collantes_C6

Pardon my ignorance but what is the difference between the 4.2 for our A8 and the rest like the one RS4 has or the Q7?

MP4.2+6.0

Return to the start of the thread and read the self study publication. Some is covered there. The Q7 version is generally similar to the one used with the A8's when they were offered. The RS4 one is similar to the R8. The "regular one" in general is tuned toward a greater torque band for everyday driving, while the RS/R one is tuned toward peak HP at high revs.

There are also (favorable) differences in the timing chain drive area relative to the more trouble prone one from the old S4. You would have to do some deeper dive searches to get at that sort of level of understanding. In general terms, the A8 4.2 is similar to the Q7, much of the C6 A6 run (for the optional 4.2 engine), and by some reports the original S5. There was a brief run for on using timing chains one the old C5 A6 based allroad; that one was an early one off. Little of this is documented well.