2x GTX Nvidia 1080 in a 5,1 MacPro without extra PSU possible ?

[serge-m]

Hi dear MacRumorians,

having a 5,1 Mac Pro with 12x 3,46 GHz and currently using an unflashed GTX 980 in it, I was wondering if my machine can handle 2 GTX 1080's... - without an extra PSU.

I ask, because "Bear Feats" did a benchmark with the new Pascal driver on Sierra 10.12.4 and on their benchmark list, they use 2 1080's in a single Mac Pro.
They said there:

"In case you haven't noticed, we added the results for a pair of GeForce GTX 1080s. We could not resist seeing how much speed they added to the 'multi-gpu aware' apps. All we needed were 6pin-to-8pin adapter cables to use the Mac Pro's factory PCIe 6pin power cables.

There is no startup screen with un-modified Pascal series GPUs. One of our GTX 1080s had a modified ROM courtesy of MacVidCards and, therefore, provided a normal startup screen."

So, sorry if this has been already discussed here (haven't found a thread for this), but can anybody tell me, if it is possible and if: where do I have to pay special attention for ?
- what kind of cards should I buy (with less power consumption)...
- do I need a flashed one not only for the booting screen, but also for a better communication for macOS...

Helpful answers are very appreciated

Edit:

I saw on a another test (http://barefeats.com/mvc_pascal.html), that he uses the power supply from the optical bay:

cMP>1080Ti*2 - DUAL NVIDIA GTX GeForce 1080 Ti GPUs in x16 Slots 1 and 2 of the 2010 Mac Pro (with power supplemented by lower optical bay)
cMP>1080*2 - DUAL NVIDIA GTX GeForce 1080 GPUs in x16 Slots 1 and 2 of the 2010 Mac Pro (with power supplemented by lower optical bay)

Is there a workaround to follow ?

 

[h9826790]

I guess what they mean is that they put an extra PSU in the lower optical bay, but not simply using the lower optical bay SATA power.

Yes, there is extra 55W avail there. However, no way to properly draw just 55W from there if the 1080 only has a single 8pin input. Also, extra 55W clearly not good enough to help to drive 2x 1080Ti.

Let assume we use the founder edition as reference. It draw ~135W from the 8pin. So even though we ignore the 75W limit form the mini 6pin. A single mini 6pin is not quite enough to power a 1080. 135W from a single mini 6pin most likely will shutdown your Mac.

Then how about SATA + 6pin? Yes, that enough power. 55W + 120W (mini 6pin real world limit) = 175W, which is > 135W. However, the cable cannot just draw 55W from the SATA port. It can only draw evenly from the power source, which means ~67.5W from the mini 6pin (which is good), but also 67.5W from the SATA port (which is bad). So, 2nd option doesn't work.

To drive 2x 1080. One of the option will be

2x mini 6pin -> single 8pin -> 1080 (A card)
3x SATA -> single 8pin -> 1080 (B card)

So, each mini 6pin only need to deliver 67.5W, which is within limit. And each SATA port only need to deliver 45W, which also within limit.

However, you lost 3 SATA port.

In fact, IMO, the best way to power 2x 1080 or 2x 1080 Ti is by Pixels mod. In that case, you can bypass all the hurdle and get power directly from the 980W PSU, which is good enough to power 2x 1080 or 2x 1080Ti.

 

[deppest]

+1 on pixlas mod, have been running a maxwell titan x and 980ti for 6 months, no problems at all

 

[serge-m]

Thanks for the detailed explanation, h9826790

There do I find a well explained workaround for this Pixlas mod. ?
With all the details like extra cables, devices and stuff ?

 

[Prince134]

See the Ultimate mac pro 5.1 mod:

https://forums.macrumors.com/threads/ultimate-mac-pro-5-1-mod-for-triple-titan-x.2043646/

Internal power for two Titan X. If you put extra 12V power, you can run 3rd Titan X. Also lots of results are provided there.

 

[serge-m]

Ok, so here I am again
I decided to go with the SATA Power version, that "h9826790" suggested: to use 3 SATA bays to pull power from. And it worked quite well so far !

