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Memory upgrade compatibility list for iMac 2019-2020

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Canaan_c

macrumors member
Jul 4, 2016
63
10
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no_fan_my_ass

macrumors newbie
Oct 20, 2020
10
0
So this is my latest report, the single core score dropped but the Multi core raised to what I expected.
So... I think this kind of report is just a reference at least it tells my iMac is normal.
Any ideas why mine are so different?


EDIT:

If I do the following:

  1. Disable second 4k display (turn off display)
  2. Disable third display (iPad 10.5 sidecar over USB)
  3. Use Mac Fan Control to put fans to Full Blast from start
  4. Close all apps
Then I get the following

Which is very similar to yours...
 
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cczhu

macrumors member
Aug 4, 2018
33
11
report my experience with crucial 64GB kit (2*32GB) CT32G4SFD8266:

I purchased 2 kits to maximize out the ram at 128GB. The new iMac 2020 (i7+5500xt) didn't turn on after the upgrade. I was able to identify one faulty kit with which the iMAC just won't turn on. The other kit worked fine. I know the slots are fine because they worked with the Apple ram.

Returning the faulty kit and waiting for a new kit to arrive soon.

I didn't go for the "for Mac" kit CT32G4S266M due to the price difference (at the time of purchase 80$ pre-tax) that I couldn't justify for simply the extra "for Mac" label.

will report back with the new kit.
 
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pldelisle

macrumors 68000
Original poster
May 4, 2020
1,793
1,217
Montreal, Quebec, Canada
report my experience with crucial 64GB kit (2*32GB) CT32G4SFD8266:

I purchased 2 kits to maximize out the ram at 128GB. The new iMac 2020 (i7+5500xt) didn't turn on after the upgrade. I was able to identify one faulty kit with which the iMAC just won't turn on. The other kit worked fine. I know the slots are fine because they worked with the Apple ram.

Returning the faulty kit and waiting for a new kit to arrive soon.

I didn't go for the "for Mac" kit CT32G4S266M due to the price difference (at the time of purchase 80$ pre-tax) that I couldn't justify for simply the extra "for Mac" label.

will report back with the new kit.
Damn. Never heard of a faulty kit from Crucial.
 
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cczhu

macrumors member
Aug 4, 2018
33
11
Damn. Never heard of a faulty kit from Crucial.
Now you hear it :p
I tried it a few times even tried the kit with different slots. iMac just won’t turn on. Must be at least a compatibility issue. I had high expeditions for crucial ram but it was disappointing with that kit. Lessons learnt "never say never"
 
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cczhu

macrumors member
Aug 4, 2018
33
11
report my experience with crucial 64GB kit (2*32GB) CT32G4SFD8266:

I purchased 2 kits to maximize out the ram at 128GB. The new iMac 2020 (i7+5500xt) didn't turn on after the upgrade. I was able to identify one faulty kit with which the iMAC just won't turn on. The other kit worked fine. I know the slots are fine because they worked with the Apple ram.

Returning the faulty kit and waiting for a new kit to arrive soon.

I didn't go for the "for Mac" kit CT32G4S266M due to the price difference (at the time of purchase 80$ pre-tax) that I couldn't justify for simply the extra "for Mac" label.

will report back with the new kit.
I got the new kit CT2K32G4SFD8266 from Amazon today. After installation, the good news is the machine turned on and has been running fine so far. Bad news is after Geekbench I got a score of ~8200, a ~10% decrease compared to running with 64GB ram.

I checked, the memory is running at 2133 (with four sticks) instead of 2666 (with two sticks). I'm truly baffled now, all 4 sticks are exact the same from crucial (DDR4-2666, CT32G4SFD8266) with speed supporting 2666, just two CT2K32G4SFD8266 kits. What could have gone wrong here?

Looks like the kit CT2K32G4SFD8266 might indeed have compatibility issue with iMac 2020 after all? Can anyone else here with the kit verify their ram speed when all four slots are filled with this ram from crucial?

I might need to go for the "for Mac" kit CT2K32G4S266M for the best speed.

mem_config.png
 
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cczhu

macrumors member
Aug 4, 2018
33
11
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?)

Hi did you install your two sets of CT2K32G4SFD8266? if so do the memory run at 2666 or 2133?

I am asking because I am having trouble with my two kits, namely the speed drops from 2666 to 2133 when running 4 sticks instead of 2. see my post above.

Thanks if you could share your experience.
 
