Sep.4 (GMM) Red Bull driver David Coulthard admits there is a difference in performance between the Renault engine in his car, and the Ferrari V8 that powers the almost identical chassis of junior outfit Toro Rosso.

The difference between the two teams' performances at Valencia recently was stark, as Toro Rosso had its best showing of the season, while Coulthard and teammate Mark Webber struggled very much amid the bottom half of the grid.

Coulthard, 37, wrote in his website column for the British broadcaster ITV that Renault's 2.4 litre unit is inferior to the Ferrari "in both the power and driveability stakes".

"That's something that we're aware of, Renault is aware of and we're trying to work through," the Scot said.

As ever in F1, Red Bull's recent problems are probably more complex than a simple power deficit, and indeed team boss Christian Horner earlier this week insisted that "tyres are proving to be the biggest factor".

Sebastian Vettel, who is making good use of Toro Rosso's run of current form, also believes Ferrari's reported 30 horse power advantage over Renault is too simple an explanation.

"I think the way you set up the car can make a huge difference for straight-line speeds," the young German told Speed TV.

"It's a bit more complicated than it looks," Vettel added.