Question Will an i7-8700k cap ram speed to 2666mhz if module is meant for higher speeds?


Feb 5, 2017
I am planning on upgrading a few components on my PC,
The CPU of choice is Intel i7-8700k *which says it supports DDR4-2666,

the motherboard I'm intending on using Asus ROG Strix Z390-E
*which supports DDR4 memory at speeds of:
  • 4266(O.C.)/
  • 4133(O.C.)/
  • 4000(O.C.)/
  • 3866(O.C.)/
  • 3733(O.C.)/
  • 3600(O.C.)/
  • 3466(O.C.)/
  • 3400(O.C.)/
  • 3333(O.C.)/
  • 3300(O.C.)/
  • 3200(O.C.)/
  • 3000(O.C.)/
  • 2800(O.C.)/
  • 2666/2400/2133 MHz
The RAM would be F4-4000C17D-16GTZR which is the G.SKILL Trident Z RGB 2*8GB
Which is 4000 modules, knowing that in the motherboard BIOS i would have to enable XMP for it to run the RAM at 4000MHz.....the RAM is supported by motherboard, the speed intended is supported 4000(O.C.)
Both RAM and motherboard supports XMP*
If i was to use this configuration, Will the i7-8700k cap the RAMs speed to 2666MHz? *at which point i can choose to enable[or not] XMP to allow speeds like 4000MHz (or other supported O.C. speeds like 3400)
According to Intel, the i7-8700k is 'stable' at 2666MHz with memory...would it be better to leave the 4000MHz RAM capped at 2666MHz ?

I'm not terribly concerned with loss of 'MHz' from the RAM, considering 16GB at 2666MHz. No problem with leaving it capped at 2666MHz for longevity if it helps or not i dont know.
Will it work? Should i leave it capped if thats the case?

Thanks for your time.


Intel states stock speeds not "stable" speed. Anything above that is technically an oc so mobos will say (oc). Ram states what speed it was tested to run at. It will default to stock speed just like a cpu will default to stock speed. Longevity is not a concern as components tend to outlive their usefulness (10+ years).


Unless you run programs that will actually benefit from faster ram speeds, for Intels there's little to no benefit, the ram is faster than the memory controller can shove it through the pc. So unless you already own the ram, it'll be far cheaper to go with 16Gb of 3200MHz, since that speed sees the highest amount of competitive sales.

But shouldn't be an issue to set XMP and let that 4000MHz do its thing.

There's 2 speeds to Ram. Default and XMP. Default speeds of DDR4 is 2133/2400/2666/3200MHz, depending on the platform and cpu. For your cpu generation, that's 2666MHz, kabylake was 2400, skylake was 2133. It's the speed set in the jedec tables when first installed or if bios is reset. Xmp is a factory OC, so as such those speeds get listed in mobo specs as (OC).
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Feb 5, 2017
Thank you for the quick responses.
Yea, i had the word 'stable' in mind instead of 'stock' for some reason

That does clarify and validate a few things i was thinking.

The reason i went for that specific RAM is because on the QVL list for ASUS only gives 2 options for G.SKILL Trident Z RGB for 16GB kits, which is the 3000MHz and 4000MHz versions...though the 3000 around $100 while the 4000 are double that at around $200. Problem is that the 3000's dont support the Z390 chipset, closest is Z370, while the 4000's DO support Z390 chipset.

I was worried the $$ put into the RAM would be wasted on the extra MHz if the CPU caps it at 2666MHz.....or that i would need to buy a new CPU to enable some setting in BIOS or that type of thing.

This helped a chunk.
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For Intel systems, memory frequency increases, above the max officially supported by the CPU (e.g. 2666MHz), only show marginal gains in gaming FPS.

For AMD systems the difference between 2666MHz and 36000MHz is much more pronounced, with gains of up to 15-20%.

If you plan to overclock your Intel chip, you would probably be better served keeping the RAM at 3200-3600MHz, especially if it allows you to overclock your CPU even slightly higher. Remember, the faster you run your RAM, the more stress you put on the memory controller, which is in the CPU. That, in turn, can hold back your CPU overclock.