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The French Man

macrumors newbie
Sep 8, 2020
20
10
Hi,

Is each new kit works fine 2X32GO on all slot (32 Go slot 1 and 32 Go slot 3) and (32 Go slot 2 and 32 Go slot 4) same test with the other (new kit).
Regards
 
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limes666

macrumors newbie
Jun 18, 2016
7
0
Hello, just thought I would add a post to this.
I originally bouight a 64gb kit CT2K32G4S266M, following the link from this list above, for my iMac 2020. It works great, machine runs very well. Installed in slots 2 and 4, runs at 2667mhz . Last week I decided to purchase the same kit again to max out the ram on the machine. Having followed this thread for some time I thought best to get the same brand/manufacturer again.
Anyway, it turned up and I installed it in the remaing slots, and it reduced the ram speed to 2133 mhz. I swapped them around so it was in 2 and 4 and the original was in 1 and 3 and still got reduced speed (see attachment). I even tried, as one of the posters mentioned above, to put original ram in 1 and 2 and new ones in 3 and 4. Here the machine did not even boot, even after witing about 10 minutes.
Anyway, I sent back to Amazon today. Not sure the reason for this as I purchased from the exact same order as before. I noticed on the memory system profile that the original memeory manufacturer is listed as Micron and the new ones as "859B". Original memory was made in China, this second order made in Malaysia. The packaging on both was identical, all crucial branded.
Anyway, sorry for long post! I Will try and order again at some point.
 

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limes666

macrumors newbie
Jun 18, 2016
7
0
I got the new kit CT2K32G4SFD8266 from Amazon today. After installation, the good news is the machine turned on and has been running fine so far. Bad news is after Geekbench I got a score of ~8200, a ~10% decrease compared to running with 64GB ram.

I checked, the memory is running at 2133 (with four sticks) instead of 2666 (with two sticks). I'm truly baffled now, all 4 sticks are exact the same from crucial (DDR4-2666, CT32G4SFD8266) with speed supporting 2666, just two CT2K32G4SFD8266 kits. What could have gone wrong here?

Looks like the kit CT2K32G4SFD8266 might indeed have compatibility issue with iMac 2020 after all? Can anyone else here with the kit verify their ram speed when all four slots are filled with this ram from crucial?

I might need to go for the "for Mac" kit CT2K32G4S266M for the best speed.

View attachment 976770
Hi, seems like same issue I am having, and I purchased CT2K32G4S266M 64GB Kit. Did you purchase from Amaozon UK?
 
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cczhu

macrumors member
Aug 4, 2018
33
11
Hi, seems like same issue I am having, and I purchased CT2K32G4S266M 64GB Kit. Did you purchase from Amaozon UK?
yes just saw your post. Looks like we have the same issue despite different kits (mine is CT2K32G4SFD8266 from crucial as well). Mine are all from Amazon too.

I thought the for Mac kit would yield better results but it looks like the same issue. Very interesting.

For me, the manufacture is all 859B. I ordered two CT2K32G4S266M yesterday. Now I don't know if I should go with OWC memories instead ....

Is this issue known?
 
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limes666

macrumors newbie
Jun 18, 2016
7
0
yes just saw your post. Looks like we have the same issue despite different kits (mine is CT2K32G4SFD8266 from crucial as well). Mine are all from Amazon too.

I thought the for Mac kit would yield better results but it looks like the same issue. Very interesting.

For me, the manufacture is all 859B. I ordered two CT2K32G4S266M yesterday. Now I don't know if I should go with OWC memories instead ....

Is this issue known?
yeah I dont know. I wonder if anyone is running 128gb at 2666mhz and if so what memory they used.
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
283
yes just saw your post. Looks like we have the same issue despite different kits (mine is CT2K32G4SFD8266 from crucial as well). Mine are all from Amazon too.

I thought the for Mac kit would yield better results but it looks like the same issue. Very interesting.

For me, the manufacture is all 859B. I ordered two CT2K32G4S266M yesterday. Now I don't know if I should go with OWC memories instead ....

Is this issue known?
Sticks need to have identical part numbers, as listed under:
About This Mac
System Report
Memory

If they don't have exact same part numbers, try them in order AABB (worked for me, as long as they are all the same size (GB))
 
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G.Co.

macrumors newbie
Oct 4, 2020
2
1
First post. Just an FYI from my own personal experience over the past few weeks with my first iMac.

