Power Consumption & Overclocking
Intel isn't using a soldered integrated heat spreader (IHS) for Skylake-X and Kaby Lake-X. Instead of the metallic solder most enthusiasts want to see, heat moves from the die to the IHS through inexpensive thermal interface material, which is just a fancy name for common thermal paste. This decision has implications for both our power consumption measurements and overclocking efforts. Fortunately, in light of the lower amount of waste heat produced by Core i7-7740X compared to Core i9-7900X, this isn't as much of a problem.
In order to produce comparable results, we used our industrial-grade Alphacool Eiszeit Chiller 2000 cooler that was needed for the Skylake-X review. A decent all-in-one liquid cooler would have been fine, though.
Power consumption is measured after the voltage converters and CPU, using points on the motherboard. These numbers need to be part of the equation for sizing your PSU. And depending on the motherboard you pick, they can differ by up to 30W. Consequently, our numbers are for the CPU only.
At idle, Intel’s new Kaby Lake-X model does a lot better than -7700K. It also pulls ahead of AMD's Ryzen processors. Most likely, a lot of this is due to the disabled HD Graphics engine.
AutoCAD 2015 doesn’t stress CPUs all that much, since it can't fully utilize their available resources. Still, Intel’s Core i7-7740X consumes more power than its predecessors. Admittedly, the difference isn't huge. However, it is larger than our measurement error range and completely reproducible.
The situation is similar in our gaming workload. Interestingly, the average frame rates are almost identical as well.
The stress test produces somewhat surprising results. Intel’s Kaby Lake-X model edges out the -7700K. We don't know why the pattern reverses under full load. However, the delta is almost imperceptible in the 2W range.
Overclocking & Stability
Does a larger heat spreader and disabled graphics engine yield a meaningful advantage to Core i7-7740X's overclocking headroom? Before we start tweaking, we’d like to provide some background by showing a graph based on a motherboard manufacturer’s binning program. The voltages needed for Core i7-7740X are very informative:
The Core i5-7640X we'll be reviewing soon made it all of the way to 5.1 GHz, putting it in the middle of the pack. In other words, we have a thoroughly average sample.
We tested for stability at every frequency step using Cinebench R15, The Witcher 3, LuxRender, and Battlefield 1. Notice the deliberate exclusion of the AVX-heavy Prime95.
The Core i7-7740X's curve stays fairly straight, which means that power consumption doesn’t explode as clock rate goes up. Rather, the CPU chugs along until it just quits all of a sudden.
A closer look at the performance and power consumption curve shows that performance scales in an almost completely linear fashion with frequency, whereas power consumption increases faster, but not extremely so. Relating single-core performance to the necessary power yields a very similar picture. The resulting waste heat isn’t as bad as some might have feared. At 5.1 GHz, we’re looking at 90W. A good closed-loop liquid cooler should be able to handle this.
In spite of our sample being average, it yielded a good overclocking experience, especially since performance gains and power consumption increases never drifted too far apart.
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