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Temperature & Thermal Problems
Cooling Nuclear Option: The Chiller
As mentioned, we had to use Alphacool's Eiszeit Chiller 2000 to achieve usable overclocking results. More conventional thermal solutions just wouldn't cut it. All-in-ones like Corsair's H100i and Enermax's LiqTech 240 hit their limits at stock frequencies under Prime95. The custom loop threw in the towel at 4.6 GHz.
Why can't those liquid coolers keep up with a CPU like the -7900X? Back in the day, a normal all-in-one was good enough to keep the Core i7-5960X running cool, even overclocked to 4.8 GHz. We measured power consumption numbers of up to 250W back then. So, why did we have to force a constant 20°C in the loop to even start experimenting?
High Temperature Differences Challenge Cooling Performance
The reason that Skylake-X is so much harder to cool traces back to the thermal paste Intel chose to use instead of solder between the processor die and heat spreader. Although paste is cheaper, it's also less than ideal for cooling performance.
Intel’s digital temperature sensors report reliable results from 35 to 40°C and up, prompting us to only include values above that range in our analysis. The difference between the water cooling block's temperature, which is held at a constant 20°C, and the CPU temperature reported by Intel's sensors shows just how bad of a choice thermal paste really was.
We measured the CPU heat spreader’s temperature the same way we did when AMD launched Ryzen 7 1800X, by using a thin copper plate. The resulting curve shows very clearly that waste heat can't be dissipated quickly enough. A solution good enough for a thermal lightweight like Intel's Core i7-7700K just doesn’t work for Core i9-7900X.
This curve represents the temperature delta, which is to say the thermal difference between Core i9-7900X’s cores and the top of its heat spreader. The outcome is shocking:
In the end, the delta between the cores and top of the heat spreader reaches 71°C, and that's using one of the best cooling setups money can buy. Naturally, lesser thermal solutions start running into trouble at stock frequencies when you run a stress test.
To illustrate our point, we plotted the temperature for all of the Core i9-7900X’s cores at stock settings running Prime95 or LuxRender. A good custom water-cooling loop does fairly well, which shouldn't come as a surprise. However, no other thermal solution will be able to keep up. Even the motherboard manufacturers we spoke to agree, telling us about their all-in-one liquid coolers running out of headroom as soon as they ran Prime95 without limiting AVX.
A Tcore of up to 65°C and a heat spreader temperature of approximately 24°C make for a difference of more than 40°C. That's at 230W. Once the 300W line is crossed, even the Alphacool Eiszeit Chiller 2000 taps out. This isn’t even difficult to do: with a Core i9-7900X running at 4.6 or 4.7 GHz, using the voltages needed to get there, even simple rendering applications trigger those levels. The highest power consumption numbers we saw were just north of 300W, which had the CPU hitting its 100°C thermal limit consistently. An emergency shutdown followed soon after.
Leakage
Next, we measured power consumption under a constant load using different coolers. For a temperature increase of approximately 40°C, power consumption increases by five percent. This isn’t just an acceptable result, but a really good one. The values above 100°C are not as reliable due to throttling. Consequently, we made an exception and used a low-pass filter that smoothed out the brief decreases.
Everything could have been great, if it wasn't for the thermal paste between Intel's die and heat spreader. Admittedly, most workstation or semi-pro users won't overclock, cutting down on the number of customers affected by this problem. But we all know that affluent enthusiasts attracted to Skylake-X's balance between high frequencies and core counts will have to face a significant cooling challenge. Your choices come down to high-end all-in-one packages or a custom water-cooling loop. Air cooling is completely out of the question if you expect the -7900X to run comfortably under full load.
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