so i have a 01 s4 now and i think its time for a new project i like the c5 style a6 just not sure the differences or the benefits of having a 2.7 or 4.2 other then the fact i know a 2.7 from from to back..just looking to make the best decision and advice would help

 

It depends on what you want to do with the car. The 4.2 is not mod friendly, very few upgrades to get more output. The 2.7T can be found with manual transmission, and is more mod friendly. I've had both, formerly the 2.7T and currently the 4.2, both tiptronic. Gas mileage was about the same for mixed city/highway driving. The 2.7T felt more agile, and the 4.2 is a great highway cruiser. In the end, you need to drive each one, and you'll know which one is the right choice for you.

 

This is a religious discussion. The 4.2 probably has the edge on engine reliability and a nice "wide body" look; the 2.7T (as you know) can be modified significantly.

 

can the 4.2 be modded with a rs6 turbo kit and software to support it or are there any other turbo kits?

 

you would be better off buying an rs6^^

There is a pes supercharger option for the 4.2 but that alone will run your upwards of 8-10 grand. and it is a possibility to swap the 4.2 to a manual. depends on how deep your pockets are, and the amount of time you are willing to spend on researching for info and parts.