Thanks! The BIOS is up to date as of a few days ago being 2.8.0 currently...

TB

On a similar note, I have an older T5820 here too that I would like to upgrade the CPU in. Currently it is equipped with a W-2133. Is there a list somewhere of compatible CPUs?  I know the Intel CL W-22## range of Xeon processors isn't compatible without a motherboard change of some kind (presumably the reason why there is now a 'CL' motherboard option on new 5820 purchases). But what about i7 and i9 X-Series CPUs? The compatibility notes just literally say i7 and i9 X-Series, but that now spans four generations. Do they really all work in this machine? 

This is all I could find on it.  The 1st screengrab is from the manual, 2nd from the spec sheet.  If interested, both are downloadable from Dell by Googling them.  It appears the 5820 doesn't take i7's or i9's.  As for the mention of dell.com/precision in the spec sheet, that just goes to Precision sales.

I appreciate the reply. I suspect I need to dig out my service tag and contact Dell directly. 

The service manual definitely suggests i9 X series processors work. There is a section in it dedicated to the difference in PCIe speeds between CPUs. 

https://www.dell.com/support/manuals/en-au/precision-5820-workstation/precision_5820_om_pub/card-slots?guid=guid-641b07b1-358e-40d2-a8c2-60ab826d654e&lang=en-us

The xeon  W-2133 uses FCLGA2066 as socket

https://www.intel.co.uk/content/www/uk/en/support/articles/000005592/processors/intel-core-processors.html

and these are the cpus that are compatible with that socket, according to intel. Then you need to dig out which ones of those

You're right about the manual, I have 2 of them.  I searched by "cpu" and "processor" vs. "I9."  And you've been right about the vagueness.

Hopefully the following link narrows it down.  If you don't see a list of CPU's, scroll down a ways to "Compatible Products."  https://ark.intel.com/content/www/us/en/ark/products/122941/intel-x299-chipset.html 

I almost should've thought of it sooner, just not used to Dell being that vague.

Remember, often the available CPU's are more limited by the chipset as compared to the socket.

so... just an update. Dell won't provide any of that information as the service tags on my machine expired about 25 days ago. 

Well, I may have something ( since dell seems hiding behind the warranty.... )

Precision-5820-Tower-Spec-Sheet.pdf (dell.com)

so, as previously mentioned, there are 2 different mainboards and if you read that pdf

Chipset Intel® C422 (Skylake/Cascade Lake W) ;
Intel X299 (Skylake/Cascade Lake X)

they differ in part for this.

You now have a xeon W, so an intel c422 chipset. and Chipset Intel® C422 these are the compabile cpus with that specific chipset ( note : the W-22xx are not supported on the first revision of the dell mb ).

Makes me think that you can upgrade to a better W-21xx , and that's all

That is some terrific info. At least we have an answer - I shall stick with the W-21## series. There are still plenty of options to choose from (and they save me money, so that is a plus). 

I've done some more internet sleuthing and found a reference to some part numbers. The T5820 requires a motherboard with part number X75JG to work with i7 and i9 X-series processors. This was specifically in relation to the 109## series of i7 and i9 CPUs.