Memory upgrade compatibility list for iMac 2019-2020

[quynhtrang]

I have a base model 2019 27 inch iMac with 8GB of RAM. I’m looking to upgrade the RAM to 16GB by buying some OEM 2 x 4GB On eBay.

My 2019 has SK Hynix HMA851S6CJR6N

I’m looking at getting HMA851S6DJR6N This is from a 2020.

Can anybody tell if this slight variation could be an issue?

 

[hoodafoo]

I have 2020 4GB x2 (HMA851S6DJR6N) pulls sitting on my desk. How much are they going for?

btw, a quick search shows that the C or D represents the die generation

 

[wilberforce]

I have a base model 2019 27 inch iMac with 8GB of RAM. I’m looking to upgrade the RAM to 16GB by buying some OEM 2 x 4GB On eBay.

My 2019 has SK Hynix HMA851S6CJR6N

I’m looking at getting HMA851S6DJR6N This is from a 2020.

Can anybody tell if this slight variation could be an issue?

Should be fine. Should be 2666 MHz if from a 2020. Hopefully you are not paying more than about $30 (if USA), because there are a lot of these taken out of 2020 iMacs.

 

[quynhtrang]

Thanks hoodafoo and wilberforce. So it’s “Die Generation”.

I’m in Australia. EBay sellers here, sell that OEM 2 x 4GB SK Hynix or Micron for about $US8 to $16. Some more optimistic sellers price it closer to $US30 or $35 or even more, but it won’t sell. That RAM with part no I quoted will sell for about $US15 plus postage ($6).

if I was more advanced user, I’d upgrade with 2 x 16gb Crucial for $US140. The only time I‘ve notice any memory swap is with using Chrome that I need to use with my work from home.

 

[unkyp]

I have not tried these specific part numbers, but yes, you can mix two different part numbers in the 2020 iMac, provided the DIMMs are all the same size and specs, but instead of the usual order in the slots 1,2,3,4: ABAB, you will likely need to install them in the order: AABB
A = the one part number
B = the other part number

The 2020 iMac wants to see the same part # in slots 1 and 2, and the same part # in slots 3 and 4, otherwise speed drops to 2133. (A weird quirk of the 2020 iMac.)

To get full dual channel, the total size in Channel A (slots 1+2) must equal the total size in Channel B (slots 3+4).

The only way to satisfy both these requirements, if using two different part numbers, is to have all DIMMs the same size and installed in order AABB.

I have confirmed this on my 2020 iMac (but not with your specific part numbers).

I am 99.9% sure it will work fine, but suggest make sure it is returnable (Crucial usually is).

btw, I suspect your iMac will only be about 1% more beastly with this upgrade, but maybe you have extraordinary RAM needs

Thanks, I can confirm I have managed to get 128Gig RAM on imac 2020 with 26667 speed, using part numbers CT2K32G4S266M in AA and CT2K32G4SFD8266 in BB. It didn't work with the usual ABAB as wilberforce mentioned. Thanks again

 

[hoodafoo]

Thanks, I can confirm I have managed to get 128Gig RAM on imac 2020 with 26667 speed, using part numbers CT2K32G4S266M in AA and CT2K32G4SFD8266 in BB. It didn't work with the usual ABAB as wilberforce mentioned. Thanks again

You might be getting full speed on each dimm, but are you getting dual channel?

 

[unkyp]

You might be getting full speed on each dimm, but are you getting dual channel?

