Hey gang,
So after much reading and answering questions I have begun my overclocking adventure on this EVGA Z390 Dark and i9 9900k processor. I was using Intel Extreme Tuning Utility Stress Test, along with CPU-Z for monitoring.
First I had already ran the Extreme Tuning stress test on my stock settings to insure I was stable at that, which went fine.
 
I began with a modest overclock of 4.9 GHz on all 8 cores.
Vcore at 1.23.
AVX - 0.
49x multi on all cores.
Vcore set to Adaptive.
Vdroop +50%.
 
I actually started little higher on vcore and kept stressing it for 15 minutes at a time, if passed I would lower a little, rinse and repeat. I stopped at 1.23v, so not sure if I could have gone lower or not.
I then raised my multi to 50x on all cores, keeping everything else the same. Ok passing that without any apparent issue.
Went to 51x on all cores still with everything else the same (1.23v), passed that for 15 minutes no problem.
Ok so then I tried 52x on all cores. Here is where I ran into problem getting into Windows. Kept upping the vcore till I got to 1.30v, still not getting into Windows. I then bailed at least for now on 5.2 Ghz - apparently I am hitting a wall there that is proving difficult to push through.
 
Ok so I decided to back it back down to 51x all cores, my 5.1 GHz overclock. This time I ran it for an hour in Intel Extreme stress test,100% CPU load. All looks fine, passed the test. Core temps peaking in the upper 70's. Not sure it would be worth my pumping significant more voltage into the CPU for an additional 100 MHz clock speed. 
 
So at least for now I am going to settle on 5.1 GHz across all cores. I will do some gaming on it now to see how it does, and how the core temps do. Hopefully it will continue to be stable. I am pretty happy with that!
One observation - whilst my Vcore setting in the bios is 1.23v, whilst stress testing and viewing the core VID in CPU-Z it was fluctuating like between 1.32-1.34v. 
 
 

Thanks will have a look.
Will go in and disable P states and Virtualization.

Yikes - so tried 5.2 again, since it had not booted into windows at 1.30v, I tried it at 1.35v.
Disabled vdroop.
Booted into Windows fine.
Started my stress test for a quick run - Intel Extreme Tuning Utility - core temps shot up like a rocket, one got to 95 and another 100, quickly stopped that.
 
Think I will be ok with my 5.1 GHz clock.

d.burnette
There were two virtualization technologies enabled in the bios, one was an Intel Virtualization something and the other just Virtualization technology.
I disabled both of them, is that correct?

yes, both.   For now, you don't need them as they just suck resources.  Once you find your daily, you can put those back. 

Ok thanks, have disabled both Virtualization Technologies.
 
Whew things back to good now temp wise with my 5.1 GHz overclock. May even try lowering the voltage a little more as I had just stopped at 1.23 vcore.
I suppose I could even disable HT as I don't think the games I run make use of it.
 
I am quite happy with 5.1 really, and my gaming in VR with this rig totally spanks what I was getting in my previous X79 4820k system. 

Delirious
to be honest,  I'm happy with 5.1 too.  Eight core going that fast.  It's pretty nice.   I got 5.2 stable a few days ago.  Around 1.408 vcore.   Temps where just 65 C.   I was able to get 5.3 up and running but temps shot into the 80-85C range and I don't really need that.   Had I delidded my CPU and changed thermal paste, I bet I could lower that 4 C  but not worth it for me for all the hassel.  

Yeah I would agree with that, sounds good to me!
Edit: You got some great temps there!

5.1 across all cores is very good. Get pumped!

So since my main purpose in this rig is gaming in VR, I disabled HT and did another run in Intel Extreme Tuning Stress Test.
Lowered my overall core temps by about 8 degrees c at my 5.1 GHz all core overclock. Highest a single core got was 71c.
 
Question on Vcore - I am running it on adaptive - is that what you guys are doing? I notice on adaptive with Vcore set in bios to 1.22v, and +50% Vdroop, during the stress test viewing vcore in CPUz it ditty bops between 1.31-1.35v. Is it safe to be running this on my 9900k?

Ok thanks, will probably just leave at adaptive. 
 
So hitting 1.32-1.35v under stress load is going to be ok right?

Delirious
yes
 

Great thanks. Been gaming in VR and loving the performance I am getting now! 

onMute
I'm able to get 5.2GHz stable on all cores - voltage hangs around 1.38 and temps in mid 60s. I set that as my daily and it has been rock solid. As others have stated, 5.1 is still very good.

What cooler do you have? Those are great temps. I just don't have good enough cooling to get that high.

onMute
Custom loop with 2 360mm radiators.

Nice!

Prime 95 with AVX on, min/max fft at 1344, and small fft in place checked. Ran for over an hour. Max temp on one of the cores was 72 but the average stayed in low-mid 60s.

AVX offset is 0. I tried pushing to 5.3GHz with an offset of 1 but after dialing in north of 1.4v and crashing cinebench, I backed off and accepted 5.2GHz as my daily. Cinebench at 5.2GHz will spike core temp to around 80c.