New iMac (mid-2020) question on memory

[ssls6]

I just bought a 2020 27" iMac 10-core with 8GB (refurb). I also bought 32GB of OWC memory. Should I remove the 8GB or populate all 4 slots for 40GB? The OWC memory is 2x16 and I assume the original 8 is 2x4. Any penalty for mixing sizes in the 4 slots?

 

[azentropy]

That is how I populated my 2019 27" iMac.

 

[glenthompson]

My wife's 2017 iMac didn't like the OWC and Apple memory coexisting. Al dort of boot problems and crashes. Pulled the Apple memory out and it's been running fine.

I suggest giving it a try and if you get problems, just remove the Apple memory.

 

[ssls6]

Is the iMac dual channel memory? Slots 1 & 3 or 2 & 4 but not 1 & 2, 2 & 3, 3 & 4, 1 & 4?

 

[wilberforce]

For the 2020 27" iMac (and only the 2020 27" iMac) there is a penalty for mixing RAM sizes. Depending on the order placed in the slots, you will either lose full dual channel operation, or the RAM frequency will drop from 2667 to 2133 MHz. There are several long threads in this forum explaining this, for those that do not believe this is true. (You can also verify yourself by running Novabench RAM speed test, that mixing RAM sizes degrades memory performance on the 2020 iMac.)

Best is to toss the original 8GB RAM, and install the 2x16GB in slots 2 and 4, like this, which is what I have done on my 2020 iMac:

 

[wilberforce]

That is how I populated my 2019 27" iMac.

It is fine to mix RAM sizes on the 2019 iMac, but not on the 2020 iMac for "optimal memory performance" (as described by Apple). Note the difference in instructions for 2020 iMac vs 2019 iMac, from Apple:

 

[ssls6]

Great information