KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Hi all! First post on this site. I received my 2020 27" iMac (tier 3 stock configuration) yesterday and have been using it happily since. Today I received some Crucial memory (CT2K8G4S266M aka 2 sticks of 8GB 2666) and I installed it in the two slots that were occupied by the Apple memory, which I moved to the free slots. When I booted the system it detected 24GB but at 2133 MHz. When I removed the Apple memory and left the Crucial it detected 16GB of 2667 MHz, when I moved the Apple memory back to the original slots and added the Crucial to the free ones it registered 24GB but still at 2133 MHz. The system seemed stable throughout all of this (and is currently stable running only with the 16GB Crucial at 2667 in the original Apple slots). I'm scratching my head wondering what's going on here. I should mention that before I added the Crucial ram, the Apple memory was running fine at 2667 MHz. Aside from juggling around the ram sticks, is there some kind of diagnostic tool that I can use to shed a little more light on this? Any suggestions?

The system is amazing by the way!
 
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pldelisle

macrumors 68020
May 4, 2020
2,248
1,488
Montreal, Quebec, Canada
Hi all! First post on this site. I received my 2020 27" iMac (tier 3 stock configuration) yesterday have been using it happily since. Today I received some Crucial memory (CT2K8G4S266M aka 2 sticks of 8GB 2666) and I installed it in the two slots that were occupied by the Apple memory, which I moved to the free slots. When I booted the system it detected 24GB but at 2133 MHz. When I removed the Apple memory and left the Crucial it detected 16GB of 2667 MHz, when I moved the Apple memory back to the original slots and added the Crucial to the free ones it registered 24GB but still at 2133 MHz. The system seemed stable throughout all of this (and is currently stable running only with the 16GB Crucial at 2667 in the original Apple slots). I'm scratching my head wondering what's going on here. I should mention that before I added the Crucial ram, the Apple memory was running fine at 2667 MHz. Aside from juggling around the ram sticks, is there some kind of diagnostic tool that I can use to shed a little more light on this? Any suggestions?

The system is amazing by the way!
Seems like you are not alone with this issue. The mac seems reeeeeally picky about RAM and mixing modules :/ We can’t predict this when recommending RAM. It is always possible the modules fall back on a lower frequency when DRAM chips are from different brands.
 
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KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Seems like you are not alone with this issue. The mac seems reeeeeally picky about RAM and mixing modules :/ We can’t predict this when recommending RAM. It is always possible the modules fall back on a lower frequency when DRAM chips are from different brands.
It just surprises me since I've always had great success with Crucial RAM. 16GB may be enough for me, but I might order another of the same and see if they'll cooperate. I had no issues with stability running off of only the Crucial RAM, and I've tried stressing it out a bit. For the record, the stock RAM is SK Hynix 4GB 1Rx16 PC4-2666V-SCO-11 HMA851S6DJR6N-VK NA AC 2012. Not that I know what most of that means.
 
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Hulip

macrumors newbie
Sep 23, 2017
16
6
Has anyone else had this problem even with other brands? May order one soon and upgrade the memory. Was hoping to keep the apple memory in as well.
 

Mcdevidr

macrumors 6502a
Nov 27, 2013
676
218
I had this issue with OWC ram. But I was even getting errors when starting the iMac with both sets in. I just ditched the Apple ram and ordered 16gb more from OWC. Hopefully it all works since they will be the same brand.
 
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KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Okay, I've ordered another set of the same Crucial RAM. I'll pop it in tomorrow and report back. I'll also try the RAM that arrives tomorrow with the original Apple RAM to see if there was an issue with the first batch of Crucial that I ordered.
 

KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
I had this issue with OWC ram. But I was even getting errors when starting the iMac with both sets in. I just ditched the Apple ram and ordered 16gb more from OWC. Hopefully it all works since they will be the same brand.
Yeah, I read about your troubles on another thread. Sorry to hear that. It's unfortunate, as 24GB of RAM is sufficient for me and I would like to use the original RAM. Apple's RAM specifications for the 2020 iMacs are the same as for the 2019 iMacs. So...

With both the Crucial and the Apple RAM my system was stable, but I ended up taking out the Apple RAM in order to run just the Crucial at 2667, and I've encountered no issues with stability.
 

Mcdevidr

macrumors 6502a
Nov 27, 2013
676
218
Thank you. Luckily ram is cheap so I just ordered more OWC. I’ve been gaming on the thing all day today in multiplayer games through parallels no less and haven’t had any stability issues since just running the OWC. Apple ram is in the keyboard box and getting put up with the box.
 

KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Thank you. Luckily ram is cheap so I just ordered more OWC. I’ve been gaming on the thing all day today in multiplayer games through parallels no less and haven’t had any stability issues since just running the OWC. Apple ram is in the keyboard box and getting put up with the box.
Out of curiosity, what is the brand of RAM that Apple shipped in your system?
 

NorCalZman

macrumors member
Jul 4, 2010
52
14
Hey, look on YouTube for 9to5Mac and the video they just posted 30 minutes ago about upgrading RAM in the 2020 IMAC. They came across the same issue with RAM speed, then changed the position of the sticks and that solved the issue.

short answer: put the 4 GB sticks in the first two slots and the new sticks in the bottom two slots.
 

KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Hey, look on YouTube for 9to5Mac and the video they just posted 30 minutes ago about upgrading RAM in the 2020 IMAC. They came across the same issue with RAM speed, then changed the position of the sticks and that solved the issue.

short answer: put the 4 GB sticks in the first two slots and the new sticks in the bottom two slots.
Interesting. The system is now recognizing 24GB of 2667 MHz, but do I not then lose dual channel? For some reason I thought that putting two sticks of the same size in the two lower slots and two identical sticks of a different size in the upper two would be suboptimal. Anyway, I think I'll just throw more of the same Crucial sticks into the machine tomorrow and have four that are identical.

EDIT: Actually, I think this arrangement (4GB in slot 1, 4GB in slot 2, 8GB in slot 3, and 8GB in slot 4) does drop the dual channel. This doesn't seem to have a "tremendous" effect on performance, and it may be better for me to have more RAM than to have dual channel. With 8GB each in slots 2 and 4 my Geekbench score was 1377 SC and 8675 MC. With the configuration just mentioned (4-4-8-8) it dropped to 1241 SC and 7143 MC. Would I ever notice this for what I do? Hard to imagine.
 
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NorCalZman

macrumors member
Jul 4, 2010
52
14

apparently they were having the same issue last year. Someone (around post 7 or 8) said it was because the ram had different CL ratings.

Does the stock RAM have that information on it? Or a model number that we can google?
 
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Brookzy

macrumors 601
May 30, 2010
4,932
5,418
UK
I have noticed this problem on mine too.

I have always upgraded the RAM myself on iMacs, and my 2019 model has two 4GB Apple sticks and two 16GB Apple sticks, and registers 40GB at 2667 MHz without an issue.

My 2020 model is stuck at 2133 MHz.

This is despite them both using Crucial RAM of the same specs, in addition to the Apple RAM.

I've played around with the sticks and these are my findings:

The 2020 iMac registers 2133 MHz using the same RAM sticks that the 2019 registers at 2667 MHz.

I took the sticks out of the 2019 model and placed them in the 2020 model, in the correct formation, and they reported 2133 MHz. So the 2020 model is more particular about RAM.

The 2020 iMac has a slightly different RAM arrangement:

Screenshot 2020-08-10 at 16.10.32.png

2020 iMac (above)


Screenshot 2020-08-10 at 16.10.26.png

2019 iMac (above)

This change is probably tied to the implementation of the T2 controller?

Putting all four 16GB Crucial sticks in the 2020 model still registered 2133 MHz.

This surprised me as they are all the same brand and spec. So I rearranged the sticks to what to me seems to be the 'wrong' way - my inherited 2019 Crucial sticks in Slots 1 and 2 (rather than 1 and 3 or 2 and 4 as you would normally expect), and the newly-bought Crucial sticks in Slots 3 and 4, and low and behold it is now running at 2667 MHz and 64GB.

The 2020 iMac is fussy even between different batches of the same RAM manufacturer and specs, not just between Hynix and Crucial. Get around this by keeping each batch/manufacturer to its own channel.

I am not sure what (if any) performance hit will come by mixing DIMMs in this way. I have always been told to install them in pairs in the first and third, or second and fourth, slots.
 
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KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Interesting. Thanks for sharing this. I wouldn't have though that it would be THAT particular about about RAM. I received my second pair of Crucial RAM yesterday and tried it with the original Apple RAM. I got the same result as the first batch of Crucial I tried with Apple RAM. I'm now using four sticks of Crucial for 32GB at 2667. I did configure it though so that the first pair that I received is in DIMM 0 and the second in DIMM 2. This restored dual channel (vs. Crucial in channel A and Apple in B) and the performance is now as expected.
 
