Rs6 C5 with S6 C7 engine
 

Is it possible to replace the engine of Audi rs6 c5 with the new engine V8 tfsi of Audi S6 c7

Also I would ask if it's possible to replace the old 5automatic transmission of rs6 c5 with the new one as 7dsg or 8tiptronic

It's possible but your probably going to have to do the engine & tranny as a pair and all of the computers and wiring harness are going to have to go with them. Also will need to check things like how far forward/aft the front halfshafts come out of the tranny compared to the other one as that will be a determining factor in the engine/tranny placement. Custom driveshaft to the rear will be required. A lot of custom work will be involved. ECU will have to be hacked to be able to work the ESP and ABS sensors.