Just installed my second 7850 today. Everything went fine, no hiccups whatsoever. I boot up, reinstall fresh drivers, and reboot. Then I go and check GPU-Z. What's this?? The second card is being choked of bandwidth! Card numero uno has a bus width of 256 bits while numero dos only gets 32 bits! What gives? I checked CCC, and yes, Crossfire is enabled, so that isn't the problem. What can I do to fix this?
Another thing, the first time I benched with two cards, I got 'correct' performance, that is to say, Heaven scores roughly doubled. But once I applied a relatively minor OC of 140mhz, things went to ***- Heaven ran worse than if I only had one card. I cut the OC, then it continued to suck.
i7 3820 @ 4.625ghz 1.3v
ASUS X79 Sabertooth
2x HIS Radeon HD 7850
Corsair Force GT 240gb
NZXT Switch 810
I am running a 7870 Crossfired with a 7850 and the second card occasionally displays a bus width of 32 bits as well. Experimenting with turning crossfire on and off and rebooting has made me come to the conclusion that gpuz isn't working 100% accurately with the card(s). Sometimes it will show the full bus width whereas other times it will show only 32 bits. Keep in mind I'm using 12.3 as my drivers because from my research online, its the best set of drivers for crossfire for these cards, and this is one of the first (if not the first) set of drivers for the 7000 series. Thus it makes sense for there to be small bugs, etc.