|By Gerhard Kuntschik and Edd Straw|
The team has been working flat-out on the design, but has opted not to introduce it as part of its new aerodynamic package because it would not have been able to exploit its performance benefit.
Theissen insisted that no date has been set for the diffuser to be introduced, although it is understood that the Monaco Grand Prix is a possibility.
Despite this, the team will have an interim diffuser modification in Spain that is expected to improve its rear-end grip as part of its raft of modifications.
"The Barcelona-spec aero package has long been in development," said Theissen. "Therefore a double diffuser introduced at short notice wouldn't have brought us any benefit.
"There is no time limit set for when we will introduce the double diffuser in our car."
"We will have an substantial update in Barcelona, from the front to the rear wing, affecting also the sidepods, there will be improvements in every detail."
Theissen added that the team has had to re-evaluate its performance targets after claiming only one points finish in the opening four races of 2009.
This season, BMW's fourth as a full-blown constructor in F1, it had been expecting to fight for the championship.
"We moved our season targets that we set in winter aside," he said. "We cannot win races at the moment, therefore we shifted our targets.
"But we won't accept the current situation. We will speed up, we will have a look at where we will end up, but we certainly will not give up on this season."
Barcelona will be the first time that the team has introduced development parts this season, something that Theissen admitted may have been a mistake.
"The main reason we turned down the chance of updates during the first races was the testing restriction," said Theissen. "The other reason was the interference with the aero package on the car.
"This approach looks to be wrong at the moment, but let's wait and see after a couple more races.
"Our performance in Melbourne was fine and it even worked in Sepang, but then the other teams passed us. That was based on a strategy: we always planned not to bring updates in the fly-away races.
"Our first target for an update was always Barcelona and this will happen."
Source : Autosport