Toto Wolff says Mercedes is at a loss to understand the abrupt performance changes from one day to the other of its W14 car, calling the fluctuations "unacceptable".
Mercedes enjoyed a positive start to its Miami Grand Prix weekend, with George Russell and Lewis Hamilton topping Friday's opening practice.
But hours later in FP2, the pair had dropped into the thick of the midfield, well adrift from their front-running rivals, while Saturday's final practice delivered a similar depressing verdict.
In qualifying, Russell was sixth fastest while Hamilton failed to make the Q3 cut after his crews mistimed the Briton's final run.
After the session, Wolff made no effort to downplay the mediocre performance, admitting that it was "worse than I thought it would be".
"We are 12 months on from when we were last in Miami," said the Mercedes chief.
"The car is just marginally better [than in 2022]. Some bouncing on the straight, that is the only thing that is better than last year.
"The car is not fast enough and we haven’t got any comprehension of why that is. Our car is P1 and P2 in one session and then the next one you are P6 and P13. It’s just unacceptable."
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"I think that the car is not a nice car, not a good car," Wolff added.
"It's everywhere. It’s the pace, the performance of the car, it’s the lack of understanding of the car [about why] it is not delivering. It is the full spread of activities. I would say the performance is just really bad."
Given its struggles, Mercedes has embarked on a heavy development programme for its W14, one that will see the team introduce wholesale changes to its car's concept.
The team's first significant upgrade package will appear at Imola in two weeks and will include, among other changes, a revised front suspension and an updated floor.
Wolff holds the hope that a step forward will be achieved thanks to the changes.
"I think what we are trying to do with the upgrade is to create a new baseline for us to take question marks and variables out of the equation and say this is not a problem now that we have come to a different spec, for example front suspension," the Austrian explained.
"We are also looking at bodywork solutions that are more conventional than others, and that will create a different airflow.
"So for me, that is almost like a reset with what would have been a good start 12 months ago and then try to add performance. But the moment it is just a lack of understanding [of the car]."
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