The 906 heads are great heads, but each factory performance head has there own unique features.... Some have bigger valves than others... Some have a closed combustion chamber... Ect ect ect.... Now as for the 906, I've seen, somewhere, that they've been listed more for racing, than the other heads that could be more "street friendly" but I don't believe everything I read.... Anyway, if memory serves me right, the unique featur about the 906 heads is the floor design?