In a nutshell: (4x4 V8 4runners)
- Front and rear differentials are OPEN
- Center differential is Torsen limited-slip that is also lockable (gear-based, instead of clutches or viscous coupling)
- Front and rear differentials are controlled by the traction control system, known as ATRAC
ATRAC: (see below for my response from a previous post)
ATRAC is there to PREVENT loss of torque to the wheel with traction.
For example, if one wheel (in mud) is spinning on an axle, then the other wheel (on dry land) LOSES it's torque because power is routed to the wheel with least resistance (the one in mud). So, power is being transferred to wheel in mud. Now, with ATRAC, the ABS sensors will sense this slippage and applies the brake to the spinning wheel. It will apply the brake UNTIL the two wheels are spinning at the same speed (in theory)...that is, it will apply the brake to slow the spinning wheel to the same speed as the wheel that is on dry land. By doing this, it tricks the differential into thinking that the slipping wheel is also on dry land...thus, preventing all power from going to the slipping wheel. It evens out the torque distribution to both wheels. This prevents the wheel with traction (on dry land) from losing torque. Thus, it will have power to move your truck forward.
Does ATRAC really allow for 50% split?? Well, in theory, yes. However, in real life, i would say that it helps prevent torque from TOTALLY going to the spinning/slipping wheel. However, because of the stress on the brakes (brakes have to go against the power flow) AND powertrain (remember, ATRAC can be quite sudden in it's operation), i doubt it. I would say that it is somewhere less than 50%. That is, ATRAC will prevent most of the torque from going to the slipping wheel. So, the wheel with traction will receive less than 50% of torque, but enough to keep your 4runner moving forward.
Thus, this is the role of ATRAC...it prevents the loss of torque from the wheel with traction...to allow that wheel to get enough power to move your 4runner forward.
This is why the 4runner has big brakes with multi-pistons and 4-channels. You need good brakes in a vehicle with ATRAC for effective ATRAC operation. In addition, 4runner powertrain needs to be beefy to take the stress of ATRAC.
How good is ATRAC?? It is as good as any limited-slip differentials (LSD) on the market. It will only lose to a true locker over severe, rocky terrain and deep mud. But then again, it is less stressful on the axles than a locker.
With ATRAC, it's like having LSD on the front and rear axles. With the center diff locked, the 4runner is quite capable! ATRAC is such an elegant and simple design...no extra stuff...it just uses the existing technology (4-channel ABS, ABS sensors, etc.) to create a LSD.
Hope this helps.
