Both charts are for dual X5675 3.06GHz Westmeres running Geekbench2 stress test (more demanding than the one in GB3) with all cores loaded in an upgraded 2009 Mac Pro. No fan control utility used in either. Mavericks 10.9.4: Yosemite 10.10.0 Some comments here have stated that the Mac Pro's fans are quieter in Yosemite than with its predecessors. In the above, Mavericks was running the CPU fans at 3,200rpm compared with 2,200rpm in Yosemite, explaining the large jump in CPU temperatures in Yosemite. The service limit for the X5675 is 178F (with Yosemite CPU A reached 151F - almost 40F warmer than in Mavericks) so if I was doing demanding video renders all day long, I would seriously consider using a fan control utility to speed up the CPU fans. On removal, after one hour of the stress test in Yosemite, the Processor tray/heat sink assembly was very warm to the touch, almost uncomfortably so. Data are much the same over many machines and many tests. Best quality thermal paste was used.