PCI-e spacing for sli gpus

Breakchimp · Jun 12, 2017

Hi there, I've recently gone and got myself the parts for a new pc build and needed some advice on gpu spacing in the pci-e slots for an sli setup. Basically, I want to get an hb sli bridge but can't decide if I should be using the two x16 slots for best performance, or using one of the cards in a lower slot to improve cooling between the two. The cpu for my new build is an intel i7 6900k and the mobo is the msi x99 gaming pro carbon. So if I'm right my graphics cards would benefit from both being in x16? The only trouble is the cards I have gone for are the evga 1080ti ftw3 x2 and they are as bulky as hell, leaving very little gap between them when both in x16. Should I get a longer hb sli bridge and drop the second card lower to keep the top card cool? Would this affect performance? Thanks in advance for any help

dotas1 · Jun 12, 2017

Just so you know, when you activate SLI, the PCI-e runs in both cards x8 mode. So if you have another PCI-e x8 lower you can connect the second GPU there with no problems.

InvalidError · Jun 12, 2017

SLI isn't officially supported in an x4 PCIe slot with Nvidia's newer GPUs. You must use slots that are at least x8 electrical, wherever they may be located. If you want to put your GPUs three slots apart, you'll have to change to an SLI-capable motherboard with the appropriate PCIe x8/x16 slot spacing.

raisonjohn · Jun 12, 2017

The i7-6900K CPU you have provides 40x PCIe Lanes. In your Msi x99 motherboard, the 4x PCIe3.0 x16 slots can be configured to run using your 40-PCIe-Lane CPU, in any of this:

1st slot (PCI_E1) / 2nd slot (PCI_E3) / 3rd slot (PCI_E5) / 4th slot (PCI_E6):

1) x16/ x0/ x0/ x0*
2) x16/ x16/ x0/ x0*
3) x16/ x16/ x0/ x8*
4) x8/ x16/ x8/ x8*

*Note: The bottom-most slot (PCI_E6) shares lanes with the M.2 socket when running in PCIe x4 mode. If you plug in your GPU at the bottom-most slot, the GPU will still run at x8 speeds, but, the M.2 will run in PCIe x2 mode only.

Note also that the PCI_E1 and PCI_E5 shares lanes. So, when 2x cards are plugged in the 1st and 3rd slot, both will run in x8/x8 speeds (which is the minimum speed for SLI).

Therefore, if you don't want the two GPUs adjacent to each other, you can plug the SLI cards in either 1st slot + 3rd slot (will run at x8/x8), or 1st slot + 4th slot (will run at x16/x8), or 2nd slot + 4th slot (will run at x16/x8).

Breakchimp · Jun 12, 2017

Thanks for the replys everyone. Looked at my mobo manual again and it all makes a bit more sense now. So given that I could take advantage of pci-e slot 1 and slot 3 for x16/ x16 (I think) would this give me notciably better performance? Or given that I'm only air cooling the cards and the gap between them using slots 1 and 3 would be around 10mm maximum, would the top card get too hot? Just wondering what people with more experience building pcs would do in this situation? I'm thinking of moving the second card down and getting the hb sli bridge for the larger gap but I don't want to sacrifice x16/ x16 if the performance is noticeably better or if the top card would be fine with such a small gap.

raisonjohn · Jun 12, 2017

Breakchimp :

You won't see a noticeable performance increase using an x16/x16 versus an x8/x8 setup (in fact, in some synthetic benchmarks, the x8/x8 scores better). https://linustechtips.com/main/topic/502425-x16x16-vs-x8x8-sli-resultsawkward/

Further reading/watching:
[video="https://www.youtube.com/watch?v=rctaLgK5stA"][/video]

As I mentioned in my previous reply above,

1st slot (PCI_E1) / 2nd slot (PCI_E3) / 3rd slot (PCI_E5) / 4th slot (PCI_E6):

1) x16/ x0/ x0/ x0*
2) x16/ x16/ x0/ x0*
3) x16/ x16/ x0/ x8*
4) x8/ x16/ x8/ x8*

Therefore, if you don't want the two GPUs adjacent to each other, you can plug the SLI cards in either 1st slot + 3rd slot (will run at x8/x8), or 1st slot + 4th slot (will run at x16/x8), or 2nd slot + 4th slot (will run at x16/x8).

This means the only setup you can run two cards at x16/x16 is plugging the GPUs on the 1st and 2nd slots (with the 3rd slot required to be unoccupied).

Since x16/x16 vs x8/x8 doesn't matter, and you prefer to have airflow between the cards, I would personally install the two GPUs at the 1st slot and the 3rd slot (x8/x8).

Breakchimp · Jun 12, 2017

Thanks for the help, I'll drop the 2nd card into a lower slot and grab a longer hb sli bridge, if there's no performance gain there's no point melting one of my gpu's

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