I'm not into over clocking, I don't really understand enough about it, but recently I've had to get some software that can control my 27-inch, Mid 2011 iMac's fans (smcFanControl) to up the RPM a little to get it to cool down some, but was warned that too high an RPM over a sustained period of time could burn out the fans!
Now I don't want to do that and have it around 1300rpm but really need to go higher, so would like to call on the experience of those who know best as how to proceed..
Thanks!
As others have noted, fooling with the fans should never be necessary on an iMac. I have the same model as you and have experienced the following heat related failures which thankfully have all been covered by AppleCare:
LCD panel x 2
Glass panel that covers the LCD x 2
DVD drive
6970m GPU
All three internal fans were also replaced with the last repairs done.
So, nobody can tell me a mid-2011 iMac doesn't have issues with heat because mine sure does and it is made exactly the same way and with the same components as any other iMac of this model year and size.
That said, cranking the fans isn't the answer as it will just burn them out before their natural life expectancy. I would recommend being sure to keep not only the vents in the back clear (a large paperclip works well for this after vacuuming it to be sure they are entirely clear) as well as the vents along the bottom which can be easily removed with a screwdriver. When your machine heats up and you can hear the fans spinning up, try holding your hand close to the upper vents on the rear of your iMac just beneath the top of it. You should feel air flow there. If you do not, that is a major problem you need to attend to in order to avoid damage to components. Even though the CPU and GPU will shut themselves off to avoid melting, that doesn't mean that prolonged exposure to heat for them and the entire interior of your iMac is anything you want to allow to happen. It should always be venting upward and outward via the fans but it won't if airflow is blocked.
It is my understanding that models since the 2011 now run cooler than the 2011's do and I am glad to hear that. I think the design of the 2011's is at least somewhat flawed based on what I've seen with my own, as much as I love the thing, love OS X, etc. I am hopeful that my next iMac will not keep melting itself the way this one has.
Sorry to go on so long but that's my experience for what it's worth. It's also why I preach to people about the value of AppleCare. I would have been really screwed without it in this case.
So in summary, I agree with the suggestion to get a decent utility to report on system temps. I like one called aptly enough, "Temperature Gauge" which provides readouts on every sensor in your iMac and also provides an easy way to see the highest it has gone since the last reboot. In addition to that you can tell it to log the temps in comma separated value format so you can import that data in Numbers or Excel and view it, graph it, etc. It's a great way to have a look at what happens when you run a demanding game for example. You can see how high the temps go. You might be surprised as I was to see that the hottest sensor consistently is not the CPU or GPU at least in my iMac. It is the power supply heat sink generally. The Temperature Gauge is available on the Mac App Store for five bucks and in my opinion is money well spent for any iMac owner. Monitoring the temps periodically can tell you about trouble before stuff starts melting allowing you an opportunity to do something about it before failures happen. I'll be watching my temps now and making sure the vents are clear for openers if I see them starting to rise and otherwise, I'll be calling my friends at AppleCare if the vents are clear and it is still melting again.
In my own case I do not have high confidence that this machine is going to make it. Why, I don't know since many 2011 iMacs do I presume. Maybe there is some issue with fan control on the logic board? Who knows but Apple owns the problem now after all the work that's been done under AppleCare and fortunately they are very cool about that, no pun intended.
Good luck and I hope a good cleaning and temp monitoring is all it takes to stabilize your system. If not, it is time to start considering what is going on with Safari in particular since you mention that app.
Oh, and to answer your original question about fan speeds since I know you were at least curious, the three fans from what I understand should be running within the following ranges in a 2011 iMac:
ODD 1150 RPM - 2700 RPM
CPU 940 RPM - 2600 RPM
HDD 1100 RPM - 5500 RPM
Although the GPU has a large attached heatsink, it does not have a fan which is why there is no report on one for it. I assume that by design heat is drawn off that heatsink by the ODD fan in particular blowing air by it. I base that on having seen the layout of the components while my system was being repaired each time.
As a point of reference, with my iMac currently idle other than Safari where I am typing this, all my fans are running at minimum values, the upper case feels warm but not hot and I can feel airflow there. Now, running a game is another story. In fact, I need to do some testing there and create some logs, etc. but haven't gotten to it. I'm not overly concerned but that is only because I have AppleCare and it is Apple's problem if this thing melts again. I am just curious about the extent to which gaming pushes my hardware.
