BMW though says that the turbo lag is not noticeable to the average driver, and that it isn't overwhelming. This phenomenon was caused due to a change in the engines software. While some units were built with the new software, others obtained it through an update at the dealership.
The constructor decided to update the software because of certain noise concerns and the by-product was the increase in turbo lag, and the decrease in noise levels.
BMW didn't recognize this as a problem for some time, but now BMW North America sent a statement that changes the situation.
BMW North America Statement:
“In an effort to address specific noise concerns, BMW updated the turbocharged engine software on 335i/xi and 535i/xi models built from June 2006 through March 2008. The updated software causes a small amount of increased “turbocharger lag” under certain circumstances and, while not substantial, the lag may be perceptible to the most sensitive BMW drivers. X6, 135i, and 3 Series/5 Series vehicles from March 2008 production onward are not affected.
Release of the new software to restore the original turbocharger response is planned for January 2009, and it will be available for vehicles produced from September 2007 through March 2008 when they are brought to an authorized BMW center for a repair which requires a programming update. Clients with vehicles produced from June 2006 through September 2007 may request installation of the January 2009 software update. Clients who get this update should understand that there is a possibility of slightly increased engine noise levels under certain circumstances.”