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Intel Core i9-7900X ‘Skylake-X’ 10C20T CPU Indepth Analysis

Our objective with CPU overclocking is to hit frequencies that we think will be achievable for daily use by the platform’s buyers. As such, we test with sensible cooling hardware in the Noctua NH-D15 and Corsair H110i GT 280mm AIO. We also used sensible voltages that lead to manageable thermal results (well, somewhat manageable as is the case for Skylake-X).

Stability is confirmed by running multiple Cinebench tests, Handbrake video conversion, and AIDA64 CPU, FPU, and Cache stress test for at least 1 hour. We do not use Prime95 as we have found it to be overly demanding as a stress test application with the more recent AVX-capable versions.

The partnering hardware of choice is the ASUS X299-Deluxe motherboard, 32GB of G.SKILL TridentZ 3200MHz CL14 DDR4, and a Seasonic 760W Platinum PSU. We chose the ASUS motherboard due to its strong power delivery system based around International Rectifiers MOSFETs and, importantly, the ability to connect an 8-pin plus a 4-pin CPU power connector.

That added 4-pin power connector gives extra peace of mind when overclocking the very power-hungry Core i9-7900X. We also point a 100mm Antec Spot Cool at the VRM heatsink to provide direct airflow for cooling, as is important when shifting such high levels of power through the MOSFETs.

The Skylake-X Core i9-7900X is both fun and frustrating to overclock. It is fun in the sense that a relatively low voltage (1.20V in our case) yields clock speeds that are very high for a 10-core chip. The frustrating part is the thermal performance which is poor thanks to Intel’s decision to use TIM under the heatspreader, rather than solder. In our testing, overclocking is limited by awful thermal performance, not the capability of the chip. A better heatspreader design would likely lead to enhanced overclocking results by being able to cool higher voltage levels.

We settled on 4.6GHz at a very low 1.20V as our best overclocking result. 1.20V is particularly low for a 4.6GHz clock frequency, even for the 7900X, but the chip was perfectly stable for extended sessions of AIDA64 stress test as well as multiple runs of our entire benchmark suite. This result was not Prime95 stable with an AVX workload and dropping down to 4.5GHz for Prime95 testing resulted in thermal throttling within seconds.

To achieve 4.6GHz at 1.20V, we were forced to use the 280mm Corsair H110i GT AIO with its fans operating at full speed (very loud). The Noctua NH-D15 could not keep the chip below 90°C which is higher than we were happy with. Backing down to 4.5GHz at 1.20V was slightly better for the D15 and would be borderline usable for daily usage, based on our preference for operating temperatures.

Pushing up to 1.25V in search of higher speeds resulted in load temperatures rushing well past 90°C, which we were not satisfied with. Put simply, if you want to push voltages to a point where higher than 4.6GHz may be possible, you are likely to need one of the best AIO liquid coolers on the market or a custom loop. That’s without bringing de-lidding into the equation, of course, which is risky business on a $999 CPU!

In short, our final Core i9-7900X overclocking settings using an ASUS X299-Deluxe motherboard were:

  • 4.6GHz on all cores.
  • 1.20V CPU VCore.
  • Level 3 LLC (1.20V load VCore in OS, according to software readings).
  • Multiple Cinebench R15 multi-core runs to validate stability, as well as AIDA64 CPU stress test.
  • DDR4-3200MHz 14-14-14-34 @ 1.35V.

Overclocking comments:

The Core i9-7900X is a strong overclocker that should be capable of 4.5GHz+ on all ten cores, provided you can cool the beast and power it. Thermal performance is limited by the poor TIM used by Intel, so pushing voltages to a level where you can comfortably surpass 4.6GHz is likely to require a custom liquid cooling loop or de-lidding of the $999 processor(!).

We found that air cooling, even with the sizeable Noctua NH-D15, will start to struggle at 1.20V and 4.5GHz on our chip. The Corsair H110i GT 280mm AIO kept 1.20V in check for frequency to be increased to 4.6GHz but a voltage uplift of 25-50mV pushes load temperatures past 90°C on the loud liquid cooler.

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