XPS 8940 SE, third party RAM cannot run at 2933MHz

@Vic384  @RoHe thanks for the feedback guys

one last thing, is there any difference between installing 2 matched RAM modules in either the white (DIMMs 1/2) or black (DIMMs 3/4) pair of slots? kinds weird that the xps 8940 service guide clarifies none of this.

@KGA4841  - To add to what @Vic384 said, memory slots have two different numbering systems.

The DIMM slot number indicates the order in which they should be filled, eg, DIMM1 then DIMM2, etc. And pairs will have matching color retention clips so RAM will run in dual-channel mode, assuming they're matched RAM modules.

The other number is a slot's physical number, counting from the CPU. If there are more than 2 RAM slots, DIMM1 is typically the second physical slot counting from the CPU and DIMM2 (with matching retention clips) is the fourth physical slot from the CPU. DIMM3 is the first physical slot, but the third to be filled...

And all that can lead to confusion...!

@KGA4841I don't think there is a difference in installing two memory modules in the white or black slots, but in previous XPS models the charts always showed if you only have two module to install them in the white slots, DIMM1 and DIMM2. Also this follows the numbering order that @RoHe mentioned.

One note is: the pair of RAM chips must be the same specification, better be exactly the same model.
That is: the white-white must be the same RAM, the black-black must be the same RAM.
If you got two RAM chips that are different, put them into different colored slots.

Dell XPS 8940 seems to have very rigid requirements on this.  I ever used a lot of different Dell computer models before, and mixed different RAM model/size/speed before, and it always worked, maybe not at the best performance, but it worked.

With 8940, if I mix a 32G chip with the original 16G chip, then Windows 10 will be screwed. The screen display is messed up, seems the shared video ram for the integrated GPU is messed. ( I am not using the 2070 super)

Even for the two RAM models I mentioned above, mixing them works fine most of the time. But one time I noticed Windows 10 display messed up. Memory diagnose didn't show anything wrong. That happened only once.

Now I got 128G Crucial RAM. Haven been running for weeks now. No issue so far.

@Fei Crane 

So, to clarify you are saying that if i install one extra crucial ram module 16GB (which crucial says is fully compatible)  in DIMM2 (white) in addition to the factory 16GB in DIMM1 (white) the computer will malfunction? I hope you are wrong but if this is the case, that is messed up. ie. to run with more than 16GB I have to toss out the factory RAM and replace???  anyway, my hope was to simply double the RAM and stick one of these in DIMM2 and be done with it  https://www.crucial.com/memory/ddr4/ct16g4dfd832a/ct19034506 

@Fei Crane  - "...mix a 32G chip with the original 16G chip..." (= 48 GB, total) is not a supported RAM config in the XPS 8940. The specs say it supports 8, 16, 32, 64, and 128 GB of RAM, total, but not 48 GB...

So your "messed up" video experience isn't surprising...

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@KGA4841 wrote:

So, to clarify you are saying that if i install one extra crucial ram module 16GB (which crucial says is fully compatible)  in DIMM2 (white) in addition to the factory 16GB in DIMM1 (white) the computer will malfunction? I hope you are wrong but if this is the case, that is messed up. ie. to run with more than 16GB I have to toss out the factory RAM and replace???  anyway, my hope was to simply double the RAM and stick one of these in DIMM2 and be done with it  https://www.crucial.com/memory/ddr4/ct16g4dfd832a/ct19034506 

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What you propose (2x 16 GB) should work OK. RAM might not run in dual-channel mode, but that depends on how closely the new Crucial module matches the existing Dell module. But the pair should run fine, even in single-channel mode.

As I said above, Fei Crane used an unsupported RAM config (48 GB). So just ignore those observations.  And I wish peeps would read the specs...

Mixed 32+16 is not supported, may or may not work. Totally agree with that.

But 32G Crucial + 32G Mushikin should work fine. I was surprised this configuration run into issue.

