Every 4WD vehicle uses a transfer case to split the drive coming from the transmission into two outputs, one directed to the front axle and one directed to the rear axle. Most transfer cases that we are familiar with are two-speed versions, having a high-range and low-range gear set.
The high-range is most often a 1:1 ratio. That is to say that for every revolution of the transmission output shaft, the front and rear transfer case outputs rotate one revolution. Factory low-range ratios can vary from about 2:1 to about 4:1. This means that the road speed in low-range is reduced by a half or by a quarter. With this reduction in speed we gain an increase in torque by the same factor.
Transfer cases have come in both gear drive and chain drive versions. Typically a chain drive case is a bit quieter, but a gear drive case is most often stronger and does not suffer from chain wear, which requires periodic replacement. Low-range reductions come in two flavors as well: gear-to-gear reduction or a planetary gear reduction assembly.
Here we've compiled a list of specs for many of the more common transfer cases that off-roaders use, swap, or modify. We've tried to included input/output shaft spline counts, specify the types of driveshaft mating, and give some idea as to the extent of aftermarket support available to these T-cases (ranging from low to high).
BW 1339 (Quadratrac limited slip)
Manufacturer: Borg Warner
Number of speeds: Two (full-time 4WD)
High: 1:1
Low: 2.57:1
Case (or housing) material: Aluminum
Front output drive type: Chain (passenger side drop)
Availability or aftermarket parts and support: Medium - Mile Marker part-time conversion & overdrive kit
Input spline count: 10
Rear yoke: Fixed
Planetary gear: Yes, four-pinion
Available U-joint sizes for yokes: 1310
Flange or slip yoke eliminator available: n/a
Output shaft spline count: 10
Shifter type: Mechanical
Found in: '73-to-'79 fullsize Jeep and CJ7s (behind AMC TH400)
BW 1350
Manufacturer: Borg Warner
Number of speeds: Two
High: 1:1
Low: 2.48:1
Case (or housing) material: Aluminum
Front output drive type: Chain (driver side drop)
Availability or aftermarket parts and support: Low
Rear yoke: Fixed
Planetary gear: Yes
Shifter type: Mechanical or electrical
Found in: '83-to-'93 Ford Ranger, '84-to-'90 Bronco II & Explorer
BW 1356
Manufacturer: Borg Warner
Number of speeds: Two
High: 1:1
Low: 2.72:1
Case (or housing) material: Magnesium
Front output drive type: Chain (driver side drop)
Availability or aftermarket parts and support: Low
Input spline count: 31
Rear yoke: Slip or Fixed
Planetary gear: Yes
Available U-joint sizes for yokes: 1330
Flange or slip yoke eliminator available: Yes
Output shaft spline count: 31
Shifter type: Mechanical or electrical
Speedo output: Cable port pre-'92 only
Length: ~18.5 inches (mount face-to-yoke center)
Commonly found in: '87-to-'97 Ford F150, F250, F350, & Bronco