Overclocking an sli system, differences to single card?

[JonJ678]

As I understand it, two cards in sli appear to the OS as a single card which runs quickly. So monitors can only be connected to one of the cards, and if each has 512mb of ram, the combination still only has 512mb of ram as it's mirrored between them.

This suggests it overclocks like a single card, evga precision for example raising the clocks on both cards at once, and stability testing failing when one of the cards can't cope anymore.

However I think I read somewhere that you should overclock the cards individually, so with sli disabled and the bridge removed, test separately, and then enable sli using whichever clock speeds both cards can handle.

Which approach is the sensible one? Both together would be convenient as I wouldn't have to unplug either, but if it gives worse results then I'll take the thing apart.

Cheers

EVGA precision seems to be behaving itself with sli enabled, but I'm still unsure

 

[setter]

Is it not dictated also by the cards maybe having different clock speeds, ie a factory oc'd card ran with a stock clocked card. Ive ran sli in the past but only with idenical clocked same model cards. Bit different from what your after maybe jon.

 

[JonJ678]

Mine are both at stock speeds now, but are both factory overclocked 8800gt's. I've flashed them both to the 600/1500/1800 stock to start with to check they both take the quadro drivers alright, and to check they'll do sli. Seems like they do (which is awesome), but now unsure how to find out what clocks they'll do before flashing them to an overclocked setting.

Furmark seems to be stressing both quite happily, and if I move the sliders in evga for one gpu, it moves them for the other. So it looks like this will be sufficient, but I think I'll be able to stress them better in isolation.

 

[setter]

Looks good so far, same clocks on my asus model, 600/1500/1800. As you suggest, try them individually, with the 7900gt's i had ages ago, iirc one clocked a fair bit higher than the other when tested individually. When i sli'ed them, both had to be kept at a lower clockspeed to suit the lesser card. The 8800gts 640's i had were both great clockers as single cards or in sli. Just shows the variance in hardware batches i suppose.

 

[Insanties_birth]

As I understand it, two cards in sli appear to the OS as a single card which runs quickly. So monitors can only be connected to one of the cards, and if each has 512mb of ram, the combination still only has 512mb of ram as it's mirrored between them.

This suggests it overclocks like a single card, evga precision for example raising the clocks on both cards at once, and stability testing failing when one of the cards can't cope anymore.

However I think I read somewhere that you should overclock the cards individually, so with sli disabled and the bridge removed, test separately, and then enable sli using whichever clock speeds both cards can handle.

Which approach is the sensible one? Both together would be convenient as I wouldn't have to unplug either, but if it gives worse results then I'll take the thing apart.

Cheers

EVGA precision seems to be behaving itself with sli enabled, but I'm still unsure

Seem to have dropped into the wrong thread by mistake. Sorry.