http://www.swedespeed.com/features/project_cars/854/part2.shtml
This was an article we recently did on our Project 854R. It contains a pretty good description of why you'd want slotted over drilled, unless the rotors were forged with the holes in them already like Porsche does.
This was an article we recently did on our Project 854R. It contains a pretty good description of why you'd want slotted over drilled, unless the rotors were forged with the holes in them already like Porsche does.