Initially when we went for an overclock in our reference review article we hit roughly 1200~1250 MHz. That was being a little careful. With voltage tweaking you can get in the 1300~1400 MHz range quite well.
We do have to say though that anything over 1350 MHz on our card really wasn't stable enough, making roughly 1300 MHz (with the dynamic clock) our maximum on this card at default RPM cooling settings. That's still a ~20% clock increase and that is huge all by itself.
Overclocking the GeForce GTX 680 will however never be the same again with the new Dynamic boost feature. You have your base-clock, add say 100 or 200 MHz and make sure that you expand the power limitation to 132% giving the card some breathing space as the TDP limitation is lowered.
From there onwards it can go two ways really, stable overclocks at a set clock frequency or you'll end up at say 1300 MHz, and due to the Dynamic boost sometimes a little lower or higher. There is a downside though, sometimes the Dynamic boost feature went nuts on us and as protection clocked down the core frequency towards the 736 MHz range. Definitely a protection as only restarting the system would get the normal clocks back on their feet again. This happened to us only once though.
As long as you don't voltage tweak your temperatures will merely gently jump up a little, as well as the power consumption and the noise levels. They all remain within perfectly fine thresholds though.
Overall the results don't lie, overclocking the GTX 680 is worth it as the card will jump up in performance quite a bit as you have been able to see. The differences are definitely significant.
It will be interesting to see custom cooled GeForce GTX 680 cards as I have no doubt a stable 1400 MHz is achievable on air cooling as well alongside a tiny bit of extra voltage.
So then, we'll not to make this conclusion any longer then needed, the GeForce GTX 680 is impressive at many fronts however we are impressed by the overclocking potential. Even without voltage tweaking, it yields results in more performance while the surrounding conditions like TDP, heat and noise remain absolutely under control. That's free extra performance, at very little extra risk or other displeasures (if you do it right).
So grab yourself some tweaking software if you feel you want that additional performance, experiment a little -- we can definitely recommend that.
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