Overclocking, Cooling & Temperature
First, we established the Core i7-8700K’s limits by cooling the chip's IHS to a constant 20°C, side-stepping potential thermal bottlenecks imposed by Intel's unfortunate use of thermal paste between the die and heat spreader. We made it all the way to 5.0 GHz without any problems. This didn't surprise us, given our experiences with Kaby Lake. At 5.1 GHz, we booted into Windows and ran a couple of games, but the processor called it quits when we tried to run Cinebench.
We’d like to point out the possibilities enabled by manual load-line calibration. Depending on the motherboard, different levels and presets are available, or the voltages can be manually adjusted for some experimentation. In this way, the core voltage can be reduced significantly without losing a whole lot of performance (so long as your chip plays along). For the motherboard we tested, the effective voltage was between 1.18V and a maximum of 1.28V when running Prime95 at stock frequencies. This lowered the package’s temperature by almost 8°C.
Unfortunately, many memory kits run into trouble if the load-line calibration is set too low; the result is general instability. CPU quality plays a key role as well. In our particular case, this affected our 5.0 GHz overclock, which didn’t hold up over time. All of the games and some of the workstation applications ran for hours without any problems, but Creo 3.0 and some of the HPC tests crashed after a few minutes.
The 5+ GHz overclocking stories are exciting, to be sure. But remember that most of them aren't validated for long-term reliability. We'd rather drop the clock rate by 100 MHz and not have to deal with intermittent crashes.
Looking at our power consumption and performance graph, we see a bend at ~4.8 GHz. Power use continues increasing with higher clock rates, but the Cinebench score levels off. A failure to continue scaling at 5.0 GHz is a good indicator that our CPU is throttling. It simply cannot dissipate heat quickly enough.
At idle, the differences in power consumption are fairly marginal. All of the processors end up just about where we'd expect based on previous reviews. AMD's Ryzen processors draw significantly more power than the Intel competition because their idle clock rate is a bit too high.
The new Intel CPU’s average power consumption in applications that combine 2D and 3D loads (like AutoCAD) is in line with the performance we observed.
The processor ends up in almost the same place during our gaming loop.
The finishing order changes dramatically once we fire up an AVX stress test with all cores running at their top Turbo Boost bins. During rendering, we were seeing the -8700K's stock power consumption at 110W, climbing to 133W under a 5 GHz overclock.
AVX without offset pushes the result as high as 170W. The Core i7-8700K at 4.9 GHz even throttles due to its package temperature. And that's in spite of our compressor cooler's efforts! Thermal paste under the IHS does us no favors.
Here’s the good news: unless you render or run Prime95 for hours on end, a good air cooler can theoretically handle 4.8 GHz in a well-ventilated case. Intel’s thermal interface material isn't desirable, but it shouldn't stop you from achieving a decent overclock.
The above graph shows that a closed-loop liquid cooler is able to keep an overclocked Core i7-8700K from throttling after 20 minutes of warming up. A good heat sink and fan combination should perform almost as well, again, given ample airflow.
Under our stress test, the overclocked processor gets uncomfortably hot, even under our compressor cooler.
While we're only measuring an average of 170W, thermal throttling keeps the 180W+ peaks from becoming our average power consumption result. At that point, even the most powerful coolers have to throw in the towel.
To be sure, it's surprising just how much power such a tiny processor can consume once it’s pushed to its limits. Nevertheless, Intel’s Core i7-8700K is relatively easy to cool, even on air. You'll just want to stay away from taxing rendering sessions and AVX-optimized workloads. At that point, you're best off with an all-in-one closed-loop liquid cooler.
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