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one question though, why aren't the risers powered?

If the cards are powered by 6/8 pin plugs and aren't strictly powered by the pcie connector, there shouldn't be a need for the risers to be powered. Seems to be the case here.
I wasn't aware of that, you learn something new every day! thanks

Hi, the risers aren´t powered because when I finished the assembly I took the photos, and then I connected it to the power.
Regards.
Hows the heat dissipation? I'd expect it to get quite warm as all the heats going into the center
Depends on the graphics cards and the heatsink design. Most of the heat from my graphics cards come out of the top of the card (EVGA 1080gtx) so it might not push as much heat into the center.