Robert Kubica has admitted that BMW Sauber's major upgrade package will probably not be enough to bring the team back into front-running contention.
BMW is introducing an array of new components for this weekend's Spanish Grand Prix, although its version of the double diffuser will not be seen until Istanbul next month.
Having slipped back into the midfield by Bahrain, Kubica welcomed the development parts, but warned it would take more than this upgrade to get BMW back on the pace.
"All the information and simulation data is giving quite big numbers – at least half a second (of improvement)," he said. "But we have to see how it feels because we are quite far away from the top so we have to make double the number of steps of the others.
"For sure we need at least two steps of improvement if the others stay at the same level, because the gap lately was nearly one second, or even more. So we need at least one second if the others stay at the same level, but we assume the others will improve.
"Also the teams who were behind at the start of the season, like Ferrari and McLaren, are already in front. We have to move forward, keep working. I think we have to have a bit more time as well, to see and analyze which direction we are going in, and try to deliver more steps in the future."
He admitted that BMW needed to make a rapid and drastic improvement if he was to retain any hope of the title, having failed to score in the opening rounds.
"Last year after seven races we were leading the drivers' championship, and we were coming to Barcelona as constructors' championship leader," said Kubica.
"This year is completely opposite so for sure for the championship it will be difficult. It's already difficult now. First of all I think we have to concentrate on more improvements, and then adapting our targets to our situation."