Hey guys, I was talking to someone and he said to lower the temps on my 9900ks (which is running at 5.2ghz) is to have only 4 cores at 5.2ghz and the other 4 at 5ghz since most games don’t use more than 4 cores. This sounds like a great idea but I have no idea to figure out which cores are the most used for games. Anyone know how I can figure out which core numbers are active during games so I can adjust in bios accordingly?

While some games don't have great support for multi core it's becoming more common to support as many cores as you have in the system. Just keep in mind games are not made to need something like the 9900K but more use the GPU. So you might be able to lower the clocks down and have it run cooler without really losing any FPS.

Sniper_3001
Hey guys, I was talking to someone and he said to lower the temps on my 9900ks (which is running at 5.2ghz) is to have only 4 cores at 5.2ghz and the other 4 at 5ghz since most games don’t use more than 4 cores. This sounds like a great idea but I have no idea to figure out which cores are the most used for games. Anyone know how I can figure out which core numbers are active during games so I can adjust in bios accordingly?

It's an idea if you need to lower your temps I guess. Your friend needs to update his knowledge base from 2012.  The vast majority of games released in the last couple of years are multi-threaded and gain from both core count and clock speed. If your temps are too high while gaming, I'd solve that rather than reduce performance.

Wow. Really good info. Thx guys. The guy works for a computer store. He was the tech guy. Anyways one more question....when running say a Corsair 150i pro 360mm AIO Watercooler, what are the temp ranges for the actual coolant not the cpu. Like what is a temp you do not want to go over? Right now I’m around 38 degrees and cpu temps around 45-65 degrees during gaming. Only problem is icue software default have my fans spinning 100% at like 35 degrees coolant temp. Is this right? What do your fan curves look like for coolant temps?

5.2GHz isn't really very much of an overclock for an i9-9900KS that has a default Max Turbo Frequency of 5.00 GHz.
 
But those things have a reputation for running on the hot side even at default frequency..You might wanna check out this thread and the GamersNexus and Hardware Unboxed videos in posts 31 and 33 about the power and thermals, seems to be a mobo BIOS issue with some boards exceeding Intel specs.

Anyway, 45C-65C during gaming is definately not an issue but with some BIOS tweaking you might get it down..Ambient temp will be a big factor as well as case ventilation, rad fan orientation (should be sucking in outside air), and Vcore at Idle/Load.

My H80i V2 liquid temps were 24C Min/25.4C Max running Superposition which is a graphics/gaming bench..CPU Cores temp were 22C Min/53C Max.
 
Running IBT which is a CPU stress test from hell..Liquid temps were 23.7 Min/28.9 Max..CPU Cores were 22C Min/67C Max.

My ambient about 20C...Temps were taken from Corsair LINK..I uninstalled iCUE because I think it kinda sux and I don't need it just for my cooler..There should be Fan/Pump presets you can choose from or make your own custom curves in iCUE like you can in LINK, I forget.