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Memory upgrade compatibility list for iMac 2019-2020

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pldelisle

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May 4, 2020
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There is already another thread for the 2020 iMac RAM compatibility issues. This thread should be renamed to 2019 iMac and earlier or something so that no assumptions have to be made in this thread anymore.
This thread is correctly named. Issues should not be here, only compatibility list and user's experiment with modules not listed in first post.
 
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MacManiac76

macrumors 68000
Apr 21, 2007
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White Mntns, Arizona
This thread is correctly named. Issues should not be here, only compatibility list and user's experiment with modules not listed in first post.

In that case anybody with a 2017 or earlier iMac looking to check RAM compatibility or anybody having iMac RAM issues of any kind should not be posting in this thread by your logic.
 
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pldelisle

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May 4, 2020
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In that case anybody with a 2017 or earlier iMac looking to check RAM compatibility or anybody having iMac RAM issues of any kind should not be posting in this thread by your logic.
I built this thread specifically for 2019-2020.

For informations on 2017, poster has to make it clear and explicit that he is looking for 2017 RAM compatible upgrade. 2017 iMac RAM isn't the same frequency (2400 MHz) so every SKU listed in this thread doesn't necessarily work (or doesn't work at all).

2019 and 2020 iMac share the exact same RAM, same specs, same SKUs, just not the same upgrade requirements.
 
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Canaan_c

macrumors member
Jul 4, 2016
63
10
Just bought two Crucial 32gb rams, CT2K32G4SFD8266, and have them running on iMac 2020 for two days.
So far, so good.
View attachment 954289 View attachment 954294

View attachment 954291 View attachment 954303

I saw on Amazon that your 2x32GB model (CT2K32G4SFD8266) is Single Ranked where it didn't indicate "PC4-21300" while the 2x16GB model (CT2K16G4SFD8266) is Dual ranked indicated as "PC4-21300". I am not sure whether your rams running at dual channel? Anyway to check or test it?


Crucial 64GB Kit (32GBx2) DDR4 2666 MT/S CL19 SODIMM 260-Pin Memory - CT2K32G4SFD8266

Also, the apple certificated one 2x32GB model (CT2K32G4S266M) on the official website indicated as "dual ranked". So this is another difference from the not certified onw (CT2K32G4SFD8266).


Btw, I am just a layman. Please advise. THANKS!!!!
 
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hoodafoo

macrumors regular
Oct 11, 2020
100
28
Lso Angeles
YES IT IS, so is it really work well for imac 27 2020 (top clock + dual channel)?
I just worry what "single rank" mean.

From what I've read, single rank is supposed to be faster than dual. It's a great deal for a 64kit but it's not my first choice. I would have preferred to fill all my slots with 16GBx4 sticks rather 32GBx2 in order to maximize all the slot interfaces. Can someone who knows more about this, chime in if my logic isn't correct?
 
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pldelisle

macrumors 68000
Original poster
May 4, 2020
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Montreal, Quebec, Canada
I saw on Amazon that your 2x32GB model (CT2K32G4SFD8266) is Single Ranked where it didn't indicate "PC4-21300" while the 2x16GB model (CT2K16G4SFD8266) is Dual ranked indicated as "PC4-21300". I am not sure whether your rams running at dual channel? Anyway to check or test it?
The 64 GB kit is said to be dual ranked because of the capacity being the double. It needs two banks of memory on each side. It’s not a major factor, it’s just « like that ». It has nothing to do with dual channel.


From what I've read, single rank is supposed to be faster than dual. It's a great deal for a 64kit but it's not my first choice. I would have preferred to fill all my slots with 16GBx4 sticks rather 32GBx2 in order to maximize all the slot interfaces. Can someone who knows more about this, chime in if my logic isn't correct?
You benefit from more « paths » from CPU to RAM, yes. Generally speaking there a small, unnoticeable gain. I personally prefer 2x32 since it allows me to upgrade in the future, but if you know you’ll never upgrade to 128 GB then yes 4x16 is a bit better on paper.
 
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Canaan_c

macrumors member
Jul 4, 2016
63
10
The 64 GB kit is sais to be dual ranked because of the capacity being the double. It needs two banks of memory on each side. It’s not a major factor, it’s just « like that ». It has nothing to do with dual channel.



You benefit from more « paths » from CPU to RAM, yes. Generally speaking there a small, unnoticeable gain. I personally prefer 2x32 since it allows me to upgrade in the future, but if you know you’ll never upgrade to 128 GB then yes 4x16 is a bit better on paper.

I checked the official website it really said CT2K32G4SFD8266 is dual ranked, so it may be amazon's mistake to marked it as single ranked. So I think we who need it can enjoy this Prime day deal :)
 
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hoodafoo

macrumors regular
Oct 11, 2020
100
28
Lso Angeles
You benefit from more « paths » from CPU to RAM, yes. Generally speaking there a small, unnoticeable gain. I personally prefer 2x32 since it allows me to upgrade in the future, but if you know you’ll never upgrade to 128 GB then yes 4x16 is a bit better on paper.

Yeah, I'm being realistic in knowing that I'll never need 128GB of ram. I've even seen some benchmark instances where it degrades performance. Maybe overhead involved managing 128GB of RAM?
 
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ninecows

macrumors regular
Apr 9, 2012
100
76
Thanks. It is Mac ram (at least it’s labeled as such).

The crucial you linked to seems sold out in Denmark and the price difference was only around 5% so I went for the Corsair.

I will report back how they behave. Still a bit surprised not to see that brand mentioned at all on 11 pages of “compatible ram”.

