Intel Max Voltages Stability Testing
Apart from disabling parts/features of the CPU that are unneeded to help stabilize your overclock (VT-d, iGPU, etc.), there are only two things you can do to help stabilize higher clocks. The first is to raise voltages, and the second is to improve cooling. Improving cooling is always good, the lower you go, the more headroom you have to add voltage, and you might even find you can reduce voltages to maintain the same overclock if the CPU is made colder.
Improving cooling is difficult and costs money, but increasing voltages is free. You can delid your CPU, use a custom water cooling loop, change thermal interface material, and even go sub-zero. While increasing voltages is free, it's also a double-edged sword. Increasing voltages increases temperatures. You might find that reducing voltages lowers CPU temperature enough, so something like 1.3v could be stable while 1.4v would cause instability at the same frequency because of the added heat. Below we have a list of Intel's recommended maximum operating voltages per their 8th Generation Core Datasheet.
Intel recommends a maximum operating voltage of 1.52v VCore, but that's just way too high. I don't know of any watercooler that could cool down a CPU being fed 1.52v, but of course, that figure from Intel doesn't account for LLC since it's not part of Intel default specifications. I recommend 1.45v as maximum VCore with non-subzero cooling, but that doesn't matter since you will probably hit a thermal limit before you hit that voltage limit.
The VCCSA (System Agent) is 1.05v stock, and the VCCIO (System IO) is rated 0.95v stock. I recommend no more than 1.35v on either of these. I honestly wouldn't run much more than 1.25v for 24/7 use. If you hit an issue with DRAM overclocking, try many different levels from 1.1 to 1.35v. I have seen memory controllers that don't like anything over 1.25v, so try increasing VCCSA first and then VCCIO, and it's fine if they are different levels.
Now, if you remember the flow chart from the first page, I have intermediate stress tests placed after each stage and then a hefty test at the end. That is because stress testing can take days at a time, some people will not call a system "stable" unless it can withstand two days of Prime95 with AVX. I recommend finding a stress testing program that can be run in minutes to an hour, and then when you have tuned all your knobs, then run something like Prime95 for a day or two.
I mainly use three stability testing programs; HandBrake encoding of a 4K video (very quick but shows performance and uses AVX), Intel Burn Test with a decent chunk of memory usage (has AVX and can be run however long you like), and Prime95 blend one day and small FFTs for another (latest version with AVX). Now, there are many out there who still use older versions of Prime95, mainly because older versions didn't utilize the AVX. If AVX is crashing your system because it is overly intensive, you should use an AVX offset, that is what it is there for. However, if you do that you should also use an older version of Prime95 or another stress test without AVX so that you can also test out your maximum multiplier.
Prime95 has a few different tests you can run. The default test is a blend test, which tests most everything, but isn't going to demolish CPU core instabilities as Small FFTs would. There is some documentation on how to disable AVX use within Prime95 by adding in a string in the local.text file created when the program runs, but from my testing, that method didn't work. Many people also use RealBench for hardcore testing. AIDA64 also has a built-in test, and you can choose what you test (core, FPU, etc.) but I would increase the amount of RAM used in that test as the default value is a bit low. AIDA64 is considered a "safe" test by many, as it's not designed to beat your CPU up, but many believe you can't become a man until you get a really good beating.
When you run Prime95 keep an eye on temperatures, as you can see I am in the danger zone (80-90C) already with only 1.25v and 4.7GHz (5GHz with -3 AVX offset). You can also see my frequency numbers in the upper right corner, showing that maximum was 5GHz, but current is 4.7GHz. All in all, overclocking is part art and part science. There are rules to follow, guidelines to abide by, and mechanisms with fixed results. However, there is also a lot of uncertainty, as you hear stories of people who pass a day or two of Prime95 but crash after 5 minutes in a game.
Stability testing does help a lot, but there is a reason enterprise-class hardware is usually overclocking locked. Intel and AMD spend a lot of money on making sure their products are rock solid at default, and the truth is if they could get an extra 500MHz out of it, they would and charge more for it as well. Overclocking, when done right, provides immense benefits and is a lot of fun, but in the end don't get your hopes up if you can't hit that 5GHz mark. It's very difficult, and 4.9GHz won't be much worse. Overall, overclocking Coffee Lake CPUs is strightforward and pretty easy. If you have any questions or comments please contact us or post in the comments.
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