Fernando Alonso says his McLaren team needs a trouble-free British Grand Prix in order to find out how competitive the updated car is.
The Spanish driver ran the updated MP4-30 during the Austrian Grand Prix, but was hit by trouble in free practice and then retired on the opening lap of the race following his crash with Kimi Raikkonen.
That meant that McLaren was unable to judge the competitiveness of the upgrades.
Alonso will be using an old engine this weekend, but he still reckons it is key for McLaren to see how strong the car is now.
"I don't know. It is a question mark also for us," said Alonso when asked what was possible at Silverstone.
"The performance of the car after Austria we were not sure. The new aero package the car feels more competitive but how competitive we don't know.
"It was not our preferred track in terms of layout.
"It will be interesting to see if we can finish the race because after four retirements I would like to finish the race if at all possible, and to see how competitive we are is an answer we need to get this weekend.
"Because we don't know if we are 12th, 17th or eighth. It is an important weekend for us.
"We will put some old engines to avoid any penalties here, which may limit the laps we can do in free practice but we need to find quality laps not number of laps and maximise the performance this weekend and see where we are."
Alonso said the post-Austrian Grand Prix test had been very useful, but conceded the new package needs more running.
"We need as many laps as possible with the new aero package," he said.
"We found some solutions in terms of performance reliability so it was a very useful test.
"Shame we don't have more tests we are really lacking time on the track and laps."
Teammate Jenson Button will also be running the updated car this weekend.