Eleven drivers have started their Belgian Grand Prix weekend on their third and final power unit of the 2021 season, moving themselves on the fringe of an upcoming grid penalty.
As the second half of F1's campaign kicks off at Spa, most teams have introduced a third piece of hardware into their engine pool.
However, not all drivers are on equal footing when it comes to the changes as most drivers still have three usable units in their inventory, while others will need to finish off the season with just two available engines, a near impossible task with more than ten races scheduled in the back-half of the F1 season.
With the exception of Ferrari customers Haas and Alfa Romeo, whose cars are still powered in Belgium by their second engines, all teams have opted to fit unit #3 to their machines for Spa, although Max Verstappen, Sebastian Vettel, Esteban Ocon, Carlos Sainz and George Russell all completed their switch before Spa.
Red Bull's Max Verstappen and Sergio Perez are also on their third engine, but Honda's decision to withdraw their second unit from their pool leaves both drivers with just two engines to complete the season.
For this weekend, Mercedes, McLaren and AlphaTauri have introduced a third unit for both their drivers while Alpine's Fernando Alonso, Ferrari's Charles Leclerc and Williams Nicholas Latifi have also made the switch.
In addition to Red Bull's drivers who will almost inevitably take on a fourth engine and a grid penalty, Leclerc and Sainz will also suffer a demotion at some point following Ferrari's decision to upgrade its power unit in the second half of the season.
The timing of the Scuderia's change to a fourth power unit is uncertain, but would likely take place after the team's home race at Monza.
Mercedes team boss Toto Wolff admitted that the reigning world champions weren't immune to adding a fourth engine to their pool.
"Every engine is degrading and obviously you have a fresh one," Wolff told Sky Sports. "It has that notch more performance and if it gets really tight then you have to have this discussion.
"But at the moment, we're still counting on finishing it with three. In any case, it's very tight with three engines this season and it's something that everybody needs to monitor, whether you want to take a penalty to just have extra power."