Mercedes left qualifying at Sepang with a major question about its new W17 aerodynamic package after both cars lost ground to their rivals at a circuit Formula 1 had not visited since 2017. The team introduced the substantial update on both cars, which meant it could not run the old and new versions in matching conditions to make a direct comparison. Team officials said the performance problem appeared to be linked to the unusually rough track surface, but the lack of comparable data has made it harder to assess and optimize the upgrade.

Championship leader Kimi Antonelli qualified fourth-fastest, while George Russell was eighth. Both drivers are set to gain one grid position because Isack Hadjar received an engine penalty. Russell was 0.741 seconds slower than Max Verstappen’s pole time, despite completing the usual two runs on new tyres in Q3.

Antonelli’s qualifying was affected by a track-limits violation in Q2. His lap was deleted, forcing him to try again on a new set of Pirelli C4 soft tyres that had been intended for Q3. With only one set left, he had one timed-lap opportunity in the final segment. At Sepang, the C4 compound had not proved durable enough for more than one attempt.

Russell said the missing back-to-back comparison left the team without a clear answer about the package. “That is the big question mark,” he said when Autosport asked whether Mercedes had found it difficult to draw out the new package’s potential. “If you could have done a direct back-to-back it would have been great to know, but that's the challenge of this sport.”

He said results at other rounds could have made the same upgrade appear successful or unsuccessful, regardless of its real effect. “If I put the upgrade package on last week, even if it was worse, we probably still would have been on pole and we'd have been saying it was fantastic. And equally, if we had the old car here, maybe the result would have been even worse.” Russell added: “So that's the challenge. As a team we have yet to have, let's say, a bad race this whole season and this was clearly a very bad qualifying for us.”

The update changes much of the W17’s aerodynamic design. It includes a revised floor, substantial changes to the floor’s leading edge and diffuser, and new components around the rear wheels. The sidepod profile is also very different. The redesign is understood to have required changes to the internal cooling architecture, making it impossible to switch between the two specifications during a race weekend.

The team’s puzzlement centred on how poorly the W17 appeared to handle Sepang’s asphalt, which is highly abrasive and offers low grip. Mercedes considered it unlikely that the update itself caused the problem, but without an old-spec car on track, it could not compare how that version would have behaved on the same surface.

Team principal Toto Wolff said the car displayed behaviour Mercedes had not encountered earlier in the season. “It's very difficult to understand,” he said. “We've been a little bit caught out on Friday with some car behaviour that we didn't see all year. But then you can say it's very hot and the tarmac is very different to all the [other] tracks.”

Wolff said the team had seen encouraging pace in final practice, when Antonelli topped FP3 by 0.278 seconds over Verstappen, who later took pole. “It looked really good in the morning [when Antonelli topped FP3 by 0.278s from Max Verstappen, who later qualified on pole]. We seem to have at least with Kimi made a step in understanding.” He said qualifying then returned to earlier difficulties: “And then qualifying was a little bit back to what we saw before. Just no performance.”

Wolff described losses on the straights and in slower corners. “It looks like we're very draggy because it's the first time we're losing a lot of time on the straights, especially to Max. And then through the low speed, we are not able to turn the car. There's just too much understeer. So overall, things we haven't seen all year.” Antonelli and Russell both appeared to fall back relative to their competitors between final practice and qualifying.

Sepang’s surface has presented a challenge to all drivers. Its aggregate is more open than that of many circuits, a feature of the track since it opened in 1999. The surface was designed to reduce slipperiness in wet conditions and help water drain and the track dry more quickly after rain. Russell said the surface was exceptional even compared with earlier visits.

“This circuit is a bit of an outlier in terms of the abrasiveness of the tarmac,” Russell said. “The lap times seven years ago were about six or seven seconds faster and at certain points on the track it feels like you're driving on gravel, which is the same for everyone. However, it's clearly affecting us more than others.”

Mercedes’ data has been more positive than its qualifying result. Russell said: “All the numbers say it's [the upgrade] an improvement, but obviously it's a sport where we need to follow the lap times and not what the numbers say. So the numbers look absolutely fine and in line with expectations. However, the result wasn't.” The team now has to interpret those figures alongside a poor qualifying and a track surface that exposed car behaviour it had not seen elsewhere this season.

Based on reporting by Autosport.