4GPU rig - one GPU at low hash: would powered risers help?

[Superdaan]

Hi all,

I have just added a 4th GPU to my rig. Previously I was running 3x 280x GPUs, each hashing at 735mh/s.
I have now added a 4th GPU (7950). Set it up so that I have two instances of cgminer running to deal with the fact that the 7950 wants 1 gpu thread, whereas the 280x's need 2 gpu threads.

The 7950 is hashing just fine (at about 700 mh/s), and two of my 280x cards are at about 740 mh/s.

However the 3rd 280x is now only hashing at about 530 mh/s.
All settings for the 280x's as are before from when they were each running at 735.

The only solution I can think of (I've done fairly extensive googling, there seems to be no consensus though hence my posting here) is that perhaps I will need a powered riser for the 3rd 280x. All 280x's are on unpowered risers. The 7950 is on a powered riser.


So basically my question is, will a powered riser have an affect on hashrate? I mean, all cards are running / hashing so you could figure they get 'enough power' to do their job. But perhaps the slow 280x is simply causing a bit too much power to be drawn from the motherboard for it to mine at an optimised speed? Could it possibly make a difference?


Thank you

[zmhaha]

I would get 2x powered riser for 4 cards. And a powered riser will not decrease the performance unless the riser itself is faulty.

Also try permute the slots and cards, if the problem is only happening to a certain card even though it is in different slot, I'd say the card itself may have some issues but don't really know a way to solve it.
 
if the problem is only happening to a certain slot and does not matter which card you put it in, then I'd say that mobo may not be providing enough power to that slot while there are other 3 cards installed. Then I will try to use the powered riser in your hand on that slot and see what happens.

I had a issues with 4x 290x on my X58 board.
any 3 card worked fine in any slots. (with only 1 powered riser)
Once I put the 4th card on, the system crashes right before the windows logo screen everytime.
And I tried any permutations of the slots/cards. none of them worked.
Then I got another powered riser(so totaling 2 powered, 2 unpowered), problem solved.