No need to go on - there is no equivalence. It's like trying to compare a diesel car to a petrol car - pointless. One (diesel/BL) is just so far ahead of the other (petrol/BR) in the efficiency league that it results in lots more performance for a given energy input. It's fair to say that current classes wouldn't be possible if we stuck with BR.
What has evolved is a number of classes for beginners, Club specialists and Nationals. For the old 27T class, 17.5 has been adopted. For 19T we now seem to have 13.5BL, and for the old 12T class it's 10.5BL. Mod is still Mod...
However, is you put these cars on the track with the old NiMh/BR combinations, the new BL classes are streets ahead. HTH
