With seven races remaining in the 2021 F1 season, it's a big ask for Lewis Hamilton to make it through the rest of the campaign without needing another engine.
Given the reliability concerns that have crept in at Mercedes, it stands to reason that the team will opt to fit the W12 with a fresh engine before the end of the season to reduce the possibility of a failure deciding the championship for them. After all, this week was the fifth anniversary of Hamilton's infamous engine failure while comfortably leading the Malaysian Grand Prix – a hugely pivotal moment in that year's title chase.
While Hamilton denied that Mercedes are considering an engine swap over the Russian Grand Prix weekend, the team haven't ruled out the possibility outright. At the moment, Hamilton has just two engines in his pool. This is the unit he started the season with (Bahrain up until Azerbaijan) and the unit introduced at the Belgian Grand Prix.
The unit introduced in Baku failed during practice for the Dutch Grand Prix, and is now out of his pool. With two reasonably high-mileage engines available and seven races to go, logic would suggest a fresh engine will be fitted, and soon.
Certainly, Red Bull are working under the assumption that the Mercedes will need a fourth power unit.
"I hope that our information is correct," Helmut Marko told Sky Sports. "According to that, Lewis will have to change again."
But, with Red Bull having used the Russian Grand Prix to get their engine penalty out of the way, there's surprisingly few opportunities left for Mercedes if they want to reduce the pain of a penalty.
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Where can be ruled out?
It's reasonably safe to assume that the change won't be made at any of the final three races. This would mean Qatar, Saudi Arabia and Abu Dhabi – simply because of how late they are in the season.
Changing that late in the season would mean running the risk of a badly timed failure with the existing engines, while also not fully utilising the life of the fourth unit.
There's also the fact that Qatar and Saudi Arabia are completely unknown quantities, while no team still in with a shout of a championship is going to take a penalty at a potential championship showdown in Abu Dhabi.
Brazil is more likely than the aforementioned three, but also not hugely likely given that the circuit isn't a slam-dunk in terms of overtaking. The straight isn't straight, and the straight-line speed clipping seen on the Mercedes at Monza could result in Hamilton finding it difficult to make his way through the field, given the challenge of overtaking through the first and second sector of the lap.
The three most likely venues
If Mercedes are going to make the change, it's likely to happen sooner rather than later. This would suggest the next race in Turkey, or either the United States or Mexican Grand Prix.
Turkey is a venue where a driver can overtake reasonably comfortably, and it was Hamilton himself who initially made a name for himself in GP2 in 2006 with an eye-catching drive at Istanbul Park. Running a low downforce setup, Hamilton spun off early in the race and fell down to 19th. Gathering himself back up, he fought his way back to finish in second place.
The United States is also a possibility, given the length of the back straight and Hamilton's prior successes at overtaking Nico Rosberg and Sebastian Vettel for race victories at the Circuit of the Americas. In 2018, Max Verstappen started the race from 18th on the grid and wound up in second place at the end after some great strategy decisions from Red Bull. A similar outcome for Hamilton would surely be regarded as a big win.
Mexico is also a track at which overtaking is a possibility thanks to the long main straight. Due to the high altitude, the teams can run higher downforce settings and still enjoy massive speeds into Turn 1. But again, there is the worry that Mercedes' engine clipping problems could curtail a recovery drive, and see Hamilton get stuck behind someone, as seen in Monza and Sochi.
The most likely scenario is that Mercedes will play it by ear, and have an engine on standby at all times. Should it appear as though Verstappen is unbeatable at any given track, they may elect for the change there and then in order to take their penalty at a track where it appears they might lose anyway.
Waiting for a bad qualifying result is also a possibility, or even waiting for news of a wet race that could introduce an element of chaos and, more importantly, the prospect of a Safety Car to negate the gap to the front.
It's a fascinating prospect and, given how successfully Red Bull managed to get through their penalty weekend, the pressure is now surely on Mercedes to respond just as well.
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