Mmmm excuse me OGELTHORPE, i think i don't get the point.
Between normal rpm (you say around 2000) and "shut down" i expect some middle step where the fan speed is increased basing on the CPU Temp.
If at 53° fan speed is 2000 i expect that at 70° fan speed should be just a little bit over 2000 rpm, and same thing at 80°....
I've reached 76° but no changes in fan rpm. And fans are right since i've downloaded SMCFan Control and now they are cooling down the laptop @4000 rpm
Perhaps first let me define what I consider 'normal'. It would include low or moderate tasks not requiring heavy CPU usage. Examples would be what we do when we correspond on this forum, general web browsing, word processing, etc. My empirical observations have been that my MBP under those conditions runs the fans at approximately 2000 RPM (it minimums peed).
When the CPU is pressed by applications such as games, video compiles and runaway applications, the temperature will increase and the fans will respond to those increases. I do not know the way the MBP fan speed is correlated to the temperature, such as at what temperature the fans will start increasing speed. I do know that the MBP should shut down if the temperature gets too high so that the MBP is not damaged.
If you have chosen to take over the control of your cooling fans, I don't see too much of a downside if you run them faster than the MBP might (perhaps they might wear out sooner? nit). My only concern would be if they would be running slower than required which could result in a premature shut down. The temperatures you have quoted are well in line for moderate usage. As I write this, my CPU temperature is about 48 degrees.
My response to you was to answer "what about normal fan behaviour? I mean, at which temperature i should hear the fan?". Perhaps I could have been clearer and stated that you can always hear them, and as they approach 6000RPM, the sound will be quite noticeable. I cannot give you any decibel levels.
My MBP 5.1's fans begin speeding up at a CPU temp of around 80 C and accelerate to top speed before 90 C, which brings the temp back down to about 85 and holds it there even with both cores at 100%. Note that i've never seen any of these temps on my machine except when I'm deliberately stress-testing it, but then I don't play games or do anything with video on it.
BTW, i'm doing something i consider as an Heavy load task, i'm video converting, so both core at 100% and proc temp @80°, the rpm is increasing slowly, depending on the amount of TIME (this is the X factor) the system works on a certain temp.
So, basically, @80° now i'm @2350rpm, and counting... untill the system is colled down to a "normale" temp. So this mean that @77° can happen that i'm now @2600 rpm (yes, higher RPM value even if temp are lower), since the system is under high temperatures from quite a bit of time.
Anyway, forcing different rpm to the fan makes me concerned about fan life, so i think i'll use SMCFan Controll just in case of heavy load, skipping any other automatic system that will increase temp basing on the proc temp (like Fan Control does).
Thanks to all
SMC Fan Control just allows you to set a higher minimal speed, the normal hardware/OS X system of fan adjustment is still at work.
The normal OS X/hardware system is when the CPU gets hot, the fans increase speed to keep the temperture down. It all depends upon what the CPU is doing and the ambient room temperture being able to remove the heat from the CPU's.
Increasing the fans speed a tiny bit, say a extra 1000 RPM's from 2000 RPM and leaving it that way, keeps the CPU cooler all the time, so when it does start working, it doesn't get as hot as quickly because it's working from a lower starting temperture.
If your gaming or using the CPU heavy, well it's going to get hot regardless and OSX is going to kick in and race the fans to 4000+ if it needs too. But most times people don't place great demand on the CPU's, they want to just be able to lower the temperture a bit so it's more comfortable and that's what SMCFanControl can do.
You shouldn't be having the fans on 4,000 RPM all the time, that's excessive and yes, could wear them out faster. However a fan is less expensive to replace than a logicboard.
You might want to run a Hardware Test, one of your fans might have stopped working and that results in a hot machine. You can replace the fan yourself, but if it's under warranty/AppleCare you should have them do it.