafts connected to driveshafts going to your rear and front axles.

Transfer case designs for off-roading mostly use a system that’s able to mechanically lock the front driveshaft and rear driveshaft together. This is so that an equal amount of engine torque is delivered to both ends of the vehicle.

• How’s Torque Delivered By A T-Case?

Delivery of torque to the front output shaft happens one of two ways: a chain or a set of gears. Which you have depends on the T-case’s design and age.

Older models are typically gear-driven, and newer models are typically chain-driven. While the change to chains is thanks to them being less noisy and much lighter than their gear-driven counterparts, many extreme wheelers tend to gravitate toward gear-driven T-cases because they don’t stretch under extreme stress and don’t break as easily under high amounts of torque. It’s a personal preference and usage thing.

• What Are The Construction Materials For T-Cases?

Various materials can be used to create the transfer case’s housing, which is also often distinguishable by the age and design of the transfer case itself.

The Dana 300 and NP205 are examples of older transfer cases that were constructed with heavy, durable, and almost impenetrable cast iron housings. This is your strongest option, but it’s also your bulkiest.

With an eye on fuel efficiency, the 80s introduced wheelers to lighter transfer cases. These housings replaced iron with much more lightweight materials like magnesium and aluminum. Hummer’s NVG242 HD transfer case, for example, has an aluminum housing. A Ford Super Duty pickup’s NVG271 transfer case is in a magnesium housing. These are two very sturdy examples to show that lightweight doesn’t automatically equate to weaker. That said, certain models, such as the aluminum NP208 and NP231, do have a deserved reputation for cracking and weakness. This is particularly true the harsher the usage conditions get.

You’ll certainly want to ensure that your T-case is up to par or swapped with a hardier housing if you’re making big modifications in torque and tire size or your activities involve super-duty conditions.

What Speed Is Your Transfer Case?

Single-speed T-cases, such as are standard to most all-wheel-drive cars and mini-SUVs, are suitable for wet pavement, graded dirt, and so forth. They’re limited when it comes to any tougher terrain because single speed T-cases don’t have a low-range gear reduction. There isn’t a direct mechanical link between the two ends of the vehicle as it splits engine power.

Most wheeler transfer cases are two-speed units. This means the case has a set of gears that can be engaged to lower the transmission’s gear ratios. When engaged in low-range, the action basically multiplies the amount of torque to the wheels.

The translation here is that you’ll want as high of a number, which is as low of a low-range ratio, possible if you want maximum pulling power and effective climbing abilities. Ultra low sidebar and other specialized applications can get you the lowest low-range ratios. Meanwhile, factory T-cases currently have between 2.50:1 and 2.70:1 low range ratios.

Is Your Transfer Case Part-Time, Full-Time, Or Hybrid?

Once upon a time, all transfer cases were either part-time or full-time. Today, you have more options.

Part-time T-cases simply enable the driver to switch back and forth between two-wheel-drive and four-wheel-drive modes based on their assessment of road conditions. Since part-time T-cases aren’t engaged 24/7, they carry the advantage of better fuel economy and less wear and tear on front-drive components.

With full-time T-cases, the driver doesn’t have to make decisions and switches; the T-case is automatically and permanently engaged in four-wheel-drive at all times. All full-time cases must allow driveline slippage between the rear and front wheels so that the driveline doesn’t bind on hardly packed terrain. Whether this is done with differential gears or a clutch/coupling between the T-case and front driveshaft, there’s usually some means to lock the slippage to still get a 50/50 torque split when needed.

Last by not least, we have the newcomer: hybrid-time. Over the last decade, manufacturers have created multiple hybrid transfer cases that allow drivers more selection and tailoring to their specific uses and needs.

Jeep’s NVG242 Quadra-Trac system on the Grand Cherokee was one of the leaders of the hybrid T-case pack. It allows for operation in both ranges, with the center diff open in full-time four-wheel-drive, and with the center diff locked in part-time four-wheel-drive. Plus, there’s a two-wheel-drive option.

In 1998, the NVG246 Autotrac case was the first hybrid offering from GM. It has an all-wheel-drive mode that’s computer controlled and both part-time 4-high and 4-low modes.

By 1999, Jeep had created the sophisticated Quadra-Drive system, which mated locking diffs in the Dana 30 and 44 axles with the NVG249 T-case.

Today, there are innumerable T-case timing combinations to suit the needs of almost any driver, and just when you think there can’t be another... one comes out.

Are You Planning To Swap Your T-Case?

Before you do any major modifications to your 4x4, such as going to a bigger tire size or adding more horsepower through the driveline, you need to consider if your T-case can handle the added torque loading, driveline stress, and so forth. If not, and you want to go ahead with the modifications, then upgrading your T-case should also be on that to-do list. Otherwise, you’ll likely still be replacing it from exceeding its limits.

In selecting a T-case upgrade, you’ll want to consider several factors.

• Price is a big deal, and older salvage T-cases are a fraction of the cost. But, you’ll want to know if it’s ready to go or needs costly rebuilding, if it’s compatible with your truck’s transmission or needs an adaptor kit, will you have to shorten or lengthen the driveshafts to get it to fit?

• Don’t let your eyes get bigger than your truck. Those big gear-driven cases are stunners, but will it provide a safe clearance and fit for your truck?

• Don’t get dazzled or frazzled by features. Do your homework and consult with your local driveline expert or 4x4 mechanic. Most T-cases are a give and take situation where you’ll have to prioritize your wants and needs against undesirable features. For example, are those must-have features worth it in a T-case with materials prone to cracking or with a weaker output shaft?

Wrapping It Up

Despite wheelers having their own varying opinions on the best 4x4s on wheels, all have one vital hunky chunk of metal under the cab to unite them in solidarity - the transfer case. It’s a technology that’s come such a long way in such a short period of time, and its future holds endless possibilities.

It’s great if the old jobs, such as cast-iron casings, work best for you. It’s great if your T-case is the latest hybrid model that has every capability short of shooting you into outer space. But, as other wheeler tech expands and and more and more 4x4 modifications become available, we just need to be sure that we aren’t neglecting upgrading the T-case to handle the extra loads and work from those choices. Your T-case is a piece of equipment that’s just too vital to go ignored.