Red Bull boss Christian Horner has expressed his “surprise” at the improvements in speed made by Mercedes over the last month.
Even though Red Bull's Max Verstappen regained the World Championship lead at the Turkish Grand Prix on Sunday, Mercedes’ Valtteri Bottas won the race and Lewis Hamilton came from 11th to finish fifth.
Red Bull recently made a complaint against the power units being used by Mercedes, which were introduced to the car in late August, but it was dismissed by the FIA.
Hamilton denies he was 'furious' with Mercedes for Istanbul strategy
Still, Horner has continued to voice his concern about how fast the Mercedes is running, especially in a straight line.
"I think their straight-line speed has taken a significant step recently," he said in Istanbul.
“Whereas we could match them with smaller wings previously now we can't get near. We saw that particularly at this circuit where Lewis in particular had a significant straight-line advantage with a bigger rear wing on the car. We've got to maximise our package as best we can. It's surprising that they appear to have made the step that they have with the power unit."
Mercedes have said that nothing has changed in their engine apart from fitting a new one to Hamilton’s car for the Turkish Grand Prix, for which they received a penalty.
Horner says Red Bull and other teams have "raised questions" over the power units to the FIA.
“We ask some questions which all teams do. It wasn’t just Red Bull, by the way, it was other manufacturers raising questions as well and they continue to be raised.
“It is something for the FIA to look at and for them to police, but the straight-line speed…when you’ve got straight-line speed that is greater with a DRS open, that is pretty impressive.”
Asked if he was satisfied with the response from the FIA, Horner replied: “It is down to the FIA to continue to look at these things.
“We’re happy with the performance we had this weekend. I think the engineering team and the recovery we had, we were out of sync on Friday, the work that we did on Friday night with Sebastian Buemi in the simulator and the set-up we used on Saturday, it worked well today.”
Mercedes F1 boss Toto Wolff says they are managing reliability against performance with their engines.
"When you start to push the boundaries with power-unit performance, at a certain stage you will experience some obstacles.
"Our power-units have been the most reliable since the introduction of the hybrid engines in 2014 and because the boundaries are being pushed we have seen examples of, let's say, unusual noises within the combustion engine that aren't completely understood at that stage yet.
"They caused some troubles in the past; we had some engines that basically failed and now it’s about containing the problem because changing parts [during a season] is not something you would tackle.”
Verstappen is six points ahead of Hamilton in the standings with six races of the season remaining.
The F1 next heads to the USA with a GP in Austin, Texas in a fortnight.
Hamilton points finger at Mercedes team after surrendering championship lead to Verstappen
Turkish Grand Prix
Hamilton tops timesheets after taking 10-place grid penalty