The Audi A6 4.2 and the S6 have the same chassis but differ in powertrain and badges. The badges are easy to get but the powertrain is not. The V-10 in the S6 is not sold as a crate engine and has to be bought secondhand from Europe. Furthermore the software that runs the S6 is proprietary to Audi and cannot be bought, so third-party software has to be sourced. It is much cheaper to replace the badges on a 4.2 A6 and upgrade the V-8's software to S6 levels than to do an engine swap.
Upgrade the A6 engine to 5.2-liter V-10 levels of performance, 435 horsepower at 6,800 rpm and 398 ft-lb of torque at 3,000 rpm. As previously mentioned, sourcing an Audi V-10 is near impossible as they are not sold as crate engines, so the only feasible path for an engine swap is to buy it secondhand, which is also near impossible given the rarity of S6s in America. Europe and now China, have the greatest number but after shipping, the cost can be as much as the A6 itself. Realistically the only way is to upgrade is to flash the Engine Control Unit (ECU) to S6 levels of performance, though this will also require upgrades to the air-intake and exhaust.
Flash the ECU with upgraded software; this is available from a number of vendors. To reach 435 hp the engine will need to be turbocharged and the exhaust replaced. Flashing the ECU is the easiest with most options just involving hooking up a software module to the diagnostic port though some must be hardwired into the ECU itself. If the flasher needs to be soldered on then an auto repair shop should install it given the danger of permanent damage.
Turbocharge the engine; preferably with the turbochargers that are used on the Audi RS6 but there are universal turbochargers that will give similar levels of performance, these just take longer to install. The highest level of the 6 platform is the RS6 which uses the same V-8 engine as the A6, but turbocharged. These turbochargers are somewhat easier to find but there are also third-party vendors that offer turbochargers as well. This installation should be done by a professional auto shop, preferably one that specializes in high-end European imports.
Upgrade the exhaust; if possible use the exhaust off the S6 or RS6 but there are many vendors that supply exhaust systems for the A6. Borla and Remus are the most popular options and again require an auto shop to complete as the header work alone can take a week.
Swap badges; Audi tends to be less prolific than other car companies in their love of badges however they still need to be replaced if the transformation is to be complete. The real badges are expensive and somewhat rare, however there are fake badges that are mirror images and cannot be told apart exceptt by an Audi technician. The S6 badges are needed on the outside and the S6 speedometer facing will need to be placed.
