Running in Crossfire with one card in PCIx16 and other card in PCIx8

[mohitbhagat]

So here is my situation. This is very urgent, and I would extremely appreciate your help.

So I recently bought a motherboard from NCIX, the Gigabyte Z97X-SLI
However, the board only comes with one PCIEx16 3.0 sot,
and the other slot for multi-graphics config is a PCIEx8 3.0
I plan on cross-firing 2, Radeon R9 290's
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So if I were to put one card into a x16 slot and the other into a x8 slot, how much would my performance diminish?
Does putting a card into a x8 slot, only give you half the performance?
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Should I return the board, and just invest another $70 and buy one with 2 PCIEx16 slots?
Here is the rest of my build - My Build

Please help !!!!!

 

[Suztera]

If you going to run in Crossfire, both slots will run at x8.

 

[jakjawagon]

PCIe 3.0 x8 still provides more than enough bandwidth for modern graphics cards. There wouldn't be any difference between two cards in x16 and the same cards in x8.

 

[ko888]

The Intel Core i5-4690K only has 16 PCI Express lanes dedicated for the graphics card(s). When using two graphics cards each graphics card will be assigned 8 PCI-E lanes to the CPU.

Any Z97 motherboards that are capable of dual x16 use a PLX PEX8747 PCI-Express gen 3.0 x48 bridge chip to multiplex each card to the CPU making it appear like there is 32 lanes. That multiplexing chip adds an extra hardware level between the graphics card(s) and the CPU so there may be some lag.

The performance difference between a PCI-E 3.0 x16 slot operating with 16 lanes or with 8 lanes is only 1%.

There's no point in spending an extra $70 with no noticeable performance difference whatsoever.

 

[mohitbhagat]

What about for 4k gaming? will there be any difference?

From what I'm hearing so far, I think I might by keeping my current mobo

 

[mohitbhagat]

Thank you very much for all of your solutions. This was really helpful for me. And I decided to stay with my current motherboard