using the cpu burner that comes with furmark (it lets you select how many thread to run) if i select just two threads and using real temp to measure the cpu core temps i see two cores at around 70 C and two cores at around 40 C.. the cores that are working run hotter than the cores that are not working..
all pretty obvious to me and how things should be.. if the OP is seeing all his cores at the same temp all his cores are working irrespective of what he thinks should be happening or what the benchmark says..
trog