...sorry to poke my nose in
BMW have quoted 12,7% loss on the Manual, 15% loss on the auto.
If it was more that 15% they wouldn't be able to keep up with the Manuals, and often in a race they have been known to be slightly quicker, what with the fast changes and the continuous Boost hold, which can't happen on the Manual.
SO
If the engine is rated at 306 BHP as per my 335i brochure, it should, deliver about 306x0.85 for the auto or 260 HP at the wheels.
This agrees nicely with many of the Dyno charts I've been seeing which run from anywhere at 255 to 275 hp at the wheels.
Now to go back to BHP you simply divide by the inverse of the loss, so
260/0.85 = 305 BHP.
Easy really :-)
Just insert your wheel HP figure or your BHP to suit.
SJ