I'd be surprised if your GTX 980s' throughput was reduced by running them at each at 8x. There is more than enough bandwidth to accommodate a GTX 980 even at PCIe 3.0 8x. Google "GTX 980 at 8x" for more information.

Same issue here!  ASUS Maximus VI Formula - Z97 board that can only support 2x8!  Lost half my FPS in SLI mode.  Selected 1 GPU for P3D and am back to full speed.  Seems to be running PhysX on the other GPU.  Waiting for new motherboard (whole system).

I use 2x8

http://forum.avsim.net/topic/468975-p3d-v25-sli-test-results-up-33/?p=3245187

Seems to be running PhysX on the other GPU

 

That shouldn't matter since P3d doesn't use PhysX, at least there has never been any mention by LM of it doing so. To test that speculation, change the PhysX setting to CPU and see if it makes any difference.

 

That fact that two people are getting similar poor results  in SLI with the same hardware setup deserves further investigation, but I wouldn't blame it on the mobo until other possibilities have been ruled out. I   would delete the P3d shader binary folder and rename Prepar3d.cfg. Then let the sim rebuild both cfg file and the shader binaries. I would try different screen resolutions. It almost seems like it's an issue with the nVidia driver and the GTX 970s in SLI.

Same issue here!  ASUS Maximus VI Formula - Z97 board that can only support 2x8!  Lost half my FPS in SLI mode.  Selected 1 GPU for P3D and am back to full speed.  Seems to be running PhysX on the other GPU.  Waiting for new motherboard (whole system).

 

I have the same board but not sli - find it hard to believe that board is not sli friendly have to look into that 

Same issue here!  ASUS Maximus VI Formula - Z97 board that can only support 2x8!  Lost half my FPS in SLI mode.  Selected 1 GPU for P3D and am back to full speed.  Seems to be running PhysX on the other GPU.  Waiting for new motherboard (whole system).

 

From what I have read running sli would be fine and no noticeable difference between 16(x) and (8x by 8x) as the cpu comes into play here also - now a Formula board for 3 way sli is not setup well for that - who cares not running 3 gpus thats for sure - you might want to look into your settings and rig setup something doesnt sound right - bios check also - not running sli so thats about all I can offer at the moment

Rob mentioned switching from full screen to window and back to reset SLI.  I did some testing and found that there is something going on between P3D and NVIDIA concerning SLI configuration.
 
Using NVIDIA recommended SLI settings:  Cockpit view - 33 fps, VC view - 21 fps, GPU1 8%, GPU2 27%
Using Single GPU settings:  Cockpit view - 74 fps, VC view - 30 fps, GPU1 30%, GPU2 22%
 
Seems like the settings are backwards.  Current solution is the set P3D to Single GPU and get a reasonable SLI effect.  All the switching between using Alt-Enter did was switch which GPU was active when using the Recommended settings.

 

I hope Rob adds this to his comments back to LM. 

 

P.S.  On a single GTX980, using the same configuration I think I was getting ~40/30 fps.

Configuration:
Sensor Data: AIDA64

Test Case: PMDG 777-20LR at KIAH gate A14, P3D executed by opening the .xfml file, changed to unlimited fps and not black out second screen

Display: 2x 1900x1200, 2x ASUS Strix GTX980
Processor: 4790k @ 4.6GHz, water cooled
Memory: 16 GB
Motherboard ASUS ROG Maximus VI Formula
P3D FSUIPC, TrafficManager, GSX, FSDT KIAH, REX, REX Softcloud, Orbx Global, FreeMesh
Networked ASN, ProATC X (Not running in these tests)

Growing pains with software support seems to be the answer not your rigs

This all seems to confirm my suspicion that nVidia's work/progress on a proper SLI implementation for P3D is far from finished.

This all seems to confirm my suspicion that nVidia's work/progress on a proper SLI implementation for P3D is far from finished.

 

Absolutely but at least its on the to do list

 

 

Absolutely but at least its on the to do list

 

No doubt, but my pessimism drives me to hope that nVidia didn't merely take a couple of swings at this issue to shut us all up and now considers the matter solved.

 

Again, I hope that the next release is more polished and provides better performance gains/scaling. I would also suspect that Prepar3D makes it into the names of titles that the driver specifically supports or enhances...

No doubt, but my pessimism drives me to hope that nVidia didn't merely take a couple of swings at this issue to shut us all up and now considers the matter solved.

 

Again, I hope that the next release is more polished and provides better performance gains/scaling. I would also suspect that Prepar3D makes it into the names of titles that the driver specifically supports or enhances...

 

We may have forgotten i may take an actual P3D update also which is in the pipeline and hoping for some major improvements on this one as 2.5 was nothing to write home about

Using NVIDIA recommended SLI settings:  Cockpit view - 33 fps, VC view - 21 fps, GPU1 8%, GPU2 27%

 

This means SLI is not working correctly when you have one GPU % much lower than the other GPU %.   I run "full screen" with Vsync + triple buffer + unlimited ... the Alt + Enter will put you back into "Window" mode, then doing Alt + Enter again will get you back into Full Screen mode and in my case resolves the low GPU1 % and they equalize out.

 

You may also want to try out Vic's NI profile here: https://www.dropbox.com/s/xcr7j7ja9qb074c/P3D25%2B.nip?dl=0

 

Cheers, Rob.

I'm using window mode, without Vsync and unlimited, and it works perfect.