Intel Core i7-8700K And i5-8400 - Overclocking And Power Consumption
We also spent some time overclocking the Core i7-8700K using the Gigabyte Z370 Gaming Ultra motherboard, to see what kind of additional horsepower the processor had lurking under its hood. Note, we didn't bother trying to overclock the Core i5-8400 because it is locked. There's some tweaking that could be done via BCLK adjustments, but if you're going to overclock Coffee Lake, a K SKU is essentially a must.
With that said, overclocking a Core i7-8700K processor is very similar to any other previous-gen Intel K-SKU processors, but like Skylake and Kaby Lake it has one additional wrinkle. Because the processor is unlocked, overclocking it is simply a matter of altering a few multipliers, tweaking voltages, and dealing with the additional heat and power requirements. These processors can also be overclocked by adjusting BCLK frequencies though, so it's possible to really fine tune the end result.
We took a somewhat conservative approach to overclocking the Core i7-8700K to give you all an approximate “worst case scenario”. Our results should be repeatable, assuming you’ve got similar hardware and everything is working properly. You shouldn’t need exotic cooling to pull off what we did here -- we used am off-the-shelf Corsair H80 AIO liquid cooler with the motherboard and processor installed in a mainstream NZXT mid-tower.
To see what our sample could do, we manually cranked the core voltage up to 1.35v and shot for a modest 4.9GHz (49x100MHz), which worked without incident. We then moved up to 5GHz, but had some instability under load, so we bumped the voltage to 1.375v and things stabilized. Unfortunately, even with more voltage, 5.1GHz (or higher) would immediately blue screen when we tried to invoke a test. After coming back down to 5GHz, things stabilized, but we were flirting with the upper temperature limits with our setup. At 5GHz, the CPU was perfectly stable and operated in the upper 80-degree range when under load. Idle temperatures were relatively cool and stayed in the upper 30's.
Overclocking the Core i7-8700K to 5GHz is only a 6% increase over the default max turbo speed, so it resulted in only minor performance gains in the Cinebench multi-threaded test, as you can see. The performance boost in the 3DMark Physics test was minor too. The increases should have been a couple of percentage points higher, but these results were repeatable and we weren't throttling, so we're reporting them as is. We suspect there's some fine tuning to be done on the BIOS level that may improve the situation, because we're aware of a couple of colleagues that saw measurably better results than our sample provided.
Throughout all of our benchmarking and testing, we also monitored how much power our Coffee Lake-based test system was consuming with a power meter, versus the other systems we used for benchmark comparisons.
Our goal was to give you an idea as to how much power each configuration used while idling at the Windows desktop and while under a heavy CPU workload. Keep in mind, this is total system power consumption being measured at the outlet and not the the individual power of the CPUs alone.
Power consumption is a definite strong point for the Core i5-8400. With that CPU in our test bed, it consumed the least amount of power of the bunch, despite offering performance on par with some of the higher-end processors in many tests. We included power numbers with and without a discrete GPU for the i5-8400 to show just how low power consumption can be with a mainstream processor like that. The power characteristics of the Core i7-8700K were also good, but were significantly higher under load. With support for 12-threads, more cache, and much higher clocks, it's no surprise the Core i7-8700K's peak power is higher than the 8400, but the test system still landed under the 200 watt mark.
We also monitored power while overclocked and saw an approximate 20 watt increase under-load, due to the higher clocks and voltages used to hit 5GHz. The jump in power while overclocked is relatively small though, which shows that Intel is already pushing the limits somewhat and being a bit less conservative with the Core i7-8700K's stock power profile and frequencies.