How do I check that? This is all I can see

 

[hoodafoo]

How do I check that? This is all I can see

oh, you should be ok as far as dual channel since you are not mixing and matching. I believe you have to use a 3rd party util to determine if dual channel is in effect

 

[wilberforce]

How do I check that? This is all I can see

The only requirement for full dual channel is that amount of RAM in Channel A equals amount of RAM in Channel B. Which you have.
The acid test is to do a memory transfer speed test, such as using Novabench. You should get about 28,000 MB/s, maybe even 30,000 MB/s with 128GB RAM. (btw, only need to run the individual RAM test, not all the tests, takes only a few seconds, repeat it several times with nothing else running)

 

[unkyp]

The only requirement for full dual channel is that amount of RAM in Channel A equals amount of RAM in Channel B. Which you have.
The acid test is to do a memory transfer speed test, such as using Novabench. You should get about 28,000 MB/s, maybe even 30,000 MB/s with 128GB RAM. (btw, only need to run the individual RAM test, not all the tests, takes only a few seconds, repeat it several times with nothing else running)

Ran both Novabench and geekbench, scores improved a lot (considering i ran the tests with **** loads open... i know i should run with nothing running and hence the skewed results i guess). Anyways, getting 28418MB/s coz of other apps running... will run another test when not busy with other stuff and see what's the max i can get.

 

[quynhtrang]

I just had 2 x Crucial CT32G4SFD8266 for my 2019 from Amazon Australia delivered. Paid $US243 for 2 single 32GB sodimms, because it was a fair bit cheaper than the 2x 32GB kit at $US329.

Crucial CT32G4SFD8266 32GB (1x32GB) DDR4 SODIMM 2666MHz CL19 1.2V PC4-21300 Dual Ranked Single Stick Notebook Laptop Memory RAM

Crucial CT32G4SFD8266 32GB (1x32GB) DDR4 SODIMM 2666MHz CL19 1.2V PC4-21300 Dual Ranked Single Stick Notebook Laptop Memory RAM

www.amazon.com.au

I wasn’t happy to see that one sodimm was made in China while the other was assembled in Mexico. They booted up ok after I checked the seating of the ram cage thing , in the same slots as the OEM SK Hynix 2x 4GB.

It turns out they are two different numbers, with one having a Crucial manufacturers code (859B) and the other identified as made by Micron. Both had Crucial stickers but one had a Micron label on the other side. I can’t remember which was made where.

Should I return them and try to get 2 with the same part numbers or spend a around $90 more and get the matched kit?

Memory Slots:
ECC: Disabled
Upgradeable Memory: Yes

BANK 0/ChannelA-DIMM0:
Size: 32 GB
Type: DDR4
Speed: 2667 MHz
Status: OK
Manufacturer: Micron
Part Number: 16ATF4G64HZ-2G6B4
Serial Number: 2DF6A7F6


BANK 2/ChannelB-DIMM0:
Size: 32 GB
Type: DDR4
Speed: 2667 MHz
Status: OK
Manufacturer: 859B
Part Number: CT32G4SFD8266.C16FB
Serial Number: E5FB19C8

 

[wilberforce]

I just had 2 x Crucial CT32G4SFD8266 for my 2019 from Amazon Australia delivered. Paid $US243 for 2 single 32GB sodimms, because it was a fair bit cheaper than the 2x 32GB kit at $US329.

Crucial CT32G4SFD8266 32GB (1x32GB) DDR4 SODIMM 2666MHz CL19 1.2V PC4-21300 Dual Ranked Single Stick Notebook Laptop Memory RAM

Crucial CT32G4SFD8266 32GB (1x32GB) DDR4 SODIMM 2666MHz CL19 1.2V PC4-21300 Dual Ranked Single Stick Notebook Laptop Memory RAM

www.amazon.com.au

I wasn’t happy to see that one sodimm was made in China while the other was assembled in Mexico. They booted up ok after I checked the seating of the ram cage thing , in the same slots as the OEM SK Hynix 2x 4GB.

It turns out they are two different numbers, with one having a Crucial manufacturers code (859B) and the other identified as made by Micron. Both had Crucial stickers but one had a Micron label on the other side. I can’t remember which was made where.

Should I return them and try to get 2 with the same part numbers or spend a around $90 more and get the matched kit?

