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Intel's Core i7-8086K CPU reviewed


Lucky number five

At Computex this past week, five was the magic number. Usually, it was describing the number of gigahertz certain Intel processors were clocked at during the company's keynote. One of those demos has touched off a firestorm about just how valid it is to demonstrate a 28-core CPU with an extreme overclock and extreme cooling, but the other 5-GHz chip Intel showed is a product you can actually buy today. In exchange for $425 at Amazon, Newegg, and Best Buy, the Core i7-8086K is meant to celebrate 40 years of the x86 instruction set architecture. It's also making a little history of its own as Intel's first processor to reach 5 GHz Turbo Boost speeds.

Intel didn't send review samples of this chip, but I wanted to see just what that 5 GHz number meant in practice for a stock-clocked CPU. I ordered one off Amazon with my own hard-earned cash to put it to the test. Ever since that shipment arrived Saturday, we've been exploring the behavior and performance of the i7-8086K.

  One core
active
Two cores
active
Three cores
active
Four cores
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Five cores
active
Six cores
active
Core i7-8086K 5 GHz 4.6 GHz 4.5 GHz 4.4 GHz 4.4 GHz 4.3 GHz
Core i7-8700K 4.7 GHz 4.6 GHz 4.5 GHz 4.4 GHz 4.4 GHz 4.3 GHz

So how did Intel get to that 5 GHz figure? Simple: Turbo Boost 2.0 relies in part on an "n-cores-active" heuristic. Put simply, fewer active cores mean the highest clocks from Turbo Boost's multi-variate frequency-scaling management. Each chip has a number of Turbo bins equal to the number of cores on the die.

With one core active, the Coffee Lake Core i7-8700K that the i7-8086K is derived from can boost all the way up to 4.7 GHz. The Core i7-8086K's top Turbo bin is 5 GHz. Once you get outside those numbers, the i7-8700K and i7-8086K are identical. Scuttlebutt had suggested the i7-8086K might have a more aggressive Turbo table across the board, but that's not the case. That fact makes sense, of course, given that the chip's TDP remained the same as its less-special sibling's.

We'll exhaustively tease out what this change in Turbo tables means for stock-clocked performance in a moment, but outside of web browsing, common desktop tasks, and the like, that 5-GHz figure probably won't make itself felt. It's nice, for sure, but I'll stop you right here: it's no reason to go spend $75 more on one of these over a Core i7-8700K if you want a Coffee Lake part. To really get the most out of this chip, you need to take full advantage of its unlocked multiplier.

Our initial overclocking efforts with the Core i7-8700K yielded a 5-GHz all-core Turbo speed with an AVX offset of -2 for SIMD-heavy workloads like Blender and Handbrake. That led to some jaw-dropping performance in CPU-bound games and single-threaded benchmarks, even if AMD's Ryzen 7 1700 could often catch up in productivity workloads when pushed to 4 GHz. The mitigations for the Spectre and Meltdown vulnerabilities have dropped some ice in the coffee since, but the overclocking headroom of Coffee Lake cannot be denied. It's the one thing AMD's Ryzen CPUs can't even hope to match.

I dived into some tentative overclocking efforts during our live unboxing and benchmarking of the i7-8086K this past weekend, and I've been refining our stable overclock since. It must be noted that every chip will overclock differently thanks to the vagaries of silicon lithography, but our sample was able to reach 5.1 GHz on all of its cores at a slightly-higher-than-conventionally-accepted-as-safe 1.38 V. Better yet, the chip can sustain those clocks with both AVX and non-AVX workloads. We have two Core i7-8700Ks in the TR labs, and neither one can stably exceed 5 GHz with the aforementioned -2 AVX offset.

I may still delid and re-paste our chip to explore its very limits, as regular workloads with those clocks and voltages are causing it to crest 90° C package temperatures under a Corsair H110i 280-mm liquid CPU cooler. I suspect I might be able to eke 5.2 GHz non-AVX clocks out of my chip with lower temperatures, as well. Perhaps we'll do that live, too. For now, let's see how the i7-8086K performs.