I actually don't need the bays, because I run a SSD Blade with 1x 960GB and 1x 512GB as my working drives and am quite satisfied.

I also did this, because it was the easiest way for me to run this card without sorting and rearranging some power cables (like the Pixels mod). Cost me also only my old card (GTX 980) and around €18 for the cables.

I upload some pics, so that you can see how it looks like.

As you can see, the two graphic cards are pretty close to each other and also to the SSD Blade.
I put some little plastic things between them (green arrows) to keep some distance for air supply:

Here are the cables:
1x PCIe Power Cabel SATA 15pin to PCI Express 6pin with extension (6pin mal to 6pin female)
1x PCIe Power Cabel SATA 2x 15pin to PCI Express 6pin with extension (6pin mal to 6pin female)

I had to adjust the holding to get them fit into my Mac Pro:

This is the GTX 980 (PC version) with is 2x 6 pin connectors:

And here's the first Octane Benchmark:

I have to say, that under full throttle the cards get "quite" warm/hot...
Is that ok ? Do I not get into trouble with overheating or maybe damaging the SSD Blade ?

So far: thanks again, Forum, for this input

 

[h9826790]

Ok, so here I am again
I decided to go with the SATA Power version, that "h9826790" suggested: to use 3 SATA bays to pull power from. And it worked quite well so far !

I actually don't need the bays, because I run a SSD Blade with 1x 960GB and 1x 512GB as my working drives and am quite satisfied.

I also did this, because it was the easiest way for me to run this card without sorting and rearranging some power cables (like the Pixels mod). Cost me also only my old card (GTX 980) and around €18 for the cables.

I upload some pics, so that you can see how it looks like.

As you can see, the two graphic cards are pretty close to each other and also to the SSD Blade.
I put some little plastic things between them (green arrows) to keep some distance for air supply:

View attachment 708448

Here are the cables:
1x PCIe Power Cabel SATA 15pin to PCI Express 6pin with extension (6pin mal to 6pin female)
1x PCIe Power Cabel SATA 2x 15pin to PCI Express 6pin with extension (6pin mal to 6pin female)

View attachment 708446

I had to adjust the holding to get them fit into my Mac Pro:

View attachment 708447

This is the GTX 980 (PC version) with is 2x 6 pin connectors:

View attachment 708445

And here's the first Octane Benchmark:

View attachment 708444

I have to say, that under full throttle the cards get "quite" warm/hot...
Is that ok ? Do I not get into trouble with overheating or maybe damaging the SSD Blade ?

So far: thanks again, Forum, for this input

Thanks for your sharing. I won't worry too much about overheating. Both GPUs and PCIe SSD has very reliable thermal throttling. If getting too hot, performance will be degraded, but should not damage anything.

 

[serge-m]

Thanks for your sharing. I won't worry too much about overheating. Both GPUs and PCIe SSD has very reliable thermal throttling. If getting too hot, performance will be degraded, but should not damage anything.

You're welcome.
So, you sure ?
I was touching the 980 during the bench test and it got quite hot on my fingertip.
Also, it smells "warm electronic" out of my Mac, that I guess should be normal, if there are two cards full running...?

 

[Flint Ironstag]

They will throttle safely if needed. I've been beating mine up with hash cat. Depending on the algorithm they can be made to throttle, but on NTLM, MD5 they just run and run full tilt for weeks (nMP and cMP).

 

[h9826790]

You're welcome.
So, you sure ?
I was touching the 980 during the bench test and it got quite hot on my fingertip.
Also, it smells "warm electronic" out of my Mac, that I guess should be normal, if there are two cards full running...?

Yes, they can safely run at >80 degree C (still run at full speed). And it's normal.

If your GPU running exceptionally hot (overheating), the GPU's fan (and PCIe fan) should spin up a lot, which easily catch your attention because of its noise.

 

[barzag]

Hi every one,

It's posible to connect two GTX 1080 in SLI in windows ?
I have a Mac Pro 5,1 with windows 11 and Opencore
Thank's