I purchased two single packs of 16GB RAM from Crucial (CT16G4SFD8266) for my 2020 i7 5500XT from Amazon Japan (much cheaper than the 'Mac Approved' version by Crucial et al).

Before even turning the machine on for the first time (or even fully removing the iMac from its original packaging), I swapped Apple's stock Micron 2 x 4GB RAM (factory installed in Slots 2 & 4).

Anyway, since then (October 14th) I have had 32GB of memory (2x16GB of Crucial's CT16G4SFD8266 RAM in slots 2 & 4) running at the correct 2667MHz without issue.

I'm no pro but have been using the most up-to-date version of Adobe Lightroom Classic, as well as Ableton Live 10 and Logic X regularly. Running 32GB of RAM seems to be more than enough for those tasks with whatever other programs I have running in the background simultaneously. No hiccups.

Anyway, I just thought I'd pass on my experience to you all.


G.

Edit: Oh and I wanted to mention that 'Mac Approved' RAM seems to be geared toward Apple's stricter environmental policies and not necessarily the actual day-to-day functionality of the RAM sticks themselves. In other words, if the specs fit, the RAM should work!
 
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Maccident

macrumors newbie
Nov 4, 2020
1
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warp9

Contributor
Jun 8, 2017
344
481
Thanks! AABB solved the issue. Now it's running at full speed 2667.

Seems no need for the "For Mac" ram.
I know your issue has been resolved but you should know, you have two different sets of memory installed. This is most likely the reason for the slot problems. The replacement you were sent was different from the other pair.

- CT32G4SFD8266.C16FB
- CT32G4SFD8266.M16FB

If anyone else has DOA issues like this, send back all the ram and get a fresh new set, because you can't count on getting a matching set.
 
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cczhu

macrumors member
Aug 4, 2018
33
11
I know your issue has been resolved but you should know, you have two different sets of memory installed. This is most likely the reason for the slot problems. The replacement you were sent was different from the other pair.

- CT32G4SFD8266.C16FB
- CT32G4SFD8266.M16FB

If anyone else has DOA issues like this, send back all the ram and get a fresh new set, because you can't count on getting a matching set.
thanks! it's annoying that crucial has two different versions under the same model name.

I understand that reorder is the best strategy to get the matching sets
 
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x83

macrumors newbie
Nov 10, 2020
3
2
Hi guys, read through this topic here and would like to say thanks to all the contributors that made this happen :)
Decided to purchase CT32G4SFD8266 from amazon, 2 sets of (32gbx2) a total of 128gb.

Received my ram today and slotted it in but got some questions I hope you guys can help with. Noted that Lime666 and cczhu pretty similar issue as me. But I am hoping to get some advice on how I should proceed please.

1. I ordered 2 sets of CT32G4SFD8266 (32gb x 2) x 2. I made separate orders for each set. However I received 1 set labeled CT32G4SFD8266 and the other 16ATF4G64HZ. Are these the same? (I checked with crucial chat on the website and they said to contact Amazon as the 16ATF4G64HZ falls under Micron and that the Amazon seller named Crucial is actually a reseller.)

2. When I installed them in the iMac 2020 today, I did 2 configs, AABB one which gave me 2667 speed and the other ABAB which gave me 2133. So was pretty happy when I saw 2667 till I watched a video regarding single and dual channel ram setup which can affect performance? totally lost at this point.

the vid I watched. jump to 6.42 shortcut in the comments

3. What should I do? Have attached a pic in the 2133 setup FYI. Should I go to the AABB setup? which gives me 2666, but if I understand correctly is not dual channel mode.

Hope I didn't miss out some solution/explanation in the previous posts but I read through a few times and where I got confused is in the single channel dual channel thing. where you could still have 2667 speed but diminished performance.

Thanks in advance :)
 

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warp9

Contributor
Jun 8, 2017
344
481
(I checked with crucial chat on the website and they said to contact Amazon as the 16ATF4G64HZ falls under Micron and that the Amazon seller named Crucial is actually a reseller.)

You got ram from a 3rd party seller. Always, ALWAYS buy only from Amazon.com. It should say "Sold by and shipped by Amazon.com" on the right side. Anything else is a gamble, like you just found out. This "reseller" (who is pretending to be crucial) shipped you whatever was on the shelf, hoping you wouldn't notice.

Send it all back and buy a new set, ensuring you are actually buying from Amazon. Don't forget to leave a one-star review on the seller page because these jokers need to be flushed out.
 
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