iMac 2019 - upgraded w 2 extra ram and the speed dropped to 2400 MHz?

Discussion in 'iMac' started by hampus, May 9, 2019.

  1. hampus macrumors newbie

    Feb 29, 2008
    Hi everybody.
    I bought the iMac 2019 27" base model with 2x 4GB ram in it. Today I upgraded it with 2x 16 GB 2666 MHz DDR4 Corsair Vengeance. At first the computer didn't boot, but after inserting the new Corsair ram into the slots the original ram was, and placing the original ram in the empty slots, it managed to boot. But upon checking that there was registered 40 GB ram, I noticed that the speed of the ram had dropped to 2400 MHz from the original 2666 MHz. Why is this?

    The ram is now placed like this (from top to bottom): 4-16-4-16

    I have attached a screenshot of the memory submenu in the system information.

    I appreciate your help!

    Attached Files:

  2. wardie macrumors regular

    Aug 18, 2008
    AFAIK all the RAM modules will drop back to the speed of the slowest as the controller needs to run at a single frequency for it all (could be wrong regarding dual channel pairs though)
  3. DeltaMac macrumors G3


    Jul 30, 2003
    Did you try moving the RAM sticks so the memory is installed in identical pairs per channel?
  4. macduke macrumors G4


    Jun 27, 2007
    Central U.S.
    Should have gone with the Crucial. It's rock solid and inexpensive.
  5. sxl1681 macrumors newbie

    Jan 18, 2019
    As DeltaMac mentioned, try pairing the capacities together, 4-4-16-16 or 16-16-4-4 to see if that helps. If it doesn't, I suspect that the Corsair Vengeance is geared for PC's with XMP support. These types of modules usually have conservative speeds programmed into the SPD chip so that the PC will boot/pass BIOS memory test, usually 2133 or 2400 speed. Once the PC fully boots up the BIOS, it loads the XMP profile to boost memory speed and then loads the OS. Macs do not support XMP and will only read what the base value is programmed on the SPD chip. Best to return the memory and buy something labeled as Mac specific or get non-XMP memory.

    Note that you can use XMP memory in a Mac since it just ignores the XMP data, but you will need to look up exact part numbers on a manufacturer's site to find what the SPD settings are and get some that is programmed to 2666.
  6. hampus thread starter macrumors newbie

    Feb 29, 2008
    Well, they are - the dual channel is A-B-A-B in the slots.

    Funny thing is that the RAM are 2666 MHz but I suspect they are only if overclocked which isn't possible on a Mac
  7. DeltaMac macrumors G3


    Jul 30, 2003
    Yes, that works, of course.
    I am still suggesting that, rather than A-B-A-B, try plugging the ram into A-A-B-B (So, pair of 16GB in channel A, and other pair of 4GB in Channel B slots.
    (Humor me, just try it)
    Do an NVRAM reset after moving the sticks around.
  8. smbu2000 macrumors regular

    Oct 19, 2014
    Have you tried only running the Corsair RAM? Also try the NVRAM reset like was suggested earlier.

    I run the exact same 2x16GB Corsair RAM kit (CMSX32GX4M2A2666C18) in my 2018 Mac Mini and it has been working fine at 2667MHz since I installed it at the end of last year. I can't run it with the original 8GB kit that was included as there are only 2 memory slots on the Mini.


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7 May 9, 2019