Upgrade CPU from 7700k to 9900k

Romulus - While I commend you or your tenacity! You face a few road blocks that prevent you from doing this.

Apple limits the system BIOS (EFI) to the CPU’s of the systems series. So technically your system is limited to the CPU’s in this set of systems iMac18,3

While Apple has just introduced a new set of iMac’s iMac19,1 and Apple now offers an iMac with a 3.6 GHz Core i9 (I9-9900K) we don’t know what Apple did internally yet.

It might be a simple swap out of the CPU if Apple only updated the current EFI firmware with the chips ID, or it may require manually updating the EFI chip.

As far as the systems cooling I strongly recommend you don’t try de-liding the chip! Intel is now soldering the lids on to improve the thermals. All you’ll do is damage the chip ($$$) So don’t do it!!

Also don’t use liquid metal TIM!! The mounting of the CPU allows the TIM to slowly leak out and the heat sink will be damaged by it as well.

Why don’t you wait a bit until the IFIXIT (or other group) teardown team has has a chance to review the new system before jumping.

If you really need this level of performance now I would recommend you just buy the new system or get an iMac Pro which offers much better performance options with the Xenon CPU’s