I should start charging every time I explain how Turbo Boost works. Your card, every GTX 770 by default, has a Temp Limit of 80c. You can check and confirm this in Afterburner or PrecisionX. What that means is that your GPU was designed to target exactly 80c as its normal operating temperature while gaming. Above that temp your card will lower its Turbo Boost clocks, decrease voltage, and increase the fan speed to maintain and stay at 80c. Below that temp and your card will ramp up more with higher Turbo Boost clocks, increased voltage, and lower fan speeds.
Note the Temp Limit is part of the overclocking/Turbo Boost parameters that can be manipulated through Afterburner/PrecisionX; it is NOT the same as the thermal threshold...