I'm back to folding on both my GTX 580. The second card (an EVGA SC ACX) was hitting 100 C during the recent hot weather, it had been overheating since I bought it some months ago. Big_Elf sent me a reference cooler, which I misplaced when decluttering with my carer. I checked the cooler a couple of days ago, and one of the fans was not turning (it was at stock speeds, 797 MHz, and undervolted to 1.00 V). I turned my pile of computer parts upside down and found the reference cooler, which I installed today. I set both cards to run at the SC stock speed and voltage, cranked the clock speed to 898 MHz, and the hot card is now at 63 C (with the fan at 85%). FAHControl in Windows estimates my ppd at around 180K with both GPUs and a 6700K on 6 threads. Next step is to put it all under water (I got two 580 blocks cheap with cards on eBay - just waiting on the mail now). Bonus is if I kill the cards, I will have two spare. I'm also hoping to have a 780 to put in my everyday working and browsing computer to fold on in a couple of weeks. Just waiting to hear back from DocJonz about that
