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Why it shows 2133MHZ ram speed on 2020 imac 27"?

L

lovecd

macrumors regular
Original poster
Aug 14, 2022
165
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Put 2 piece 4GB apple PC4-2666mhz ram (removed from 2019 imac 21.5" desktop) into my 2020 27" iMac, but system shows it is 2133mhz only, if I removed the original 2 pieces ram but keep those two pieces there (the one from 2019 21.5"), restart the imac, it can show 2666mhz speed correctly, but shows 2133 again if I put the original 2 pieces back though 2666mhz is printed on all chips itself.

Can anyone explains why like that?

thanks,
 
The 2020 iMac is super finicky with RAM speeds. You have to get the sticks in the right slots to make it behave.

I think mine has

16GB
4GB (factory)
16GB
4GB (factory)

to get full 2666 speed.
 
lovecd said:
Put 2 piece 4GB apple PC4-2666mhz ram (removed from 2019 imac 21.5" desktop) into my 2020 27" iMac, but system shows it is 2133mhz only, if I removed the original 2 pieces ram but keep those two pieces there (the one from 2019 21.5"), restart the imac, it can show 2666mhz speed correctly, but shows 2133 again if I put the original 2 pieces back though 2666mhz is printed on all chips itself.

Can anyone explains why like that?

thanks,
Click to expand...
Is your iMac 20.1 or 20.2?
 
GMShadow said:
The 2020 iMac is super finicky with RAM speeds. You have to get the sticks in the right slots to make it behave.

I think mine has

16GB
4GB (factory)
16GB
4GB (factory)

to get full 2666 speed.
Click to expand...
Thanks for the hint, I changed to this order, factory/factory/upgrade/upgrade, it shows 2666 now. Feels really strange to Apple.
 
I hate to resurrect an old thread, but I'm having the same issue: 20.1 2020 i7 iMac, 2133mhz speed RAM. If need be, I'll start a new thread.

Brief history: I have been having trouble with this Apple Refurb machine, which came loaded with Monterey, since 3 months after I bought it. First the SSD went bad, took it in, got that replaced (I assume the entire motherboard was replaced, as the SSD is soldered on in this model).

When I picked the repaired computer up, the newest version of Ventura had been installed. I decided to leave that rather than regressing.

Then, less than a week later, the Mac decided it did not like my 4 16 GB OCW Ram sticks that had seemingly worked fine before (In Monterey). It wouldn't boot with them in it. I could get it to boot with the 4 GB Apple sticks, with 2 of the OCW sticks (but only a certain 2; luckily, I numbered all four so I could keep track), or with any 2 of the OCW sticks and the 2 original.

However, it will only run at 2666 MHz with either the two original OR the two OWC sticks. When I combine them in any order, it drops to 2133.

I decided maybe one OWC stick was bad, so I returned them. With a new pair of OWC 16 gb sticks, it so far has booted and has not crashed. However, I'm back to 2133 MHz again. I'm currently running Memtest via Rember, which should take a jolly long time to complete, but I anticipate no issues. Even the 'bad' OCW showed no problems when tested. So I thought I'd get into this thread now.

I deem it unlikely that I've gotten another bad stick, but if I'm incorrect in assuming this, I'd like to know.

Meanwhile I'm trying to decide if I should settle for 2133, try to get it repaired yet again, or try to get Apple to refund me and buy a new Silicon Mac Mini of some flavor (probably M2 Pro 32GB for longevity).

My original thought was to try to run 32 bit games/apps (I have a few 32 bit Apps that have not and probably will not ever be upgraded) in a VM on the iMac, but I've been since told that won't work, or will work as well in Silicon as on this machine, but it occurs to me with a mini, I could get a KVM switch and run the old stuff on the old machine. If, of course, Apple will do a refund. Any thoughts on that would be welcome as well.
 
Familiar Purrson said:
I hate to resurrect an old thread, but I'm having the same issue: 20.1 2020 i7 iMac, 2133mhz speed RAM. If need be, I'll start a new thread.

