JOCean is correct.  Here is another Dell released info on a similar Inspiron model supporting 10th gen cpu:  NOTE: The computer is shipped with 3200 MHz memory. However, the memory speed is limited to 2666 MHz in computers shipped with Intel Core i3/i5 processors and 2933 MHz in computers shipped with Intel Core i7/i9 processors. Re: Would it matter if the speed doesn't match between the two modules, even if the system will step the higher speed module down anyway? in general, it is not a great idea to mix Dell ram with Crucial ram of different spec (speed and latency settings).  If we know the original manufacturer model number of Dell ram, you may be able to buy the same sped ram from third party vendor.  Alternatively, replace the Dell ram with crucial ram (not mix them).  Lastly, it is possible you may get lucky that PC will POST with mixing Dell and Crucial ram. The following Dell 8GB ram P/N is what I pulled from a 3891 system spec using genuine service tag It is quite likely your pc has the same DP/N.  When you know your service tag you can type it in Dell support to verify that.  9CXF2 is a familiar Dell 8GB ram used in many models.  Thanks to Dell Chris M, here is Dell released info: 9CXF2 8GB DDR4, 3200MHz, 1Gx64, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted Adata AO2P32NC8W1-BD3SHC, Hynix HMAA1GU6CJR6N-XN, Kingston K9CXF2-MIB/K9CXF2-MIE, Micron MTA4ATF1G64AZ-3G2B1/MTA4ATF1G64AZ-3G2E1, Samsung M378A1G44AB0-CWE/M378A1G44BB0-CWE So here are the original manufacturer's ram model number for the Dell ram.  Dell just validates these ram and ensures quality control, and charge consumer higher price. If that part number is your Del ram, you may consider buy another 8GB of same model to upgrade to 16 GB.  [IMAGE]      
I just purchased the Insprion 3891 for my son, and because of Dell's ridiculous lack of "options", I just picked a ready-to-ship build.  Buy "just purchased", I mean just clicked "purchase" on the online site about 30 minutes prior to making this post. It's the cheapest one with the i3, 8 GB ram, and 256 GB ssd. This is replacing a 14 year old Inspiron 530 that was handed down to him.  It came with 2 GB ram that has since been upgraded to 6, and a 512 GB hdd that every once in a while will get stuck clicking, but will come out of it and continue to run until some point in the near future when it may decide to just die for good.  It's done this for over a year now so I know our luck is running out - just a matter of when. However, there is no way to really customize the purchase without everything else changing with each option that can be set.  Like, why is there no option to add the GT 1030 graphics card that the XPS offers - the 1030 does not need extra power like the 1660 that is offered does.  Why can't I add ram or a hdd option individually without automatically changing the CPU, adding a couple hundred dollars to the price in the process? (My assumption is that the options matrix just jumps the the next pre-configured box with that option bringing along all the other changes with it - instead of being real individual "customization" options). So, what I'm doing is buying a couple of upgrades now to add in when the machine gets here. I'm adding a 1 TB hdd that I'm getting from Newegg, and I'm looking at ram too - but the ram is the question. The ram speed is DDR4 2666 MHz, but looking at Dell's site, all they offer for the 3891 is 3200 MHz ram.  I know the system will run the ram at 2666 anyway, but is there a reason only 3200 is offered?  If I buy Crucial ram, should it be 2666 or 3200 (there's only a buck difference anyway)?  Is the ram that comes in the machine 3200? Thanks,
Inspiron 3891 RAM type for upgrade