Water Methanol Injection is a somewhat common and cost effective power modification for turbo engines tuned to a higher level of performance. By injecting a fine spray of 50% Methanol and 50% Water into the inlet air stream of the engine, the charge air is dramatically cooled and the effective octane of the fuel is raised. Both help to eliminate detonation — the enemy of any engine, especially those using today's lower octane fuel (frequently with 10% ethanol) under higher levels of boost.
There is no rule of thumb on how much timing or boost can be increased when running a water methanol injection system. Logging data will tell you just how it is reacting with your tune settings, as each engine and car will react differently. In general most engines can usually run the equivalent boost and timing of settings of a 100 octane tune while on pump gas. I've been able to run an increased level of boost and timing as mentioned, and most noticeably able to maintain power levels when ambient temperatures increase. The RS6 is very prone to heat soak due to the cramped engine compartment, so having a cool spray injected maintains a cooler more accurate combustion. This is good for both power and the engine as it can reduce internal temperatures.
I've been using a kit from DevilsOwn for the last few years now and it's worked very well. They use an engine displacement calculator to help determine the total amount to be sprayed. Once you determine the approximate nozzle size, two D05's for the 4.2L V8, it's then up to the user to fine tune the progressive controller to perfect the timing and amount of the spray. Since the RS6 is a two turbo engine, with one turbo on each side, I have mounted one nozzle on each side of the engine's air intake; all of this being post Intercooler. The Y-pipe takes the air from each intercooler and brings it into the engine throttle body. As shown in the picture below, the best point of delivery is each side of the Y-pipe. This allows for dispersion of the water/meth before entering the engine. This work is best done with most of the front clip removed due the Y-pipe being difficult to access; along with all of the metal shavings needing to be cleaned out.
The progressive controller mounts discretely in the fuse box, or a more open location if you want easier access. For boost reference (so the digital controller knows what the engine is doing), we tapped into the engine vacuum boost line, and the controller is adjustable for the amount of spray depending on boost level it senses. There is a port in the firewall that allows the vacuum lines and fluid lines through. The heart of the system has been custom installed in the trunk, in the location where the stock battery was. The stock battery has been replaced with an Optima PC925c which is enclosed in this same housing. Under the custom cover with a liquid level window is the 2.5 Gallon tank with sump, with the primary pump and the car battery nestled right behind it. All of the electrical wiring and water/meth hoses run along the edge/floorboard of the car, similar to wiring in a rear subwoofer for a stereo. You will also need to consider power wiring options for both the controller and the pump. This is an enclosed and effective system for upgraded performance in all driving conditions, and most importantly keeping full power in the warm summer months when heatsoak of the engine becomes a common problem.