XPS 8940, memory upgrade of 2x8GB

@bigfraggle007 The 3000 MHz Ballistix kit should work, it is listed when Crucial's 100% compatible memory finder is used. In this forum post, a user brought a 16 GB 3000 MHz Ballistrix kit and although he had a bad memory module he eventually got it to work.

@RaymondGarcia  have you finally been able to make your 4 x 8gb run at 2933 mHz ?

I will receive my new XPS 8940 next Monday, with an i7 10700 and 2 x 8gb of RAM. I wish I had seen this post before ordering, I would have rather ordered 1 x 16 gb instead of 2 x 8 gb, but I wanted it to run dual channel... I thought it would be easy to add 2 more 8 gb sticks to get 4 x 8 gb. I can't understand why this 4 x 8 gb RAM configuration isn't supported.

Please keep us informed if you ever find a solution.

Thanks

@sck90  Is it a question that 4x 8 GB isn't supported or that the 2 pairs of 8 GB aren't well matched so they have to clock down to a slower speed where they can work together...?

@RoHe  well I think he wrote that he ordered the ram sticks from Dell...so I guess they must be fully matched with the 2 x 8gb his computer was shipped with

Thanks for the precisions @RaymondGarcia . It kind of annoys me thinking I wont be able to upgrade my 2x8gb ram without throwing these to sticks away. Is this a BIOS limitation ? Why cant 4x8gb be supported when there is no problem with 4x16gb or 4x32gb ? 

So the unfortunate part is the 2x8GB RAM I got with my XPS 8940 were not matched well (see graph below).   One was Micron one was SK Hynx.  Now it worked fine and ran at 2933 MHz (at 2x8GB).  When I went to 4x8GB (again RAM from DELL SK Hynx) it dropped the speed down to 2400 MHz.  Interesting any combination of 2x8GB (Micron or SK Hynx) worked at 2933 MHz.  Going to 4x8GB goes down to 2400 MHz (tried all sorts of order in the slots it made no difference).  Also the BIOS is up to date and apart from that you don't have much more you can do.  

@Jon-62 

Ok, good advice. Conversely, what memory do I need if I want to achieve the full 2933 MHz?

And: how do I know the regular speed of RAM modules without overclocking? I checked again at Crucial and found no information. Or is this information hidden behind the "PC4-24000"?

The 3000MHz given for the Crucial Ballistix memory is the XMP 'overclocking' profile frequency.  Dell does not provide the capability to overclock the memory.  The SPD profile for the Crucial Ballistix 3000MHz CAS 15 memory is 2400MHz CAS 15, so the memory will not work at a higher frequency than 2400MHz in your (and my) Dell XPS 8940 computer. 

Thanks @Vic384 I did download and run Memtest86.  After about 4 hours no errors.  So it seems that the 4x8GB is stable but it will only operate at 2400 MHz,   @sck90  I am as disappointed as you are.  Thought I was smart going to 2x8 instead of 1x16 so I could operate at dual channel.  Like you I didn't look closely enough at the memory chart provided by DELL and that is posted at the start of this chat.  It is odd that 4x8GB isn't supported but 4x16 and 4x32GB is supported.  I am not a hardware engineer but I can only assume it is a cost saving thing.  Similarly not supporting XMP is a bit of a drag.  So if you really need 32GB running at 2400MHz is better than using a bunch of virtual RAM even with an SSD.  But if you don't then it certainly isn't worth it.  Tossing all the RAM you have and going to 2x16 is an option but is an expensive one.  Bit concerning as well given that DDR5 is coming out and finding RAM with SPD timings at 2933 MHz that don't require XMP profiles is already difficult I fear that if you wait a few years trying to find 16x2GB that are actually compatible might be a problem.   If you are interested my TimeSpy 3D mark scores didn't change much.  See below

with 2x8GB of Ram

Score with 4x16GB of Ram.  I ran it several times and the score always varies by 50-100ish points.  Seems like it was consistently about 100 lower with the 4x16GB.

I'm sure I found the information on the Crucial site.  And yes, the 24000 in the designation is a giveaway.

The advice from others is to use Crucial memory, but this should be 'standard' memory, not the gamer memory, which tends to be sold using the XMP values rather than the SPD values (and dressed in a fancy aluminium coat).

Here in UK, I'd probably just go to Mr Memory and put in XPS 8940 and pay their price rather than take any risk.

@Jon-62 

Ok, good advice. Conversely, what memory do I need if I want to achieve the full 2933 MHz?

And: how do I know the regular speed of RAM modules without overclocking? I checked again at Crucial and found no information. Or is this information hidden behind the "PC4-24000"?

@bigfraggle007 You need memory with an SPD speed greater than 2933 MHz, Since Crucial does not list the SPD speed in their specifications you may have to call them to find that information. 

@Jon-62 

Thanks!

So 2x 16 RAM would be CT2K16G4DFRA32A?

Crucial part numbers are:

1 x Crucial 8GB DDR4-3200 UDIMM - CT8G4DFRA32A

Crucial 16GB Kit (2x8GB) DDR4-3200 UDIMM - CT2K8G4DFRA32A

@RaymondGarcia At least your Memtest86 results show that your 4 x 8 GB is stable at 2400 MHz. I don't think you will see a noticeable difference in performance. See this video about whether or not RAM speed really matters: Does RAM speed REALLY matter? - YouTube