Can I upgrade my i3 8100 to a i9 9900t?

Thanh - Be careful! While this person in the YouTube appears to have installed a i9 9900 CPU in a 4k 21.5" iMac. The systems logic board and power supply was not designed to support such a hot running CPU. While it may work the systems lifespan will be reduced! Take the time to review the TDP of the chip.

Think of it this way one pep pill or can of Red Bull gets you through the day so take two or more will allow you to work through the night! Thats if you live through the night without having your heart fail you. Sometimes too much of a good thing is just as bad!

Lastly, he used the wrong chip as Apple needs the intergraded graphics (iGP) for some of its functions.

In my opinion, we can install i7 or i9 with K/KF but CPU can not use all the strength because of mainboard and cooling system. I think i9 9900T with 35W TDP and 1.7GHz turbo boost to 3.8GHz is the best choice. Last year, the man in this video tested Core i7 7700K in iMac 21.5 inch 2017 and performance was not satisfactory. Remember that Power Supply of iMac 21.5'' is 185W and i9 9900/9900K is 95W while i7 7700K is 91W.

Check out this video: https://www.youtube.com/watch?v=u7MU5aWu...

@Thanh Tùng Nguyễn - You're mixing different wattage measures that don't apply across. The power supply wattage is a very different measure than the TDP measure of wattage.

The power supply power budget needs to be looked at by current load of all of the devices within the system. Remember Ohms Law

Now look at what TDP is all about Thermal Design Power which only applies to the CPU chip alone.

It's a highly flawed measure! In the old days with a single processing core the measure had standing, over the years as the number of cores increased the method of measure has become useless in gaging the true load of the multi-core chip. Remember this is still at a different voltage (depending on the given CPU's running voltage) you will need to scale it to the voltage you are referencing the wattage of the power supply.

Just took a stock i3 (8100) that has TDP of 65w and swapped with i9 (9900) that is also TDP of 65w, same microcode family (all are coffee lake), same socket (lga 1151) and works fine. Removed all thermal paste on processor and graphics and reapplied with arctic MX-4 paste.

cinebench went from 2900 to 9600 and the machine flies! Fan ran during testing and immediately slowed, stopped after.

also upgraded ram to 32 gb and changed HD to Samsung 1 TB SSD.

want do same upgrade my iMac 21.5 4k 2019 with i5 3.0 to i9 9900 and 16gb memory. also want upgrade ssd to Samsung evo 980 512gb and anyone knows will it work or not? as I checked i5 8500 has TDP 65w and i9 9900 has same 65w TDP so anyone tried this upgrade?? will this upgrade damage iMac after long usage? theoretically this i5 and i9 has exactly same TDP and it should work.