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pob42

macrumors member
Jul 5, 2011
35
13
Sheffield, UK
I have the new i7 8-core model. Came with 8GB. 32GB (2x16) added.

When installed in the order 4-16-4-16 or 16-4-16-4 the system reports 2166MHz memory speed. Geekbench 5 results are 1270 for single-core and 8400 for multi-core.

When installed in the order 16-16-4-4- the system reports 2400MHz memory speed (!). However multi-core Geekbench results drop to 6930 (approx 17% drop). So although the reported speed is higher (although still not 2666MHz), it must be dual-channel memory access that disappears.

With just the 2 x 16GB (Corsair Vengeance 2666MHz) the system reports 2667MHz memory speed and multi-core Geekbench goes to its highest figure of 8900.
 

pldelisle

macrumors 68020
May 4, 2020
2,248
1,488
Montreal, Quebec, Canada
I have the new i7 8-core model. Came with 8GB. 32GB (2x16) added.

When installed in the order 4-16-4-16 or 16-4-16-4 the system reports 2166MHz memory speed. Geekbench 5 results are 1270 for single-core and 8400 for multi-core.

When installed in the order 16-16-4-4- the system reports 2400MHz memory speed (!). However multi-core Geekbench results drop to 6930 (approx 17% drop). So although the reported speed is higher (although still not 2666MHz), it must be dual-channel memory access that disappears.

With just the 2 x 16GB (Corsair Vengeance 2666MHz) the system reports 2667MHz memory speed and multi-core Geekbench goes to its highest figure of 8900.
I read this behaviour a few times already. Really weird how Apple maps memory channels on this iMac. I don't understand. I think the best is to have a single brand RAM modules finally ...
 

KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
I have the new i7 8-core model. Came with 8GB. 32GB (2x16) added.

When installed in the order 4-16-4-16 or 16-4-16-4 the system reports 2166MHz memory speed. Geekbench 5 results are 1270 for single-core and 8400 for multi-core.

When installed in the order 16-16-4-4- the system reports 2400MHz memory speed (!). However multi-core Geekbench results drop to 6930 (approx 17% drop). So although the reported speed is higher (although still not 2666MHz), it must be dual-channel memory access that disappears.

With just the 2 x 16GB (Corsair Vengeance 2666MHz) the system reports 2667MHz memory speed and multi-core Geekbench goes to its highest figure of 8900.
Yes, this was my experience...sort of. My speeds dropped to 2133 when I added compatible RAM to the two original Apple sticks. I found that I had to keep matching pairs in DIMM 0 and matching pairs in DIMM 1 to keep dual channel, but this reduced the speed. I found that mixing pairs in the DIMMs resulted in 2667 speed, but dropped dual channel. I keep hearing that it was possible to simply add compatible RAM to previous iterations of iMac (2019 and earlier) with compete functionality (and I did with a 2007 and a 2013 iMac), but perhaps something is different about these 2020s. In any case, I just shelved the Apple RAM and went with 4 sticks of Crucial for 32GB. I now have 2667 MHz and dual channel and Geekbench scores of about 1390 SC and 8990 MC. That's about all I think I can pull out of this system.
 
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gtosnipey

macrumors newbie
Jun 26, 2018
9
6
Just installed OWC 32GB and I also get 2133mhz when leaving in the Apple ram. So what is everyone’s final decision who isn’t buying new ram? My assumption is it’s better to have 40gb at 2133 than just the 32GB at 2666 and putting the Apple memory in a drawer. Thoughts?
 
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KrazyKanuck

macrumors regular
Original poster
Aug 8, 2020
130
136
Just installed OWC 32GB and I also get 2133mhz when leaving in the Apple ram. So what is everyone’s final decision who isn’t buying new ram? My assumption is it’s better to have 40gb at 2133 than just the 32GB at 2666 and putting the Apple memory in a drawer. Thoughts?
Depends on what you do with it, of course ;). Depends also on what configuration you went with. If you got the i9 for its multi-core performance but are throttling it without dual channel RAM, then you aren't really getting your worth. For me, I would rather have capacity over speed, but that's based on my use patterns. The difference between single channel and dual channel appear "huge" in benchmarks but may not be noticeable at all in real world use. For that matter, eating into your swap with SSD speeds like those in these iMacs also goes some way towards negating insufficient RAM capacity. Also, the difference between 32 and 40GB may not be significant in your case. Why don't you try the 32GB in the same DIMM for a while to take advantage the speed and the dual channel and keep your eye on the swap usage. If you routinely find yourself eating into that, try adding the Apple RAM back into the system for more capacity.
 
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