Brief history: I have been having trouble with this Apple Refurb machine, which came loaded with Monterey, since 3 months after I bought it. First the SSD went bad, took it in, got that replaced (I assume the entire motherboard was replaced, as the SSD is soldered on in this model).

When I picked the repaired computer up, the newest version of Ventura had been installed. I decided to leave that rather than regressing.

Then, less than a week later, the Mac decided it did not like my 4 16 GB OCW Ram sticks that had seemingly worked fine before (In Monterey). It wouldn't boot with them in it. I could get it to boot with the 4 GB Apple sticks, with 2 of the OCW sticks (but only a certain 2; luckily, I numbered all four so I could keep track), or with any 2 of the OCW sticks and the 2 original.

However, it will only run at 2666 MHz with either the two original OR the two OWC sticks. When I combine them in any order, it drops to 2133.

I decided maybe one OWC stick was bad, so I returned them. With a new pair of OWC 16 gb sticks, it so far has booted and has not crashed. However, I'm back to 2133 MHz again. I'm currently running Memtest via Rember, which should take a jolly long time to complete, but I anticipate no issues. Even the 'bad' OCW showed no problems when tested. So I thought I'd get into this thread now.

I deem it unlikely that I've gotten another bad stick, but if I'm incorrect in assuming this, I'd like to know.

Meanwhile I'm trying to decide if I should settle for 2133, try to get it repaired yet again, or try to get Apple to refund me and buy a new Silicon Mac Mini of some flavor (probably M2 Pro 32GB for longevity).

My original thought was to try to run 32 bit games/apps (I have a few 32 bit Apps that have not and probably will not ever be upgraded) in a VM on the iMac, but I've been since told that won't work, or will work as well in Silicon as on this machine, but it occurs to me with a mini, I could get a KVM switch and run the old stuff on the old machine. If, of course, Apple will do a refund. Any thoughts on that would be welcome as well.
Click to expand...
The trick to get 2666 speed is to use identical 4 pcs with the order of 1/2/3/4, not 1/3/2/4.
 
lovecd said:
The trick to get 2666 speed is to use identical 4 pcs with the order of 1/2/3/4, not 1/3/2/4.
Click to expand...
So if the supposedly identical 16 gb sticks are not identical enough, therein lies the issue?

I know I was probably confusing, but that's what I have in it now: 4 16 gb sticks, all from OWC
 
Familiar Purrson said:
So if the supposedly identical 16 gb sticks are not identical enough, therein lies the issue?

I know I was probably confusing, but that's what I have in it now: 4 16 gb sticks, all from OWC
Click to expand...
Try switching those two chips between lot 2 &3 and see if it makes the difference.
 
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To get 2667 MHz memory speed on a 2020 iMac, the part number in slot 1 must exactly match the part number in slot 2, and the part number in Slot 3 must exactly match the part number in slot 4 (when using four sticks).
Go to About this Mac, System Report, Memory, and look at the reported part numbers.

See this post for background: #22
 
Last edited:
wilberforce said:
To get 2667 MHz memory speed on a 2020 iMac, the part number in slot 1 must exactly match the part number in slot 2, and the part number in Slot 3 must exactly match the part number in slot 4 (when using four sticks).
Go to About this Mac, System Report, Memory, and look at the reported part numbers.

See this post for background: #22
Click to expand...
Okay, I see what you mean. Two don't have a part number (only '-') and two do, but they're not in the correct slots. I shall try that.

I don't know where I got the weird idea about the banks, since that goes against the way RAM banks have been set up in every other Mac I've ever owned (I have owned Macs upon Macs all the way back to a Macintosh Plus, and the banks in it are set up the same way you've explained). I think frustration is beginning to cloud my thinking.
 
Familiar Purrson said:
Okay, I see what you mean. Two don't have a part number (only '-') and two do, but they're not in the correct slots. I shall try that.

I don't know where I got the weird idea about the banks, since that goes against the way RAM banks have been set up in every other Mac I've ever owned (I have owned Macs upon Macs all the way back to a Macintosh Plus, and the banks in it are set up the same way you've explained). I think frustration is beginning to cloud my thinking.
Click to expand...
The 2020 iMac is unique among Macs, in requiring matching part numbers in the slots I stated.

