Adding Memory to My Mac Pro

Discussion in 'Mac Pro' started by fastlanephil, Apr 23, 2015.

  1. fastlanephil macrumors 65816


    Nov 17, 2007
    My 2009 5.1 3.46GHz 12-core Mac Pro has slots 1,2,3 and 5,6,7 filled with 2GB and 4GM 1333MHz DIMMs. I'd like to go from my current 16GB or RAM to 32GB of RAM. It looks like installing 8GB DIMMs in the two empty slots would be the most economical solution but I remember reading that filling slot 4 and slot 8 will slow the RAM down to 1066MHz?

    If true, would this be a problem or just a slight RAM speed hit?

  2. h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    No, in most of the real world usage, you won't feel anything.
  3. IowaLynn macrumors 65816


    Feb 22, 2015
    I think you use to need to have 3&4 be matched, and to put high capacity DIMM in lower slot (1 and 5). Or even put 2x8GB on one bank and 4-4-2-2 on 2nd.
  4. MacVidCards Suspended

    Nov 17, 2008
    Hollywood, CA
    It is possible to have all slots poplated and stay at 1333. Someone else knows the conditions needed, I just know I did it and it works.
  5. ActionableMango macrumors G3


    Sep 21, 2010
    Over the years I've read a lot about this in the forums. A lot of things can cause your 1333 to drop to 1066 with all slots populated. I don't remember all the reasons, but using quad-rank memory or having mismatched memory sizes are two very common reasons.

    Several people have had success:

    OWC seems to know what the magic formula is, but you'd have to replace all your existing RAM and not just add a couple of sticks.
  6. flowrider macrumors 603


    Nov 23, 2012
    The CPUs in the cMP use three channel memory. The third and forth slots are wired together to form the third channel. I would put the "2GBs" in the 3rd and 4th slots (7th & 8th) (4GBs for channel 3) Move the 4GB to slot 2 (6) and put the new 8GB Dimm in slot 1 (5). That should keep you at 1333MHz. It's not optimum, but should work out.

    BTW, I would stay away from, OWC. My memory supplier since 1986 has been:

  7. AndreeOnline, Apr 23, 2015
    Last edited: Apr 23, 2015

    AndreeOnline macrumors 6502


    Aug 15, 2014
    Admittedly, I haven't thought too much about it, but the 'wired together' was news to me.

    When I first got my Mac Pro I was advised to stay away from slots 4 and 8 due to 3 channel and performance.

    I currently run 3+3 8GB modules for 48GB (1333GHz) in a flashed 4.1-->5.1. Could I add another 16GB by filling up all the slots without timing penalties?

    EDIT: and by timing penalties I don't mean dropping to 1066—I know I'll stay at 1333. I wonder more about the 3 channel setup…
  8. IowaLynn macrumors 65816


    Feb 22, 2015
    You have 5,1 firmware so that is one requirement, along with Westmere X56xx cpu which is another. All DIMMs supporting 1333.
    Ranking, unbuffered, number of DRAM chips on each.
    The difference between 3's and 4's is usually outweighed by amount, but in the case of 2x2GB maybe not so much ;-)

    Another good source: [As mentioned above, better prices and less issues with RMA returns etc]
  9. carylee2002, Apr 23, 2015
    Last edited: Apr 23, 2015

    carylee2002 macrumors member


    Jul 27, 2008
    You currently have a 12 core machine with another 12 virtual cores. You should be adding 2 gb of ram for every core so you should be looking at 48gbs ram if you use your machine for any aftereffects or heavy grading. Now it is best to try to fill the first 3 slots on each side with equal amounts of ram to keep the tri channel configuration. IF filling the 4th slot then you should do it both sides and be even. It is best if you get the same ram and not mix with different speeds or manufacturers.
  10. fastlanephil thread starter macrumors 65816


    Nov 17, 2007
    I'll give that configuration a try.


    I've been buying my memory from for some time with no issues. They say all memory is Mac tested.

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