World Champions Mercedes are set to fit what is known as 'Phase Four' of the development engine that was fitted back in Monza to Hamilton's car. During the First Practice Session in Monza, Hamilton was a clear 1.5 seconds ahead of the rest, and when asked to push during the closing stages of the race, he pulled away in an alarming fashion.
It is said that this new Power Unit that Mercedes has introduced is a step in a different direction to which they have previously gone. Therefore, coordinated testing is required within the works team, before it can be made available to it's customers.
Bahrain International Circuit - Winter testing