Underclock R9 270x for dual GPU on internal power?

Discussion in 'Mac Pro' started by electonic, Mar 18, 2014.

  1. electonic macrumors member

    Mar 18, 2014
    I'm exploring dual GPU options for my 2010 Mac Pro (Currently a GTX 680 flashed running in it)

    The Radeon 7750 and 7770 will work out of the box, but just not enough power for me. Then there's the R9 270 and that won't run at all.

    The 270x runs, and one guy on Netkas even managed to make EFI for it.
    The problem is: the 270x is just barely over 150 Watts. I think it uses about 180 Watts and needs 2 x 6 pin. No dual GPU on internal power.

    So here's my idea:
    Modify a R9 270x BIOS to downclock it - yes, down clock - it two make it use less power so it can run from Slot and 1 x 6 pin.

    And here's my questions for you guys, before I do it:
    Is that possible? Do you think clocking it down to 270 speed will be enough?
    Will it still need BOTH Pins connected, so do I have to use one "dummy" 6 pin, or some kind 1 x 6 pin to 2 x 6 pin adapter?

    I you guys say it's possible in principle I WILL try to pull it of, buy the cards, flash them and post everything here. Dual 270x, even under clocked, would be very, very good for Pro Apps.

    So, what do you think?
  2. snarfquest macrumors regular

    Jun 7, 2013
    I think you should buy a power supply and not worry about it :)
  3. macproredux macrumors member

    Mar 3, 2014
    PS's that fit in the drive bays just don't cost enough to risk it
  4. electonic thread starter macrumors member

    Mar 18, 2014
    Hey, thanks for the reply. But that doesn't help. External PSU is out of the question.

    All I really need to know is what what I have to do with the cables.
    If somebody could give me quick advice if I'd need to connect both 6 pin ports and if they you think down clocking is enough to get power down that would be appreciated.

    The thing is: if you help me pull it off, the worries for dual GPU in old Mac Pro are actually pretty much over.


    These Drive bay PSUs aren't available here. Or I just haven't found any. And: it's still a really messy solution.

    2 x 270x would be enough power. All I need is to safe 2 x 30 Watts and I need to know what I have to do with the cabling.
  5. ActionableMango macrumors G3


    Sep 21, 2010
    I do not have experience with this, so it's speculation only.

    I doubt the cards will work without power connected to both connectors. On other cards it has been proven that there are separately powered zones within a single card. So not connecting power means some board components will have no power at all.

    I also doubt that underclocking will result in a significant enough change to the power draw in wattage, but I don't know how much to expect. You could experiment by measuring the actual power draw of your current card (or preferably a similar card to the one you want) while running a benchmark, underclock the card, and measure the power draw again. This should give you a much better idea of the effect of underclocking on power draw.
  6. electonic thread starter macrumors member

    Mar 18, 2014
    Thanks for this.
    Can anybody confirm / not confirm?


    Btw.: The crazy Bitcoin Miners under volt their GPUs to safe power.

    Look at this. Same clock speed, just less voltage already 15 Watts saved:

    That's why I think this is doable! :D
  7. 0x2102 macrumors member

    Feb 1, 2014
    I will be able to tell you tomorrow.

    I just recently purchased 2 x XFX 7850 Black Edition for my Mac Pro to do the same thing. The card has two 6 PIN connectors but won't boot with only 1 connected. Power consumption is approx. 135 W for 1 card and that should be no problem for 1 PCIE 6PIN and the slot itself.

    I have ordered 6Pin to 2 x 6Pin Y cables.

    I just looked at some of the 270x bios version from ASUS, MSI etc for the Radeon 270x most of them show a TDP of 141W + 20% PowerTune - which will then equal to 169W. You can lower the TDP and/or the PowerTune to stay within the 150W limit.

    Another options is a pair of overclocked Radeon 7850 - like I did. You will need to modify two Kernel extensions and update the device id prior to installing the cards - to support them with the native OS X drivers.

    EFI is needed for boot screen.
  8. electonic thread starter macrumors member

    Mar 18, 2014
    Finally! Thanks a lot.
    That's exactly the info I needed.
    Let me know how your project turned out.

    I'll order the two 270x then, when I know how your cabling turned out.

    Let's pull this off! There's still a lot of life left in our babies ;)
  9. 0x2102 macrumors member

    Feb 1, 2014
    The 6Pin to 2 x 6Pin cable works fine on my Radeon's 7850 Black Edition.

    Ran FurMark stress test for 30 minutes for now - no issues.

    With the 270x you will need to be a bit more careful and watch TDP with Hardware Monitor while you stress test it.
  10. electonic thread starter macrumors member

    Mar 18, 2014
    Thanks a lot for the information and your help.

    When I compared the 270 and the 270x from the specs I just thought that clocking down the 270x to a little less than the 270 should be enough. I'll try to under volt them slightly at the same time.

    The 270x runs at 1000 mhz with boost to 1050 Mhz and needs 180 Watts.
    270 is 900 and boost to 925 Mhz.

    What I would do now is set the 270x to 900 Mhz and never boost. Hell, I'd even go down as low as 850 Mhz, if I need to. Still much better than a single 7950 and better than my gtx 680 for OpenCL.

