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A2115 / 2020 / Processors from 3.1 GHz 6-core i5, up to 3.8 GHz 8-core i7. Released August 4, 2020.

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Best arrangement of stock and 3rd party RAM?

I bought the 3.8GHz 8-core 10th-generation i7 variety 2020 iMac, with the base config 8GB of 2666MHz DDR4 memory. I’m now trying to install 2x 16GB sticks of HyperX HX426S15IB2K2 (Kingston) and have discovered an impossible optimisation problem.

The stock RAM comes as two 4GB sticks. I tried the following configurations of RAM stick in each slot (remember slots 1 and 3 are DIMM 1 and slots 2 and 4 are DIMM 0):

  1. Empty
  2. 4GB
  3. Empty
  4. 4GB

Result: 8GB @ 2667 MHz

  1. 4GB
  2. 16GB
  3. 4GB
  4. 16GB

Result: 40GB @ 2133 MHz

  1. 16GB
  2. 4GB
  3. 16GB
  4. 4GB

Result: 40GB @ 2133 MHz

  1. 4GB
  2. 4GB
  3. 16GB
  4. 16GB

Result: 40GB @ 2400 MHz

  1. Empty
  2. 16GB
  3. Empty
  4. 16GB

Result: 32GB @ 2667 MHz

Here’s a photo of the last configuration to provide physical orientation. Up to towards the top of the computer.

Block Image

In summary, it seems the only way I can retain the full 2667 MHz speed, is to leave some of the RAM out! Oddly, cross-matching them (putting different size sticks in a single bank) reported a higher speed than matching sizes in a single bank, but I suspect I’d lose out in other ways like parallelisation.

If I want to take full advantage of my RAM bandwidth potential, but would otherwise more RAM than less, what’s my best option? Is there a way to have it all? Is the drop to 2133 MHz real? Is the smaller drop to 2400 MHz a false hope?

In case it’s relevant, the HyperX RAM wasn’t my first choice, but I was steered by stock limitations. It’s not exactly “approved” for use in the iMac by anyone, but matches up spec wise. It has a slightly better CAS Latency of CL15 than the required CL19, but is otherwise much of a muchness. This thread raises interesting points, although nothing hard and fast: What Ram to order with the new 2019 IMac?

Update (08/20/2020)

Well whaddya know, here’s a very detailed thread on why this occurs: iMac 2019 - upgraded w 2 extra ram and the speed dropped to 2400 MHz?

Still nothing conclusive (whether it’s an XMP/overclocking issue, or a single/dual rank issue seems unresolved), and doesn’t really explain why removing one pair restores full speed, but good background nonetheless. Still looking forward to your advice on the best way to proceed (short of buying more RAM).

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I have a mid-model iMac 2020 with 1TB SSD. I bought a pair of OCW 16GB, and it’s work with my pair of 4GB Apple ram. Below is my ram arrangement.

Block Image

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Marco, that's a bad configuration. Run benchmarks like that and then with just the 2 16gb sticks in the same DIMM, and you should see that you are crippling your performance with this config.

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Heath Raftery sarà eternamente grato.
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