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High End Ram + FCLK Questions

Level 7
Hello ROG Fam,

*I recently built a new desktop PC, to replace a 12 year old system.

360mm AIO CPU Cooler Corsair h150i Elite Cappelix*
32GB (2x16) Corsair 3600Mhz CL18 (18-22-22-42)
X570 Asus Rog Crosshair VIII Hero (non WiFi)
1TB WD SN850 Gen4 NVME
Corsair RM1000x PSU
EVGA RTX 3080 Ti FTW3 Ultra*

1440p w/ 165hz Refresh Rate w/HDR *

Sort having some buyers remorse regarding the RAM, wanting to eek out
that last bit of performance so I’d be pretty much set for a long while
by having the higher end of what’s available now. Also to eek out that extra couple FPS.

I was looking at these:
G.Skill Trident Z Neo 4000Mhz CL14 (14-15-15-35)
G.Skill Trident Z Neo*3800Mhz CL14 (14-16-16-36)

I’d prefer the 4000mhz RAM but I’m concerned
about a few things:

How would one know if their FCLK would go that high
before purchase? Can you OC ur FCLK beforehand with
lower speed Ram first, if it posts then I’m good to go?
Neither is on the QVL list, anything 4000Mhz on the QVL
is like CL17-19*

I learned that XMP of 3600Mhz is already an overclock
and that the FCLK running 1:1 @1800mhz is overclocked to accommodate.

Does anybody who has this board have higher speed RAM above 3600mhz
and could get that XMP profile to post without issue ?

If I confirm I can’t post, that would mean I’d have to just seek RAM with lower timings?

*I tried Ryzen DRAM calculator to try and tighten timings and once I told it I had Micron
die or whatever it said not supported. I’d rather just buy lower latency as stock tbh.

I was able to get my FCLK to 1866Mhz and RAM to 3733Mhz
although my timings exploded to (26-26-58-88) *just to mention.

**I’d appreciate any advisement**

Level 12
FCLK ideally runs at half the memory speed and in sync, as a general rule. So memory running DDR@3200 will have FCLK of @1600. The FCLK is the speed of the memory controller, the Infinity Fabric and actually the RAM memory, however the RAM memory is read twice per cycle and therefore FLCK of 1600 is DDR@3200 speed.

That explained, going to talk with memory speeds and assume the above rule with FCLK (is half and synced) from now on.

In the above graphic, the line in the sand sits DDR@3800 to maintain 1:1 sync ratio however Dark Hero motherboard, Ryzen 5000 series and G.Skill kit have pushed this line to DDR@4000 and few people that are good at overclocking game on other systems and there maybe other appoved systems (not kept following consider this general information about the top of the line being DDR@4000). Your system can run upwards to least DDR@4800 however it comes with using 2:1 divide and this makes for a latency penalty in the Infinity Fabric (ie: not worth running or considering). I have run my system to DDR@4800 2:1 with my old samsung b-die (not the ones currently use) and the graph suggests AMD ran to DDR@5133 ?2:1? on Ryzen 3000 series.

For gaming (for best FPS results), you will want

2xDual Rank (DR) memory kit; or
4xSingle Rank (SR) memory kit

So of the kits you have selected they are both top tier (4xSR Samsung B-die kits) and you should be able to run DDR@3800 one without issue. Since I own DDR3800 one and fair few others, if you want to save coin and see very little difference get DDR@3600 2xDR or 4xSR at lowest CL willing to pay (think that is CL14) or if money is no object consider your current selection of DDR@3800 CL14. I would not consider the DDR@4000 or higher, myself however I did find this information (graphic below) which suggest the Infinity Fabric changes in 5000 series support higher DDR@4000 speed (note the Good luck! on the slide).

Hope you find this helpful.