Hi,
TLDR version:
Check the CPU Voltage for your 7700K. My motherboard was giving unnecessary high voltage to the CPU out of the box(did not change any settings in motherboard or anywhere). My FPU stress test temperatures were at 91C-94C within 2 seconds. After checking out this forum and Gamernexus review, I was able to reduce the CPU voltage and bring the temps down to ~67C! Currently running 4.8GHz at ~80C and very happy with it
Long version:
I got a Gigabyte Z270XP-SLI (Rev 1.0) to pair with my new 7700K. For cooling, I'm using Corasair H100i v2. This was a completely new PC with a clean install and all out of the box configurations in the motherboard. After installing Windows 10, I did stress testing on the CPU and GPU(My EVGA GTX 1080 SC was quite cool at 65C under furmark).
As soon as I started the CPU and FPU stress tests on AIDA64 Trial version, the temps jumped to 91C-94C in less than 2 seconds. Scared out of my wits, I shut down the system after 2 minutes of same steady temps and reinstalled my AIO cooler carefully. Repeated the procedure 4 more times with no improvements in temps.
By the way, my idle temps were perfectly normal at around 29C-31C.
Joined this forum and found this amazing post by Streetguru in this forum. Apparently Gigabyte Z270 motherboards applied waaaaay to much CPU voltage right out of the box so when doing a stress test, the temps went skyhigh before you could say wtf. The motherboard came with BIOS version F2(dated 11/26/2016). Once i upgraded the BIOS to F4(02/22/2017), the CPU voltage dropped to 1.25V and the temps reduced to 70C-73C.
Tried a few more variations and here are the results for the stress tests:
Stable 4.5Ghz @1.2V with temps at 67C
Stable 4.8Ghz @1.242V with temps at ~80C [Current setting]
Stable 4.9Ghz @1.248V with temps at ~82C-83C
BSOD 5Ghz (didn't want to up the voltage beyond 1.3V. I might be able to do it, but this speed is not necessary for me. )
Another article linked by Streetguru thta I found very helpful: http://www.gamersnexus.net/hwreviews/2744-intel-i7-7700k-review-and-benchmark/page-4
Hope that helps for anyone worried with their temperature reading. Im a newbie here, let me know if my post had any issues and I will edit it. Thanks to you guys I fixed my issue!