Chosen Solution
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): Empty4GBEmpty4GB Result: 8GB @ 2667 MHz 4GB16GB4GB16GB Result: 40GB @ 2133 MHz 16GB4GB16GB4GB Result: 40GB @ 2133 MHz 4GB4GB16GB16GB Result: 40GB @ 2400 MHz Empty16GBEmpty16GB Result: 32GB @ 2667 MHz Here’s a photo of the last configuration to provide physical orientation. Up to towards the top of the computer.
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).
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.
When it comes to RAM, best to match it all with a matched module set - ESPECIALLY MACS. Apple is particular about this to the point one bit it doesn’t like=POST error. You can’t mix and match with Apple… At all, generally. Buy a matched 3rd party RAM set from a company like Crucial (MUST be Mac memory to be sure, unless you’re a gambling man), keep your factory RAM for warranty purposes or buy memory from Apple - again, note which ones were added by you and remove them for warranty purposes but keep the stock RAM installed.
