Carlos Sainz and Daniil Kvyat are concerned that Toro Rosso is falling further behind its midfield rivals as the 2017 season progresses.
Sainz believes that they are holding their own against comparable teams such as Force India. But he worries that they are losing ground to squads with bigger budgets.
"McLaren and Renault are the ones who are bringing loads of downforce into their cars and making the difference in the corners compared to us," he told Autosport magazine this week.
He particularly noted the difference made by a new floor introduced by Renault at Silverstone. "All of a sudden this half a second [gap] appeared," he pointed out.
"I tend to think that from now onwards, McLaren and Renault are likely to be ahead of us in qualifying," he commented. "Especially when we go to tracks less engine-dependent, McLaren will take a big step forward.
"Renault is half a second in front, but on the other hand Force India, Haas - we can control them," Sainz added. "We have still an upgrade to come, so hopefully this will help us."
His team mate Daniil Kvyat agreed that Toro Rosso had not been able to keep pace with their midfield rivals when it came to development in 2017.
"To be completely honest it hasn't been as fast as expected, our development," the Russian driver conceded. "We are lacking a little bit in that respect."
He said that the difference in performance was clearest in qualifying. Early in the season, both Toro Rosso cars were competing to make it into Q3. But last week in Spa, Kvyat was eliminated at the end of Q1 and Sainz was almost half a second off progressing at the end of Q2.
"Qualifying is very important this year, so we need more," Kvyat confirmed.
At least both drivers are looking forward to Toro Rosso's home Grand Prix this weekend.
"I grew up in Italy and it's a country I know well. It's like a second home for me," said Kvyat. "I lived in Rome for some time when I was younger. I'm fluent in Italian, so I always enjoy going back.
"[Monza] is a legendary circuit. It has very long straights, which means we drive at very high speeds, and it's always an interesting race.
"The only thing I don't like is the fact that the Parabolica doesn't have a gravel trap any more," he pointed out. "It's now all asphalt, so it's not such of a challenge and a compromise if you go off track. In the past it could end up meaning that your race was over."
Sainz quipped that the Italian Grand Prix weekend was "frustrating".
"You want to spend the whole weekend eating pizza and pasta but you can't as we're there to race!" he explained. "Apart from this, I have to say it's an amazing weekend."
It's made even more special for Sainz as it coincides with his birthday - he'll turn 23 on Friday.
"It's a good way to spend this special day," he said. "Doing what I love to do and celebrating it with the team, which is like a second family as we spend so much time together."
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