Memory Slots:
ECC: Disabled
Upgradeable Memory: Yes

BANK 0/ChannelA-DIMM0:
Size: 32 GB
Type: DDR4
Speed: 2667 MHz
Status: OK
Manufacturer: Micron
Part Number: 16ATF4G64HZ-2G6B4
Serial Number: 2DF6A7F6


BANK 2/ChannelB-DIMM0:
Size: 32 GB
Type: DDR4
Speed: 2667 MHz
Status: OK
Manufacturer: 859B
Part Number: CT32G4SFD8266.C16FB
Serial Number: E5FB19C8

Should be fine. Not necessary for part numbers to match. Crucial is a brand of Micron.
Suggest do memory speed test using Novabench. Not sure what 2019 iMac should get, but probably about 28,000 MB/s, with no other process running.

You say "BANK 2" - should be BANK 1?

btw, for the 2019 iMac, you can put the 2x4GB DIMMs in the remaining two slots, no problem. It is only in the 2020 iMac you should not do that.

 

[quynhtrang]

Thanks wilberforce. I’ve decided to send them back and try again to get a pair with same part number. I’m going to need to sell this iMac sometime this year and the RAM will fetch a better price on its own.

It was interesting to see the memory usage with that 64gb. Over 20gb was being used with Chrome, Brave and Safari browsers open (I need different browsers for work), a PDF document open in Preview, Mail open, and a 720p HVEC movie open in VLC (for quiet periods at work). With only the 8gb reinstalled, with the same tabs and apps open, it’s using 6.4gb with 350mb memory swap.

 

[VegetarianNachos]

I have a 2019 iMac with the original Micron DIMMs (4GB each) in banks 0 and 2. I would like to add a pair of Crucial DIMMs 2x16GB and am unsure about the optimal banks to put them in. Do I move the 4GB modules to banks 1 and 3, and put the larger Crucial modules in banks 0 and 2? From what I have read that sounds like what others have done. Is there an optimal configuration for the proposed 40GB of memory?

 

[wilberforce]

I have a 2019 iMac with the original Micron DIMMs (4GB each) in banks 0 and 2. I would like to add a pair of Crucial DIMMs 2x16GB and am unsure about the optimal banks to put them in. Do I move the 4GB modules to banks 1 and 3, and put the larger Crucial modules in banks 0 and 2? From what I have read that sounds like what others have done. Is there an optimal configuration for the proposed 40GB of memory?

Either of the following order of slots will give the same performance:
16-4-16-4
4-16-4-16

So the easiest is just to put the new ones in the empty slots.

 

[VegetarianNachos]

Either of the following order of slots will give the same performance:
16-4-16-4
4-16-4-16

So the easiest is just to put the new ones in the empty slots.

Okay thanks!

 

[VegetarianNachos]

Successfully upgraded my Early 2019 27" 3GHz (base) iMac to a total of 40GB of memory using the 2x4GB OEM (Micron) memory with 2x16GB of Crucial

CT16G4SFRA266​

from Amazon. They are all running at 2667MHz and dual channel mode.

Geekbench scores went from; 1102 single core, 4802 multi-core with 8GB ram
to; 1003 single core, 5475 multi-core with 40GB

Novabench shows ram speed of 27,102 MB/s

 

[quynhtrang]

I finally managed to buy 2 x 4gb SK Hynix that had been pulled from a 2019 iMac. There is a surprising amount of competition on ebay for ddr4 ram. Paid around $US20. No memory swap so far with 16gb.

I was just curious about whether a matched pair of RAM is preferable to 2 single sodimms or doesn’t it matter.

 

[wilberforce]

I finally managed to buy 2 x 4gb SK Hynix that had been pulled from a 2019 iMac. There is a surprising amount of competition on ebay for ddr4 ram. Paid around $US20. No memory swap so far with 16gb.

I was just curious about whether a matched pair of RAM is preferable to 2 single sodimms or doesn’t it matter.

For a 2019 iMac it doesn’t matter, provided they are the same size and spec.
For a 2020 iMac it does matter. As you don’t have a 2020 iMac there is no need to get into a long explanation, other than to say it is a quirk of the 2020 iMac.

