Know Your Voltages
Apart from increasing the frequency with multipliers and BCLK, you also need to increase voltage to sustain the higher clock frequencies. As the CPU works faster, the transistors need to be able to hit certain electrical highs, and increasing voltages help this.
CPU Input Voltage (CPU VRIN): This is the voltage that the motherboard provides to the CPU, and the CPU then takes this voltage and produces all the other voltages. The default for this voltage is 1.8v, however, increasing this voltage can help a little bit. I would stay under 2.1 on air cooling, as increasing this voltage will increase the delta between this and the other voltages, which will increase the internal CPU VRM's temperatures.
CPU VCore: This is the main voltage to the CPU Core domain. Default for these CPUs is around 1.00v, I suggest staying under 1.35 to 1.4v on air/water, as cooling anything above 1.35v seemed to be impossible with 100% load. These CPUs do throttle between 80C and 90C.
CPU Cache/Ring Voltage: This is the voltage that is responsible for powering the cache. Increasing this helps with increasing cache frequency (Uncore in certain BIOSes) - its default is 1.05v. This voltage helps to also stabilize CPU core domain overclocks too, so if VCore causes too much heat, try increasing this voltage, while dialing back the VCore a tad - you might be surprised at the gains. The cache voltage can be increased to the same levels as the CPU VCore, but I would try to stay under 1.3v on air.
CPU System Agent Voltage: This is the voltage to the system agent, which is where the integrated DDR4 memory controller is - default is +0.000v. I think ASUS has changed this into 1.000v at default; regardless, adding +0.3 is like setting 1.3v on an ASUS board. This voltage is one that helps a lot with memory overclocking on the CPU side.
I have found that adding +0.500v didn't seem to have any detrimental effects for clocking over 3100MHz on the memory. However, I would suggest staying under +0.3 to +0.4v on air, but if you want to get the most out of your IMC for high clocking challenges, you can try +0.5v. This voltage helps with BCLK as well in some cases.
CPU VCCIO Voltage: This seems to be one of those useless voltages this time around. This voltage should provide power to the CPU IO analog and digital portions of the CPU, like the PCI-E controller and other IOs. I increased and decreased this gradually to see if it would help with CPU, BCLK, or memory overclocking, but it didn't seem to make any difference. I would just not touch it.
The DRAM voltages need to be changed twice, one for each set of two to four DIMMs which reside on either side of the CPU.
DRAM Voltage: This is the main voltage for DRAM overclocking and the only one 99% of you will need to touch to increase your DRAM overclocking margin. JEDEC specifies 1.2v as the default, with 1.35v being the upper-end for XMP high frequency modules (2800MHz and above).
On air, I have found increasing towards 1.5v or even up to 1.6v can be acceptable for quick benching and testing, but I would stay under 1.4v for 24/7 operation. Higher voltage might be needed if you are trying to stabilize lower timings. I found that my Micron brand ICs don't like over 1.4v, but Hynix modules seem to be okay with over 1.4v.
DRAM VPP Voltage: This is the voltage for the electrical high for DRAM row access. This isn't a technical article on the power delivery for DDR4, but I would think some of you would want to know what exactly this voltage does. DRAM has both row and column access, and the control line for the row is referred to as the word line. In the past, to activate the access transistor, the word line required a higher voltage, which was pumped up from the 1.5v (now 1.2v) DDR voltage.
For DDR4, JEDEC decided to introduce a secondary external VRM that provides a 2.5v electrical high voltage for the word line. Since the word line voltage is no longer pumped up from the DDR voltage, the inefficiencies of pumping up the DDR voltage are gone, and instead you get power savings. Increasing this voltage doesn't do much since it is already really high, but increasing it a tiny bit can help with very high DDR4 clocks like 4GHz.
DRAM VTT Voltage: This is the VTT, which is the termination voltage for the DDR. This should be half of the DDR voltage. This should automatically be half of the DDR voltage, but if you are encountering instability at higher DDR voltage, you can try to increase this like 0.001v above half the DDR voltage.
PCH Core Voltage: This is the PCH's core voltage, at 1.05v it should be more than enough to reach 103MHz, but if you want to go further, you can try increasing this to 1.15-1.25v.
PCH IO Voltage: This is the voltage for the I/O of the PCH. On the Z97 chipset, a little known trick to maximize BCLK overclocking was to lower this voltage as low as possible, and since default is 1.5v, I used to decrease it to 1.05v, but on X99 this doesn't work. In fact, this voltage seems to have little effect, as I am guessing Intel has totally changed the PCH I/O for the X99 PCH. I wouldn't touch this voltage.
Every motherboard is different, as some use digital PWMs and some use analog, however, all of them should have some sort of LLC (Load Line Calibration) which will control the voltage drop of the CPU VIN voltage. I would stabilize the CPU VIN voltage as much as possible, and set this to its highest value.
Since many motherboards now use International Rectifier PWMs (ASUS and GIGABYTE do across all their X99 motherboards), I would just mention that you should set the current capacities and PWM control schemes to benefit higher current flow into the CPU. Do provide some sort of airflow over the CPU VRM, as you don't want CPU VRM temperatures to throttle your CPU.
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