The difference comes from the fact that the A6 has dual position ( variable ) valve timing. Both A4 and A6 have variable intakes.

Think of a solenoid positioned between the cam sprokets. By expanding, it applies tension to the timing chain. More to advance for high rpm, and less for good torque at low rpm. The advantage is that if the system fails, nothing else breaks, and the solenoid costs about $80.

April