Previous gen IvB i7 in multithreaded tests you can google with stock fan gets to 59degC and haswell in the same test stock fan can get to 78degC so there is quite a difference. Imac has only one fan as far I know so cooling is very limited, also fan threshold is set to start increase speed at around 95degC core temerature, so it will heat up more than stock fan in other desktop pc before cooling will kick in. So taking all of this together in theory we should expect haswell in imac to be warmer than haswell mounted in normal PC. Also I read that from i7 4770k asus has tested 70% can hit 4.5Ghz , 30% hit 4.6Ghz and only 10% hit stable 4.8Ghz, so even though imac has got 4771 with locked overclocking I expect that the same rule would apply: some lucky people will get cpu that can work at slightly lower temperatures than others. And of course environmental factor mentioned in one post should also matter, in warm rooms or even during summer times when average temerature is much higher fan may be used more often.
This is a pure theory, how it applies to the real application is a different story. I tested Starcraft II 8 player map with 8 bots highest ai added and fan never kicked in through the whole game, max core temperature was in low 90degC but average cpu temerature was much lower, possibly Starcraft doesn't use all 4 cores properly. I tested also another rts : DOW2 with biggest map and all expert bots, fan was quiet again through the whole game. I don't have BF3 so if someone could run 64 player map it would be a great test because it usues 4 cores or even hyper threading to some extend. CPU demanding games can be a good test because fan has to cool not only cpu but also gpu which can get itself to 80degC or more. Also would be great if someone could test video processing, 3d rendering or sound processing it would give us more indication if haswell in imac can handle high cpu load for a long period of time with low fan speed.
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