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I've found a similar question but it's about the 2016 iMac, so I want to double check for the current (2019) model.

Can I mix new RAM (Crucial CT2K16G4SFD8266) with Apple's stock RAM? I am planning to buy an additional 32GB of memory, so it'd be a total of 40GB - is that even a number the 6 core iMac can handle?

2 Answers 2

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Yes, you can mix them. There are 4 slots, so you can keep the Apple RAM, even if it's in 2 slots.

According to the reliable EveryMac, your Mac can take up to 128Gb !!

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  • 1
    I totally endorse this. More ram is always better than less ram. The system will beep or not run when the memory isn't compatible.
    – bmike
    Jun 29, 2019 at 14:19
  • Just in case someone stumbles upon this: I've done the upgrade with the mentioned RAM and the iMac runs fine with 40gb of memory. Jul 1, 2019 at 16:10
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my test results with 2019 iMac 8GB and 32GB

I tested a mix of two kinds of memory sticks with different size capacities and different benchmarks results

A) 2x4G Original Apple SK Hynix

2292 15/36 https://www.memorybenchmark.net/ram.php?ram=SK+Hynix+HMA851S6CJR6N-VK+4GB&id=11986

15/34 https://benchmarks-stat.com/page_2_ram.php?id_ram=11986&ram_name=SK_Hynix_HMA851S6CJR6N-VK_4GB

B) 2x16GB Kinsgton HyperX Impact KHX2666C15S4/16G

3202 17/32 https://www.memorybenchmark.net/ram.php?ram=Kingston+KHX2666C15S4%2F16G+16GB&id=10472

17/32 https://benchmarks-stat.com/page_3_ram.php?id_ram=10472&ram_number=4&ram_name=Kingston_KHX2666C15S4/16G_16GB

configurations (note: the bottom slot is Bank 0 the top slot is Bank 3)

#1: 8G Bank 0 Channel A DIMM 0 4GB Bank 1 Channel A DIMM 1 Bank 2 Channel B DIMM 0 4GB Bank 3 Channel B DIMM 1

#2: 32GB Bank 0 Channel A DIMM 0 16GB Bank 1 Channel A DIMM 1 Bank 2 Channel B DIMM 0 16GB Bank 3 Channel B DIMM 1

#3: 40GB Bank 0 Channel A DIMM 0 16GB Bank 1 Channel A DIMM 1 4GB Bank 2 Channel B DIMM 0 16GB Bank 3 Channel B DIMM 1 4GB

#4: 40GB Bank 0 Channel A DIMM 0 16GB Bank 1 Channel A DIMM 1 16GB Bank 2 Channel B DIMM 0 4GB Bank 3 Channel B DIMM 1 4GB

test results Novabench - each test run three times and since results vary a bit, I am reporting an approximate range (for RAM test only)

#1 24.5K to 26.5K MB/s #2 27K to 29.5K MB/s #3 26.5K to 27.5K MB/s #4 16.8K to 17.2K MB/s

test results Geekbench 5 (single/multi-core)

#1 1218-1220/5510-6856 #2 1310-1326/7850-8220 #3 1187-1197/7760-7880 #4 1187-1192/6250-7750

my analysis:

the Apple Hynix is slower than the Kingston HyperX Impact as can be seen by comparing #1 and #2, where #2 was the fastest of all.

for the mix, #3 where the sticks are staggered was the fastest, but not as fast as #2 (apparently the Apple sticks being slower, bring the overall speed down???)

#4 mix (which if I am not mistaken is what Apple recommends) was the slowest by far in Novabench, 16K vs 26K MB/s, yet about the same as #3 mix in single-core Geekbench and bit faster than #1 in multi-core

see https://discussions.apple.com/thread/251751930?answerId=253366511022 for what Apple recommended

for a while, I was running #2 with the 32GB Kingston, however, it seems that apps now require more and more memory, and so I just switched to configuration #3 with 40GB

hope this helps

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