CMSA64GX4M2A2666C18 seems to work.

Did geekbench and got 1366/8762. I have the top tier w i7 and 5700XT and now with 64 GB ram.

Should the score have been higher?

People list all sorts of scores here but not much about what iMac model it’s based on ;-) Maybe it’s just me? :)
 
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kashiteiwa

macrumors newbie
Jul 20, 2020
8
6
I have ordered two sets of CT2K32G4SFD8266 (32GB X4 in total, 128GB)

Will this be quad channel or dual channel, will this work on iMac 2020?

What does 21300 21333 25600 even mean? will it be a problem if it's not 21333 as apple required?

**just figured out it's brandwidth, will PC4-25600 memory be perfect in iMac 2020? if not im returning this.

I just wanna order hassle free 128GB kit for my iMac.

Does CL19 or C18 makes any difference in macOS? I heard iMac would slow any latency below (for example CL15, 2666) to CL19 2666 anyway. is this true?




Should I just spend $200 Australian dollar for this kit? which has better latency (does it matter in macOS, will it be downgraded to CL19?)

 
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pldelisle

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Original poster
May 4, 2020
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Montreal, Quebec, Canada
Will this be quad channel or dual channel, will this work on iMac 2020?
Dual channel. Quad channel is for higher end Mac Pro with Xeon CPU.

It will work.

What does 21300 21333 25600 even mean? will it be a problem if it's not 21333 as apple required?
Self answered.

Does CL19 or C18 makes any difference in macOS? I heard iMac would slow any latency below (for example CL15, 2666) to CL19 2666 anyway. is this true?
The only CAS latency supported is CL19. So any modules not CL19 will automatically adjust to CL19 or simply won't work (less likely thought). So this statement is true.

will PC4-25600 memory be perfect in iMac 2020? if not im returning this.

It won't work or it will adjust itself to 2666 MHz. Don't play with that. Take only 2666 MHz RAM.

Should I just spend $200 Australian dollar for this kit? which has better latency (does it matter in macOS, will it be downgraded to CL19?)

Crucial makes the best RAM. Stay with it. You've already been lucky to order some since stocks are very low. See above answers.
 
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kashiteiwa

macrumors newbie
Jul 20, 2020
8
6
Dual channel. Quad channel is for higher end Mac Pro with Xeon CPU.

It will work.


Self answered.


The only CAS latency supported is CL19. So any modules not CL19 will automatically adjust to CL19 or simply won't work (less likely thought). So this statement is true.



It won't work or it will adjust itself to 2666 MHz. Don't play with that. Take only 2666 MHz RAM.



Crucial makes the best RAM. Stay with it. You've already been lucky to order some since stocks are very low. See above answers.
On the crucial offical page of this model, it says PC4-25600 while it also says PC4-21333 2666mhz
I don’t know which one to trust already :
Please take a look:
 
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pldelisle

macrumors 68000
Original poster
May 4, 2020
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Montreal, Quebec, Canada
On the crucial offical page of this model, it says PC4-25600 while it also says PC4-21333 2666mhz
I don’t know which one to trust already :
Please take a look:
This model works and have been tested by many on this forum.
 
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fly0nawall

macrumors newbie
Oct 15, 2013
5
0
I checked the official website it really said CT2K32G4SFD8266 is dual ranked, so it may be amazon's mistake to marked it as single ranked. So I think we who need it can enjoy this Prime day deal :)
I just found this thread after both ordering a new iMac 27" 2020 model and finding the Prime Day deal for the 64 GB of RAM for $164 (Crucial 64GB Kit (32GBx2) DDR4 2666 MT/S CL19 SODIMM 260-Pin Memory - CT2K32G4SFD8266). This was just $29 more than the 32GB of OWC RAM some other article recommended so I thought I'd investigate further. After crucial.com said it wasn't compatible I chatted with a Crucial support agent who basically just deferred to the automated compatibility checker:
the part CT2K32G4S266M is not showing to be compatible with iMac (Retina 5K, 27-inch, 2020) and could be similar to CT2K32G4S266M which is showing compatible. but Crucial can only suggest you memory which is showing compatibility in the compatibility list.

I then asked him what the difference was between the two models:
CT2K32G4SFD8266 is PC4-25600 and CT2K32G4S266M is PC4-21300

I then asked, "In general a PC4-25600 model should work in a system designed for PC4-21300 (but may just run slightly slower than it's max capability), right?" He confirmed.

I then found this thread where it shows multiple forum members have confirmed that it works, so I went ahead with the purchase. I now just need my iMac to show up in a few weeks so I can test it out and post back here!

Thanks all for the very helpful information here!
 
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The French Man

macrumors newbie
Sep 8, 2020
20
10
The only CAS latency supported is CL19. So any modules not CL19 will automatically adjust to CL19 or simply won't work (less likely thought). So this statement is true.

Hum not exactly ... CL16 is OK
HyperX Impact DDR4 HX426S16IBK2/64 Mémoire RAM 2666MHz CL16 SOIMM 64Go works fine with my 27' 2020
 
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The French Man

macrumors newbie
Sep 8, 2020
20
10
perhaps you're right ... 👍


Capture d’écran 2020-10-15 221228.png
 
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hoodafoo

macrumors regular
Oct 11, 2020
100
28
Lso Angeles
Yes crucial factories are in Mexico, but engineering is in USA. I think the DRAM chios are made in USA, but the final assembly on PCB is in Mexico. SSDs too are made in Mexico.

Isn't the soldering the weak point in mfg? Over and over again, I see various threads about tossing devices/boards in the oven for 10 mins to 'fix' something
 
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