I've been thinking of an estate, A6 avant of 5 series touring Audi has an advantage of quattro But BMW engines seem to get better reviews and on paper more power, economical etc. i.e. 530d vs 3.0tdi the 530d wins on performance and mpg etc. Then the BMW top trumps Audi with the 535d, a twin turbo 3.0d. Why has Audi not responded with something similar?