McLaren are reportedly working hard to adapt their 2018 car to the very different Renault engine.
When the British team began the 2018 project, it might have expected to stay with Honda power beyond this year.
Instead, after three years of struggling with their Japanese works partner, McLaren is moving from Honda to Renault.
However, Auto Motor und Sport reports that the Renault power unit has a "completely different architecture".
McLaren designer Matt Morris confirmed: "We had to redesign the clutch and the gearbox."
The cooling and radiator requirements are also completely different, with Morris admitting that McLaren will simply rely on Renault's recommendations for the first year.
"Only with more experience does it make sense to develop something of our own," he said.
McLaren have earned 24 points with Honda in 2017.
