iMac Retina 5K 27-inch 2020 RAM Upgrade comatibility

Question

I would like to update my RAM from 40 Gb to 96 Gb

I have a iMac Retina 5K 27-inch 2020 running BigSur 11.6 with the following RAM:

Slot1: Kingston KCP426SD8/16 16GB

Slot2: Micron 4ATF51264HZ-2G6E3 8GB (original)

Slot3: Kingston KCP426SD8/16 16GB

Slot4: Micron 4ATF51264HZ-2G6E3 8GB (original)

At the moment, the RAM run all at 2133 MHz.

I found this post regarding the lower speed than expected (2133, not 2666 MHz) https://forums.macrumors.com/threads/imac-2020-showing-ram-running-at-2133mhz-but-tech-specs-say-2666.2250601/

Now I wonder if I get 96Gb 2666 MHz if I replace the original RAMs (8Gb) with

A) Kingston KCP426SD8 2x32 Gb

or

B) Kingston Fury KF426S16IBK2 2x32 Gb

I think A) and B) only differ in CAS latency (19 vs 16). However, https://support.apple.com/en-us/HT201191 does not mention any information about CAS latency and therefore, I am not sure if this has an impact on my desired setting.

Answer 1

As I am sure you are aware from the MacRumors thread you quoted:

1. For maximum compatibility with 2020 iMac, you need four identical memory sticks.

2. Get them from a dealer that guarantees full compatibly (including 2666 MHz) with the 2020 iMac.

Anything else, and it is a matter of experimenting.

You might notice that Apple Support page says "For optimal memory performance, DIMMs should be the same capacity, speed, and vendor" for the 2020 iMac, but not for older iMacs.

Studying published memory specs (e.g. CAS latency) is not sufficient for RAM in 2020 iMacs.