Cross firing two 7870s between a PCI express 3.0 and a 2.0 slot.

[radicalpancakes]

Hey guys, I recently got a good deal on a gigabyte radeon 7870 at microcenter, This was to replace my 6850 that I had been running before. I was thinking that I might crossfire it in the future with the same card but my motherboard has only one pci express 3.0 slot. This means that I would have to pair up the cards on a 3.0 and a 2.0 slot. How will this affect the performance and what are your suggestions for what I should do? My mobo is a z77a-g41 from MSI.

 

[spawnkiller]

your motherboard don't support X8 (it's X16 but support only X4) on the second slot so it'll be horrible for performance of the second gpu...
http://www.msi.com/product/mb/Z77A-G41.html#/?div=Detail

It's only your motherboard that is rather cheap as it would work with a X8 pci-e 2 and a X16 pci-e 3 without issue, the X4 however will bottleneck a high-end gpu like the 7870, it will work but you will not be really happy with it as the second card will never be capable of using all it's power as it'll always wait for informations to run trough the slot and the bandwith will not be enough...

 

[blazorthon]

your motherboard don't support X8 (it's X16 but support only X4) on the second slot so it'll be horrible for performance of the second gpu...
http://www.msi.com/product/mb/Z77A-G41.html#/?div=Detail

It's only your motherboard that is rather cheap as it would work with a X8 pci-e 2 and a X16 pci-e 3 without issue, the X4 however will bottleneck a high-end gpu like the 7870, it will work but you will not be really happy with it as the second card will never be capable of using all it's power as it'll always wait for informations to run trough the slot and the bandwith will not be enough...

PCIe 2.0 x4 does not inhibit gaming graphics performance much more than PCIe 2.0 x8 and that's still not much at all. Most information between cards in Crossfire is transferred through the Crossfire bridge, not the PCIe lanes, at least when you're not in something like some extremely high-end triple or quad Crossfire solution. Crossfire isn't nearly as bad as SLI is when it comes to PCIe bandwidth for the secondary cards.

 

[RazberyBandit]

That board is technically CrossfireX capable, but the PCIE 2.0 slot is limited to x4 speed. Essentially, the card in the 3.0 slot would have the maximum x16 PCIE-3.0 bandwidth allotted by the Z77 chipset, but the bandwidth of the card in the 2.0 slot would be lessened quite a bit in comparison. Such a setup wouldn't achieve ideal Crossfire performance, but there would be a gain in performance. Typically, you can expect a 7870 Crossfire configuration to scale upwards of 70- to 90-percent (sometimes nearing 100-percent) in many titles. A Crossfire setup using your motherboard would not yield the same levels of Crossfire scaling due to the slightly limited bandwidth of the 2nd card.

What you'd ideally want for a Crossfire or SLI configuration is a board that has two identical x16 slots, which would then split the x16 into a x8 + x8 configuration. MSI's Z77A-G45 and above boards have such a setup - they all use two PCIE-3.0 x16 slots. (So do many boards from other manufacturers.)

Lastly, you could potentially run into further bandwidth limitations for the 2nd card since it shares its 4-lanes (x4) with the remaining PCIE-x1 slots. Should you make use of additional cards in those slots, you would end up taking away some of the already slightly limited bandwidth.