Dual GPU in cMP without power mod

Discussion in 'Mac Pro' started by Lauwie, Aug 13, 2015.

  1. Lauwie, Aug 13, 2015
    Last edited: Aug 13, 2015

    Lauwie macrumors regular

    Jun 17, 2011
    Hey guys !

    I'm not really into modding my macs power supply, adding an additional PSU or performing the mod Pixlas came up with.
    Because of this I went into search for using another method to supply both GPUs with power.
    At this moment I'm using the SATA power of the optical drive area (can't really care about losing 2 drive bays)
    I'm planning on buying PCIe extension power cables to wire the power cables via the back of the case up to where the power cables are located behind the removable wall in the optical drive bay (having the lid of isn't ideal nor sexy with those cables sticking out)

    I've also tried using the SATA power in the numbered drive bays but that doesn't seem to work :/

    I've also come across a possible solution where you don't lose 2 SATA ports which would involve using one of these (the price is stupidly expensive but you get the idea of the cable), and then wiring the SATA cable to the optical bay area and use SATA power splitters on the SATA cables of the optical bay area so you can mount 4 SSDs in there. (I've done my best to explain this hope you understand :) )
    Screen Shot 2015-08-13 at 17.38.49.png
    Anyways I hope this is of help to a lot of people !

    Cheers !
  2. SoyCapitanSoyCapitan macrumors 68040


    Jul 4, 2015
    Nothing cleaner than what you see guys doing in Pixlas thread
  3. Lauwie thread starter macrumors regular

    Jun 17, 2011
    Yeah but there are people like me who are not willing to mod the wires and leave the system as is.

    Never mentioned it being cleaner then other solutions..
  4. h9826790, Aug 17, 2015
    Last edited: May 14, 2016

    h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    Just finish my dual 7950 setup without power mod. Each card connect to a single 6pin source by a mini 6pin to 2x6pin adaptor.

    Overall power draw from the PSU is just 430W, not even reach 50% of the 980W limited of the PSU. During this test, Handbrake + OpenCL simulation to achieve 100% CPU loading. For GPU, it's running Luxmark to simulate normal GPU heavy loading.
    PSU Max.jpg
    The terms in Hardware monitor is misleading. The Slot 1 Line is actually the 6pin power source from the mobo, Slot 2 Line is another 6pin power source. My HIS R9 280 draw 77W from a single 6pin cable, just 2W over the limit, not significant at all. And it draw the remaining 38W of power from the slot. The Sapphire 7950 Mac Edition draw less power from the slot, only 25W, but it draws 86W from another 6pin cable. It's over the limit, but I am still happy with this figure. If both card can craw a bit more from the slot, and a bit less from the cables, then it will be prefect. However, there is no way to control it.

    I limit both card to 130W TDP in the BIOS as safety precaution (assume the card can only draw 30W from the slot, and able to draw 100W from the cable), but none of them reach 130W in this test. I am sure it can if I run Furmark, but I don't want to do that for stress test. No real world application will use that much power anyway. And I did try to run Furmark after the initial setup is completed. No problem at all, won't over load the Mac and shut itself down. However, I only have a quick check from iStat (7.99A as expected from a single 6pin cable) but didn't take any other data like the Luxmark test. It seems the 130W TDP safety precaution able to protect the Mac Pro to certain level.

    And here is the Luxmark result. Close enough to single 7950's result x2. So I consider this is a success.
    V3 10.10.5 CF.jpg
    Since both card limit to 130W TDP and tuned to minimum stable voltage. Even the reference cooler can easily handle the heat. However, to achieve better cooling, I didn't install the card in slot 1 and 2, but install the 2nd card in slot 3. It's just a x4 slot, however, almost no impact on Luxmark (and zero performance degradation in Win 10 Valley Benchmark with crossfire enabled). Performance of course is important, however, I am willing to sacrifice some performance for much much better cooling. The R9 280 stay quiet all the time, and the 7950 only produce very little fan noise. On the other hand, the PCIe fan can reach 2000RPM when both GPU under full load after few minutes. However, IMO, the noise still at an acceptable level. (Update: MacFanControl can fix the PCIe fan at any available speed. And I now fix it at 1300RPM, which is virtually silent in my place and provide adequate cooling for the GPU and HDD etc.)

    During the test, I found that the PCIe fan speed may not only related to the power draw from the card, but also the GPU temperature (even though there is no way to monitor it in OSX). If I put both GPU in slot 1 and 2, then the PCIe fan will idle at around 11xxRPM. I assume it's because of the poor cooling for GPU in slot 1 cause the high idle. If I put the 2nd GPU in slot 3, than the idle will back to 800RPM. So, obviously this idle fan speed is not only related to the total current in both 6pin cables.

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