Normally, the requirement is to simply match the amount of RAM in Channel A (Slots 1 + 2) to the amount of RAM in Channel B (slots 3 + 4), in order to get full dual channel operation. The 2020 iMac has this requirement too, in addition to the matching part number requirement I stated earlier (i.e., the part number in slot 1 must match the part number in slot 2, and the part number in Slot 3 must match the part number in slot 4, to get 2667 MHz speed)

Hope this is not too confusing
 
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wilberforce said:
The 2020 iMac is unique among Macs, in requiring matching part numbers in the slots I stated.

Normally, the requirement is to simply match the amount of RAM in Channel A (Slots 1 + 2) to the amount of RAM in Channel B (slots 3 + 4), in order to get full dual channel operation. The 2020 iMac has this requirement too, in addition to the matching part number requirement I stated earlier (i.e., the part number in slot 1 must match the part number in slot 2, and the part number in Slot 3 must match the part number in slot 4, to get 2667 MHz speed)

Hope this is not too confusing
Click to expand...
By Ceiling Cat, it actually worked! 😹

Thank you to everyone who commented on the issue. I was so far at sea on this that the waves from Hawaii were getting to Australia faster than I was to grasping a solution.
 
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Familiar Purrson said:
By Ceiling Cat, it actually worked! 😹

Thank you to everyone who commented on the issue. I was so far at sea on this that the waves from Hawaii were getting to Australia faster than I was to grasping a solution.
Click to expand...
There’s a large thread from when the 2020 iMacs went on sale where this whole RAM setup situation was figured out as it was driving people rather crazy. @wilberforce gave you the cliff notes version.

forums.macrumors.com

Memory upgrade troubleshooting in 2020 27-inch iMacs

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...
forums.macrumors.com forums.macrumors.com
 
wilberforce said:
The 2020 iMac is unique among Macs, in requiring matching part numbers in the slots I stated.

Normally, the requirement is to simply match the amount of RAM in Channel A (Slots 1 + 2) to the amount of RAM in Channel B (slots 3 + 4), in order to get full dual channel operation. The 2020 iMac has this requirement too, in addition to the matching part number requirement I stated earlier (i.e., the part number in slot 1 must match the part number in slot 2, and the part number in Slot 3 must match the part number in slot 4, to get 2667 MHz speed)

Hope this is not too confusing
Click to expand...

Why do they have to match so much? It doesn't make sense from a hardware standpoint, as you normally need the sticks to just be the same size and speed to get dual channel, in any computer. But now they have to match part numbers too in the 2020 iMac (something that is nearly impossible by chance to do, the only way is if you are buying brand new RAM from a store and making sure they even send you the matched pairs (else what? Are you going to return the RAM? That is an extra hassle), is this matching requirement done via the software side by Apple?

If yes, that would be a bit devious on their part, then I wonder if there is some hack to remove the software matching pair numbers check, then there would be one less requirement to satisfy, and then a lot more RAM would be available to be used in dual channel, everybody wins.
 
Last edited:
2aw said:
Why do they have to match so much? It doesn't make sense from a hardware standpoint, as you normally need the sticks to just be the same size and speed to get dual channel, in any computer. But now they have to match part numbers too in the 2020 iMac (something that is nearly impossible by chance to do, the only way is if you are buying brand new RAM from a store and making sure they even send you the matched pairs (else what? Are you going to return the RAM? That is an extra hassle), is this matching requirement done via the software side by Apple?

If yes, that would be a bit devious on their part, then I wonder if there is some hack to remove the software matching pair numbers check, then there would be one less requirement to satisfy, and then a lot more RAM would be available to be used in dual channel, everybody wins.
Click to expand...
Good questions.
Apparently it is an Intel thing, see this post:
#454
As to why, whether it is really necessary, or whether it is devious, this can be debated, but the fact of the matter is that's just the way it is. It will likely take more energy to devise a hack than just buying matching part numbers. Note: not all 4 sticks need to match part numbers, only each of each pair need to match, provided all are same size. Normally people buy DIMMs in pairs anyway, so it is not a huge problem.
 
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