    My steps now would be:
    1. Get the y cables ;) ... and the 2 x 270x, of course ;)
    2. Dump bios. And what tool? VBE7, right? Edit speed and voltage with that.
    3. Add EFI to the modified with script from Netkas.

    Anything I missed?
  11. 0x2102 macrumors member

    Feb 1, 2014
    I noticed that some 270x cards have a UEFI bios - that might cause issues with the mod you have in mind. I don't know for sure.

    Best thing is to test and download the bios from the exact same card you want to buy and open it in VBE7 to see if you get the UEFI warning.

    Download / search your card here:

    1.) Get ATI_WinFlash (remember to right click and run as Administrator)

    2.) Get VBE7 - Version 0.0.7b (latest version)

    3.) Get GPU-Z latest version to monitor GPU related things

    4.) Get MSI Afterburner 3.0 (latest beta 18)

    4.) Save 270x existing ROM from ATI_WinFlash or with GPU-Z to a file

    Open the rom with VB7 and change the TDP under PowerTune from the default to something like 125W to be safe. Then lower the clock for state #4 and #0 to maybe 925 and keep everything else. You might want to lower the voltage a bit - which will bring down temperatures. Keep the rest and re-flash your new bios.

    Reboot with the new GPU bios. I would do one card at a time.

    Then run a stress test like Furmark:

    Look at the Frames per second for a simple test in 1024 x 768 or something and then run MSI Afterburner and play with the clock speed to find the sweat spot. There is no need to clock high if you lower the TDP but there is a certain spot where clock and TDP provide the highest scores.

    After you found your setting take a look to add EFI for boot screen.

    I don't know how to write/modify an EFI - so I just used the existing 7950 EFI and coupled it with my 7850. This worked well under OS X but caused a blue screen in Windows when I activated Crossfire. I did some testing and as far as I can tell this is related to the EFI. I ended up with two cards without EFI (boot screen).

    Hope this helps as a start.

    Good luck.

    PM me if you need any further help.
  12. electonic thread starter macrumors member

    Mar 18, 2014
    Thanks a lot for your help. Great advice. I appreciate this a lot!

    Best way indeed seems to first do the TDP, frequency band voltage mod with regular bios and only if that works then try to add the EFI part. I guess I could live without boot screen... ;)
  13. electonic thread starter macrumors member

    Mar 18, 2014
    Just a quick update:
    have been too busy to do go on the with the "project", but I'll do it soon.

    News is that 10.9.3 beta now has device ID for your 7850. So I think no kext modification necessary anymore?

    Makes me almost reconsider what way I'd look to go for dual gpu. ;)
  14. electonic thread starter macrumors member

    Mar 18, 2014
    Update: I'm now going for dual Radeon 7950. I've already bought one to look at TDP and at EFI. EFI and Boot Screen is really easy on these. And Underclockling is really easy, too. They actually use way less power than I though. With the stock 800/1250 frequencies my card's bios says 146 Watts.

    Which tool can I use to check usage before I install two of them with you y cable trick (Already have the cables, too).
  15. 0x2102 macrumors member

    Feb 1, 2014
  16. ActionableMango macrumors G3


    Sep 21, 2010
    I have to admit I'm curious what application you'll be using this for.
  17. electonic thread starter macrumors member

    Mar 18, 2014
    I'm just trying to future-proof my Mac Pro 5,1 with a Dual GPU setup.

    I've sold my GTX 680 because the performance was really, really bad in OpenCL apps. With the one 7950, which is supposed to be a less powerful card, the speed up in FCPX alone is so big, it's unbelievable. Nvidia is really just good for games and if your job requires you to use CUDA-only apps.

    The Apps I use every day are:
    FCPX, Aperture, Pixelmator, Photoshop ...

    The first three are all Dual GPU optimized already.
  18. ActionableMango macrumors G3


    Sep 21, 2010
    IIRC, Nvidia did something between the 5xx and 6xx series that really crippled OpenCL and maybe even CUDA on their gaming cards, in order to protect the Quadro line. So yes, the 680 is really terrible for that as you noticed.
  19. Chung123 macrumors regular


    Dec 5, 2013
    Whatever became of your project? Did you go for the Dual 7950s? Did you require an aux power supply?
  20. electonic thread starter macrumors member

    Mar 18, 2014
    I bought two 7950 an tried. I got them to run but had random shutdown. That made me really scared so I sold the second card and I'm now on one slightly overclocked 7950. Luxmark right now for that one card is 1850 points.

    I would like to go for dual GPU, but I'm really confused.
    It's said that there's support for many new AMD GPU in Yosemite, but I can't figure out which ones.
    Does somebody know if R9 270 (without X) is supported by Yosemite now?
  21. crjackson2134 macrumors 601


    Mar 6, 2013
    Charlotte, NC
    Well, I just installed an HD 7970 from MacVidCards. Of course this card has already been modified in all things needed, and it runs like a champ. Really nice upgrade from the HD 5870. It is supported OOTB in ML, MAV, YOSE.

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