As an introduction to different tuning stages I thought of a piece that would cover things broadly and then an individual piece for each level describing perhaps what it would entail for different cars and etc.
This is the introduction to the tuning stages vernacular. In general many people describe various tuning stages, and while this may not constitute a specific definition this does aide in communication amongst similar minded people. I am sure a strong epistemological argument can be made that these stages are meaningless and etc, but in practical measures this is what the various tuning stages implies for late model bi-turbo Mercedes Benz vehicles. This holds true currently, but it may evolve over time as different driveline technologies are employed. Which is a different rabbit hole for a different post.
Stage 1 Tuning Definition
Approximately 60hp-130hp gain from ECU remap/tune, air box/filter, and optionally exhaust.
Stage 1 tune often employs an ECU tune, an airbox upgrade as needed (risers, UPD kit, ROW box) with better filters. Some people also will do exhaust in stage 1.
Although many tuners put this together and describe it as stage 1. Adding airbox risers, filters, and deleting resonators/mid-exhaust should result in 23-35 at wheel hp according to UPD and other dyno runs for such engines as the m278.
The ECU tune, or remapping will try to optimize the engine based on generic information, or specific information. Increasing pressure and boost the ECU tunes can improve HP by a fairly large amount. For example AMS performance, some say Renntech, and OE try to deliver 585 crank hp from the ECU tune on a M278 which is at stock likely 450-460hp although rated by mercedes at the crank at 402 in some applications.
Stage 2 Tuning Definition
The above video by weistec is an example of stage two tuning.
More than 50hp gained from turbo back pipes (2 parts), mid resonators, ECU Remap/tune
While stage 1 may include exhaust, in stage 2 exhaust is 100% addressed. The turbo downtubes are replaced and often the cats are replaced as well. This can be done in two parts and some manufactures do sell that way. Although the turbo back cat delete, for a race car or as local laws permit, can be fabricated and installed at an exhaust shop for approximately $200 a piece so $400-500 is a fair price while shipped it can run upward of $1000.
This stage 2 tune is a difference because it will throw codes without some modification back to stock if the stock ECU is used. While Stage 1 tuning can be completely reversible, Stage 2 will require modification to return back to the stock ECU.
The stage 2 tune removes the CEL when these "primary" cats are removed and may also change the tune based on the idea that the cats are removed. Expect a gain of >40 awhp for an M278.
There are two different ways parts, one is a turbo back which will improve restrictions and produce a good power gain of 30-40hp. This part connects to a cat and 02 sensor, this part can be replaced separately at a muffler shop for off road use or replaced with upgraded higher flow cats. This part costs about $200 per side.
Stage 3 Tuning Definition
Approximately >70hp-300hp gained from improving turbos & methanol.
Stage 3 tuning takes the physical mods beyond the mere unbolting (on many Mercedes) of the cats, and physically modifies the Turbos. Mercedes choose a design of turbos that does not actually maximize power, but provides a nice balance of drivability, emissions, efficiency, luxury, and power. AMG and Brabus further optimize that balance toward power.
However, there are great amounts of power that can be gained by changing or upgrading the turbos which may change the drivability. As you add more turbo power much of the power is from boost which requires a certain amount of exhaust pressure so the power delivery is increasingly non-linear. A part of this stage will be modifications to support the additional power delivery, including swapping to AMG style fuel delivery. Many will also upgrade the brakes at this point.
Billet turbos take the stock turbo application and modify them, and add different turbo rotors that are manufactured to much tighter tolerances. Combined with an ECU change based on individual pulls, an m278 engine should be able to achieve 600 awhp with VRP system.
Turbo Swap / Better Turbos
M278 engines can use M157 turbos they bolt up completely and can be had for $1500-3000. Build the most powerful CLS thread starter went in this direction before going and building completely custom turbo setup with bigger turbos.
Weistec sells aftermarket turbos for several mercedes applications delivering immense power. As you approach 800-1000hp in an M157 Weistec suggests upgrading the rods. They suggest their W3 turbo mod can deliver more than 1400 hp in the M157 if the engine internals are properly addressed.