Many folks have reported problems where DDR4-3200 rated RAM won't operate at the rated speed in an XPS 8940 (or at the 2933 MHz like the stock RAM).  Also, when paired with the stock RAM shipped with the machine, all memory is reduced to a lower operating speed.

This appears to be related to how RAM is speed-certified (JEDEC vs XMP).   See this link for more details: https://www.crucial.com/support/articles-faq-memory/what-is-xmp

The XPS 8940 doesn't support XMP and prefers 1.2V RAM so any RAM that requires XMP or 1.35V to reach the advertised speeds may not work. In those cases, the XPS will look for a JEDEC (SPD) 1.2V profile that your existing RAM and new RAM have in common and will operate all DIMMs at that speed.

To summarize, the XPS 8940 seems to like RAM that operates at 1.2V with CL 22 and a JEDEC (SPD) speed rating of 3200 MHz.  Here is a list of DIMMs that have been shown to work based on articles I've read and data I found on: https://www.userbenchmark.com/System/Dell-XPS-8940/196842

XPS 8940 RAM known to support 2933 MHz 

8 GB DIMMs 

8ATF1G64AZ-3G2J1 or MTA8ATF1G64AZ-3G2J1

HMA81GU6DJR8N-XN

CT8G4DFS832A.C8FJ 

CT8G4DFRA32A.C4FE

16GB DIMMs

HMA82GU6DJR8N-XN

16ATF2G64AZ-3G2J1 or MTA16ATF2G64AZ-3G2J1

CT16G4DFRA32A.C8FE

CT16G4DFD832A.M16FJ

CT16G4DFD832A.C16FJ

32GB DIMMs

HMAA4GU6AJR8N-XN

16ATF4G64AZ-3G2B1 or MTA16ATF4G64AZ-3G2B1

CT32G4DFD832A.16FB1

MSL4U320NF32GX2

Crucial Kits

CT2K16G4DFRA32A 2 x 16 kit

CT2K8G4DFRA32A 2 x 8 Kit

CT2K16G4DFD832A 2 x 16 Kit

Has anyone used this crucial memory kit :

CT2K8G4DFRA32A 2 x 8 Kit

....to go from 16GB to 32GB with this configuration?

XPS 8940 with 10th Gen Intel® Core™ i7-10700 processor

I'm really confused if it's compatible or not.

@SomeCaliGuy 

My xps i9 came with 2*8 go Sk hynix on slot 1 and 3

i bougth  2*8 crucial 3200 cl 22 1.2v to have 32 go ram (ct8g4dfra32a)

first i install thèse new ram in slot 2 and 4 => 2400 mhz

then in test with only crucial ram in place of hynix => 16 go ram at 2933mhz

After i test to put hynix in slot 1 and 2 (Black and white slot) and put crucial in slot 3 and 4 otherwhite and black slot

thats mean that i have not the same brand of memory  in white slots. neither on black slots

the dual Channel stell work in this configuration and the frequency is 2933 only in this configuration 

Big question for everyone here: is it worth it to upgrade from 16gb to 32gb? I play mostly video games but also dabble in video editing. Or should I just go for it and go for 64gb? 

I was looking through the comments here and saw a few recommendations of 32gb that work on the XPD 8940 (at the best optimized speed) but may have glossed over 64 recommendations so if anyone has one, don't be shy. 

Help is greatly appreciated! 

@Fadadas Your advice worked 100% for me!

I bought two of these 8GB UDIMMS from Newegg ("Crucial Brand" ct8g4dfra32a).  When I staggered them, overall speed dropped to 2400.  But when I did as you suggest, all speed was retained (2933 MHZ) and dual channel too. 

Ran the memory test (twice!) and it passed with flying colors.

Check it out.

I think it works because both sets of different ram share one JDEC state that matches up exactly.  Very happy as it works great and saved me nearly $100 USD when buying more pricey memory.