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Time has come to clear things out... I would like to know the difference between a center differential and a transfer case.

I know that a transfer case engages the front axle with the rear axle in 4H and 4L.(correct me if I'm wrong) As far as I know, vehicles with transfer case are RWD. Now when I thought about a center differential, I just thought of a differential (that typical circular one) in the center of the bottom of the car. When I had a chance to look underneath an AWD car, I found no "center differential".(Even though I saw a driveshaft and a rear axle)

But there are also vehicles with a transfer case that acts like a normal AWD car...so it confuses me overall!

So my question is: What does a center differential look like, and its location, and its function (like a transfer case)

Thank you all.

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The center differential in a typical Subaru transmission is housed inside of the main transmission case. For the MT models, these are typically viscous-coupling that control the torque split between the front and rear differentials (which may be open, viscous/clutch/Torsen LSD, etc.). The AT models may have different things that function as a center differential that I'm not as familiar with.

Some overview on the different AWD systems Subaru has:

Center Differential with Viscous LSD

In this AWD system, the center differential is implemented by a bevel-gear, utilizing a mechanical viscous coupling device acting as limited slip function. This type of differential is commonly known as VLSD (Viscous Limited Slip Differential).

Subaru VLSD Center Differential

In the above diagram, (10) is the center differential, and (11) are the viscous coupling plates.

Active Torque Split + Multi-Plate Transfer Clutch

This is the most common type of AWD in the market. In this system, a multi-plate clutch pack acts as power coupling, and is responsible for transfering power to the front wheels. The ECU uses multiple sensors’ input to determine the optimal torque split ratio, then it controls the hydraulic lines to apply suitable pressure between the input and output clutches, therefore achieving the goal of actively adjusting front/rear torque distribution ratio.

Active Torque Split

Variable Torque Distribution (VTD)

For this AWD system, Subaru installs a planetary gear-set as the center differential, and uses a multi-plate clutch to implement the limit slip functionality.

This system is fundamentally different than the above #2 type. For the VTD system, the power transmitted to front wheels is passed through the planetary gear-set, not the clutch pack. The multi-plate clutch only acts as limit-slip protection, and it will not kick in unless wheel slip is happening.

Variable Torque Distribution

In the above picture, (23) is the planetary gear differential, and (24) is the electronically controlled clutch-pack.

Driver Controlled Center Differential (DCCD)

The DCCD AWD system is specifically for the WRX STI (6MT). In fact it is the VTD, plus an extra feature to let driver manually select front/rear torque distribution, and both of the front/rear differential are also LSD type. Default front/rear torque distribution is 41:59; maximum adjustment ability is 50:50.

Double clutch-pack of DCCD

The above picture depicts the two LSDs used in the DCCD. The electrical LSD allows the driver to adjust the torque bias between the front and rear in real-time. Here's an official document from Subaru explaining the DCCD system in detail (ca. 2016).

Source

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  • Will do. Thanks for the tip! Would this website be the best and the fastest to ask any automotive questions? – user38723 Jul 11 '18 at 22:33
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