Originally Posted by xoleras
IMO, its probably because you're using 2 cards in SLI; Single card overclocks are always better by a lot. My instability temperature threshold is low on GPU1 when running SLI, I have to keep it very very low in temps for complete stability... i'd take a guess if you removed your 2nd GPU altogether you'd get a bigger overclock with a single card. Thats how it always works usually.
I've oc'ed quite a few dual gpu setups and you'll never OC SLI to the same level unless you're on water generally speaking. In fact, i've gotten both of my cards over 1400 in single card quite easily but in SLI GPU1 becomes temperature limited because GPU2 is blowing hot air onto GPU1 and temps creep up slowly. My temperature instability threshold is low so obviously there's something in addition to core and VRM temps that is important - If I go past 65C on GPU1 in SLI I get artifacts while over volting. That is why I say 100% manual fan is an absolute requirement for OC and OV for maximum overclock benchmarking...
I want to see how my bottom card responds, so I'm going to put that one in the top slot and leave the other out for testing. But when disabling SLI and testing the top card alone, even though I didn't remove the 2nd card, temps were noticeably better, but still got a horrible increase in performance from a ton of voltage. I'll see what happens with the bottom card tested alone later on tonight and let you know.