Vitaly Petrov says the Renault team needs more reliable testing mileage before it knows how competitive its R31 can be this season.
The car has shown flashes of impressive speed with Nick Heidfeld setting the quickest time on one day at Jerez last week. It was often closest to Red Bull in the fast corner speed traps, but it has also been plagued by reliability troubles and has not completed as much running as the team had hoped.
Renault was sidelined several times at Jerez with KERS/braking-related troubles and lost time again on Friday at Barcelona due to another KERS problem.
"We've still not found the limit or something close to it," Petrov told AUTOSPORT. "I didn't do many laps. I had a lot of problems with brakes, KERS and leaks so I didn't run so much. For me it's very difficult to compare how it is. I didn't push.
"We still need to work on a lot of things. We had a lot of problems with the KERS and it was difficult to find where this problem was."
Petrov says he suffers from the same braking issues with the R31 as he did with its predecessor and identified that as the team's main area of concern.
"The braking is just locking too much at the rear – I think we just need to have the right shape map on this curve," he said. "That's why we need to test. We were struggling all last year with the brake stiffness and retardation, now we came back in the beginning with the same problem so we need to sort it out.
"As soon as you approach the brake, the rears start to lock. When it's rear locking, you lose performance from the tires and when you have rear locking, after two or three corners the rear tires are already too hot and there's no grip there. It's the main problem."
Renault is still yet to test its car over longer runs, with Petrov adding that the team's attempts so far have had to be aborted.
"We've tried already since the first test and the car is not working," he said. "So cross fingers, we hope we can do long runs here."