group a racing was the top echelon of racing way back when
pretty much unrestricted engine size, power, engine capacity, 2wd/4wd etc etc etc
as a consequence there were really fast cars and cars that werent so fast
that my understanding why GrpA racing was canned (like gary said - uneconomical and too vast a difference between the good and the not as good)
the japanese GT500 and the aussie v8 supertaxis all stem from the GrpA - but restricted engines, engine capacity, production based cars (so no 4wd supertaxis) etc etc
somebody correct me on teh finer points of it, but that to me is the general gist of it