Tired of the conjecture and conflicting stories, I decided to do my own research...
Hi, I'm new to the forum, I'm a Mazdaspeed 6 owner and I've conducted a number of tests that confirm my suspicions--everything posted on this thread (especially crossbow's comment above) about the AWD system, and most of what Mazda says about it, is absolute BS.
First of all, the system is not "AWD on demand", some torque is ALWAYS going to every wheel unless one or more is slipping. A multimeter connected to the center "differential" (clutch pack) solenoid wires shows that the voltage varies between zero (handbrake applied or extremely low speeds) and 5.85 (full lockup). During normal driving, the voltage is constantly changing. The center diff is in fact a wet clutch pack that can go from about 10% clamped to full lockup, though the latter only occurs during hard, straight acceleration. When the clutch solenoid gets zero power, there is still some torque transfer to the rear because there is a static load on the clutch discs. At maximum voltage, the center diff is essentially locked, making it entirely possible to transfer 100% of the engine's torque to the rear axle, if the front is slipping. The center is even energized when coasting with the throttle closed or the clutch disengaged, so all 4 wheels participate in engine braking, too.
In dry conditions, the torque split front/rear varies from approximately 90/10 to 20/80. The system can easily transfer more torque to the rear axle, because a locked differential (as in the case of a spool or transfer case) transfers ALL of the torque to the output shaft which has the greater load, that is, the slower one, or the one with more traction. Because the rear axle never goes faster than the front, attempting to drive the rear at the same speed as the front results in a rear torque bias. This is why drifting is possible in the MS6 but not in many other AWD sports cars. This is not a Haldex system. Haldex is a cheap end-around designed for economy, not performance.
It is true that the system has many sensors. It senses engine torque output, throttle position, steering angle, yaw rate, and acceleration at several points in the vehicle. All the systems work synergistically with the DSC but the AWD still operates when DSC is turned off. "Normal" "Sport" and "Snow" modes aren't really "modes" but rather 3 different behaviors for the computer. Normal is front-biased for gentle driving, limiting driveline wear, noise, and fuel consumption. Sport is rear biased, and engages when accelerating or cornering with any kind of aggression, and maintains the car's responsiveness and grip. Snow mode maintains a torque bias based on the vehicles weight distribution (slightly nose-heavy) to reduce traction loss on slippery surfaces. The system is constantly monitoring and adjusting torque regardless of what mode it is in.
Further proof lies in the roller-ramp test. Subaru uses this machine to demonstrate their AWD systems, but only one of their cars is sport-tuned. With DSC turned off, the MS6 can climb a slope if both front wheels OR either rear wheel has traction (the rear LSD is a preloaded torque-sensing Tochigi). If only one front wheel has traction, it cannot climb, because the front diff is open (50/50 torque split all the time, so both wheels receive equal torque) but when you turn the DSC back on, the traction control applies the brake to the front wheel that is spinning, creating enough counterforce for the other wheel to drive the car.
Obviously these are from my personal findings, so I invite you to replicate these tests and see for yourselves! You will see that what I say is true. I have some suspicions as to why Mazda says it's a 100/0 to 50/50 torque split. For one thing, many people confuse speed with torque. At full lockup, the cender diff does provide equal SPEED to both axles, but that had nothing to do with performance. Another possibility is when the system sends more torque to the rear, the whole driveline necessarily binds up (as the rear axle is constantly trying to overtake the front) and this results in the rear differential mounts snapping off. The drivetrain itself is robust and sturdy, but the mounts are woefully inadequate. A true center differential gear, which applies equal torque to both axles regardless of speed, would never cause binding like that, but would not permit such a high degree of traction and control. Center differentials are more popular on 4x4 trucks and cheaper Subarus.
The AWD on the CX series SUVs incorporate the same machinery, but the computer's behavior is not sport-oriented. They don't have the same power and handling as the MS6 but they do share the ice-climbing ability of Subaru's fancier models. Those systems are meant to prevent torque steer and wheel spin, not to get you around corners faster.
I suspect the high cost of replacement parts is the reason this system was only released for 2 years. So far, I've taken the warranty company to the cleaners. So I guess the MS6 goes in the archives along with the Millenia's supercharged Miller cycle, and the Cosmo's twin-turbo, 3-rotor Wankel. Even so, this car of mine makes me very, very happy!