 

[haddy]

For a 2019 iMac it doesn’t matter, provided they are the same size and spec.
For a 2020 iMac it does matter. As you don’t have a 2020 iMac there is no need to get into a long explanation, other than to say it is a quirk of the 2020 iMac.

I have a 2020 iMac (20.2) 16Gig in slot 2 and 16Gig in slot 4. I am getting two 32Gig RAM modules... can I just fill slots 1 and 3 with them? Will I get dual channel and 2667 Mhz? If not is there any way of arranging the order to get both?
Cheers

 

[wilberforce]

I have a 2020 iMac (20.2) 16Gig in slot 2 and 16Gig in slot 4. I am getting two 32Gig RAM modules... can I just fill slots 1 and 3 with them? Will I get dual channel and 2667 Mhz? If not is there any way of arranging the order to get both?
Cheers

In a 2020 iMac there is no way to get full dual channel and full (2667Mhz) speed unless all DIMMs are the same size.

If you have to compromise, dual channel is much more important than full speed. Which means do as you plan (add the 32GB DIMMs to slots 1 and 3), but the speed will drop to 2133Mhz. Which is not as bad as it sounds, and really only decreases effective real world speed like 5 to 10%, which may be OK for you if you really need more RAM than 64GB. (Do you? If you don't, you will be better off just installing only the 2x32GB DIMMs. Or it would be cheaper to buy 2x16GB DIMMs instead, and add to the existing 2x16GB DIMMs.)

 

[wilberforce]

To clarify, the "rules" (deduced by testing) for full speed and full dual channel using four DIMMS in the 2020 5k iMac are:

1. Part # in slot 1 must match part # in slot 2, else speed drops to 2133
2. Part # in slot 3 must match part # in slot 4, else speed drops to 2133
3. Amount of RAM in slots 1+2 (channel A) must equal amount of RAM in slots 3+4 (Channel B), else lose full dual channel.

It is impossible to satisfy all three rules if you have two pairs of different sizes

 

[haddy]

To clarify, the "rules" (deduced by testing) for full speed and full dual channel using four DIMMS in the 2020 5k iMac are:

1. Part # in slot 1 must match part # in slot 2, else speed drops to 2133
2. Part # in slot 3 must match part # in slot 4, else speed drops to 2133
3. Amount of RAM in slots 1+2 (channel A) must equal amount of RAM in slots 3+4 (Channel B), else lose full dual channel.

It is impossible to satisfy all three rules if you have two pairs of different sizes

Thank you..... will try at first with using all the DIMMs for dual channel and lower speed

 

[haddy]

In a 2020 iMac there is no way to get full dual channel and full (2667Mhz) speed unless all DIMMs are the same size.

If you have to compromise, dual channel is much more important than full speed. Which means do as you plan (add the 32GB DIMMs to slots 1 and 3), but the speed will drop to 2133Mhz. Which is not as bad as it sounds, and really only decreases effective real world speed like 5 to 10%, which may be OK for you if you really need more RAM than 64GB. (Do you? If you don't, you will be better off just installing only the 2x32GB DIMMs. Or it would be cheaper to buy 2x16GB DIMMs instead, and add to the existing 2x16GB DIMMs.)

Well well well I have just installed, in my 20.2 27" iMac running Catalina 10.15.7 with all the updates:

slot 1 16GB DIMM / slot 2 32GB DIMM
slot 3 16GB DIMM / slot 4 32GB DIMM.
Total 96GB.

And the speed for all 4 DIMMS is 2667Mhz and ok!!!

What do you figure?

 

[wilberforce]

Well well well I have just installed, in my 20.2 27" iMac running Catalina 10.15.7 with all the updates:

slot 1 16GB DIMM / slot 2 32GB DIMM
slot 3 16GB DIMM / slot 4 32GB DIMM.
Total 96GB.

And the speed for all 4 DIMMS is 2667Mhz and ok!!!

What do you figure?

Well that is very interesting, possibly one of the updates corrected the issue? Which would be good news! I will try again with mine, although I am on monterey now, and have iMac 20,1. Thanks for reporting