BMW Sauber's head of engineering Willy Rampf says there is still a lot of work to do for the team to extract the maximum from the upgrade package introduced in Singapore.
The Hinwil-based team is running a major upgrade this weekend, hoping it will help its drivers take a significant leap forward.
Rampf admitted, however, that the challenge is not easy, as the whole team has to get used to the new parts.
"We brought a comprehensive development programme for the F1.09 to Singapore," said Rampf.
"From an aerodynamics point of view, this comprises a new front wing, more deeply undercut sidepods, a further developed double diffuser and a modified rear wing. Added to which, the F1.09 also has a totally new and more compact gearbox.
"For the drivers it is, of course, a challenge to get used to the heavily modified car. We gained important indications, but are not where we want to be.
"There is still a lot of work ahead of us. However, this is not unusual if you bring such a large modified package."
Nick Heidfeld posted a promising fourth fastest time in today's practice, while team-mate Robert Kubica finished down in 10th.