Overclocking Skylake: The Power, Performance, and Peculiarities of Intel's Core i7-6700K

Intel’s long-awaited Skylake processors are finally arriving in the channel. Details about the new architecture are distributed a bit unevenly, with reviews going up well before information on the actual architecture itself did. You can play with the Intel Core i5-6600K and Core i7-6700K now, but what are you getting into? As it turns out, Skylake is a little bit different from its predecessors.

What Changed

One of the major differences between Skylake and the preceding Haswell/Broadwell generation is the removal of the Fully Integrated Voltage Regulator. Voltage regulation is off-die and back on the motherboard again, giving motherboard vendors another way to differentiate products. The upshot of this is that it should reduce CPU temperatures.

A minor change is a full range of BClk adjustment. The BClk has been decoupled from the peripherals and PCI Express bus, allowing you to fine tune your overclock. It’s not strictly essential – the multiplier has been getting the job done for years now – but it’s useful for extracting a higher overclock from your CPU and DRAM.

Another major difference we’re finding is high VID. With the move to a smaller, 14nm manufacturing process, we expected lower voltages and tighter ranges, but that’s not the case. Our i7-6700K at stock settings alternates between 1.296V and 1.312V, voltages that would push last generation’s processors close to their limits on air cooling.

The Haswell-E-based i7-5960X wound up being a solid overclocker because it tended to start at a low VID – around 1.1V – but the architecture could handle going up pretty high if you were willing to pay for the wattage. With the i7-6700K, we’re left without a whole lot of headroom.

Finally, while the i7-6700K at least appears to run fairly cool (or at least our sample does), per core and package CPU temperatures don’t appear to be as accurate this generation, with our reported per core temperatures idling a few degrees below ambient.

That said, a good cooler will apparently work wonders. Even if you assume the reading to be a beefy 10C lower than what it actually is, the i7-6700K still runs at a reasonably safe temperature.

Test Platform

• CPU: Intel Core i7-6700K (4GHz, turbo to 4.2GHz, 14nm, 8MB L3 Cache)
• Motherboard: ASUS Z170 Deluxe (BIOS 0504)
• Graphics Card: NVIDIA GeForce GTX 980 (Reference)
• DRAM: 4x4GB Corsair Vengeance LPX DDR4-3600 CAS 18 1.35V
• CPU Cooler: Corsair Hydro Series H110i GT
• Storage: 480GB Corsair Force GT SSD
• PSU: Corsair AX1200i 1200W 80 Plus Platinum Digital PSU
• Chassis: Corsair Carbide Air 540

All testing was done in a thermally-controlled laboratory with a steady ambient temperature of ~19C.

Power Consumption

We used OCCT 4.4.1 to stability test our overclocks, but note that the temperature monitoring in OCCT hasn’t kept up with Skylake yet. As a result, our power consumption and thermal monitoring was logged through Corsair Link.

With all that said, despite a bump to 1.375V on the CPU core, power consumption in OCCT only increased about 30W. Our fully loaded system drew 169W under OCCT load.

Despite fairly incremental jumps in voltage to hit 4.6GHz, even 1.45V wasn’t enough to get 4.7GHz stable, and I wasn’t willing to risk the chip to go past that even though it seemed to have the thermal headroom. Incidentally, these results map fairly well against the good Dr. Ian Cutress’s experience over at AnandTech. When looking at his results, though, it seems like Intel may be setting unnecessarily high VIDs on the i7-6700K, as he was able to solidly undervolt his chips and still overclock, while my own overclocks didn’t require much massaging of the VCore until ~4.5GHz.

Performance

In a weird way, overclocking the i7-6700K is a lot like polishing a cannonball. Intel has made great strides in making Skylake an overclocker-friendly architecture, but performance was already high with Devil’s Canyon and Skylake just drives it higher.

We can still get some good performance scaling going from stock to 4.6GHz, but that’s only 500MHz off of stock Skylake’s quad-core turbo speed, or about 12.2% faster. Interestingly, bordering on inexplicably, Adobe’s suite gets some good love out of hitting higher speeds, and we’ve found that it does get some extra juice out of high speed memory.

Conclusion

Intel’s new Core i7-6700K is screaming fast, but weirdly underwhelmingly so. Unlike the past couple of launches (Broadwell, Haswell, Ivy Bridge), we’re able to hit the high clocks we absurdly crave and see top end performance increase generationally, but the crazy high clocks we want – 4.8GHz and better – still seem to be out of reach.

On the bright side, it’s still fairly efficient, and our high end coolers are absolutely up to the task of taming this beast. Users on Devil’s Canyon systems are probably perfectly fine unless they want the enhanced connectivity of the Z170 chipset, while users with Haswell-E systems are going to be perfectly content.

The interesting part of Skylake is how much of a jump from Devil’s Canyon it’s not. In most benchmarks, the i7-6700K is only slightly faster than the i7-4790K. Because it’s not a major architectural leap in performance, and because the Z170 chipset doesn’t offer appreciably more important features than X99, Haswell-E continues to be a viable enthusiast alternative. Skylake and its faster memory subsystem show the most benefit in content creation tasks, where Haswell-E’s quad-channel memory bus and extra cores will still go much further.