Just trying to make sure I understand how the AWD system on my 98 Impreza works.

 

During normal conditions most of the power goes to the front wheels and just a small fraction goes the back wheels. If the front wheels start to slip, more power is transferred to the rear wheels.

 

Do I have that right so far?

 

So how much power is transferred?

 

What about side to side traction differences. If one front wheel slips, is more power transferred to the othe 3 wheels, or is it transferred just to the other front wheel?

 

And thanks to everyone for helping with the newbie questions

Hi. Maximum power split is 50% power to the rear wheels. There are some STI transmissions that can put more than 50% power to the rear.

 

Other than those certain STI trans's, the front wheels always have power; they are directly driven off the transmission. The trans can vary the amount of power applied to the rear wheels. So if it detects front wheel slippage, more power is directed to the rear wheels. If rear wheel slippage is detected, less power is directed to the rear wheels. This is a basic description; the mechanics are different for AT's and MT's. When there is no slip detected, in the AT's anyway, the power split is controlled by a map.

 

Side to side traction differences depend on the differential type you have and whether or not you have TCS. The normal is the open diff, where one wheel can sit there and spin and the other won't turn. Unless you have a limited slip diff or locking diff. If you have TCS, the system can use the ABS pump to apply the brake a little bit to the wheel that is slipping thus giving more power to the wheel with traction.

Is your 98 AT or MT?

for automatics read these

 

http://www.ultimatesubaru.org/forum/showpost.php?p=630282&postcount=11

 

http://www.ultimatesubaru.org/forum/showthread.php?t=77389

Sorry, should have mentioned that it's a manual transmission. It's the L model and stock, so I assume nothing fancy as far as the differentials go.

you have open front and rear differentials and a center differential with a viscous coupling to limit slip. The viscous coupling will transfer 28lb-ft with a 100rpm speed difference between the front and rear.

 

I don't know how the transfer capability changes with different speed differences or the upper limit of how much can be transferred to the front or rear. If, for example, you had the front or rear wheels up on rollers, the VC would not be able to transfer the full power of the motor to the wheels with traction (assuming they had enough to not spin).

 

http://www.ultimatesubaru.org/forum/showpost.php?p=618964&postcount=41

you have open front and rear differentials and a center differential with a viscous coupling to limit slip. The viscous coupling will transfer 28lb-ft with a 100rpm speed difference between the front and rear.

 

I don't know how the transfer capability changes with different speed differences or the upper limit of how much can be transferred to the front or rear. If, for example, you had the front or rear wheels up on rollers, the VC would not be able to transfer the full power of the motor to the wheels with traction (assuming they had enough to not spin).

 

http://www.ultimatesubaru.org/forum/showpost.php?p=618964&postcount=41

 

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

 

AWD 101

 

nipper

During normal conditions most of the power goes to the front wheels and just a small fraction goes the back wheels. If the front wheels start to slip, more power is transferred to the rear wheels.

Under normal driving its 50/50. There is a center differential. The front end doesn't get more power until it starts to slip. When that happens the viscous coupling to the back heats up its fluid and starts to transfer more power rearward.

 

So how much power is transferred?

Many variables to get an exact answer. Usually the answer is usually "enough", unless you get stuck . One factor that changes the maximum amount of limited slip action in the center diff is the condition of the viscous coupling (they can be worn out).

 

What about side to side traction differences. If one front wheel slips, is more power transferred to the othe 3 wheels, or is it transferred just to the other front wheel?

Open differentials front and back. So it acts just like a 2wd drive car on both sides. If one tire spins it just spins faster with more power. When the overall speed difference of the front and rear is great enough it will activiate the viscous coupling mentioned earlier. This just tries to keep the power even front to back.

 

It's not very complicated or active when it comes to power distribution but it provides plenty of traction 95% of the time. Rear limited slip differentials are easily found in other models though, it just has to match the final drive of your transmission.