A given processor's maximum stable frequency is mostly determined by the limitations of the particular chip on hand—also known as the silicon lottery—and the CPU cooler one straps on top. Still, you want a motherboard that is going to help you along on the path to peak clock speeds, not one that will hinder you.
We put the Z170A SLI Plus to the test with a Core i7-6700K CPU and Cooler Master's Nepton 240M. The Nepton has a 240-mm radiator, and before it was superseded by the company's MasterLiquid Pro 240, it had a $110 asking price. This probably puts it toward the high end of the range of coolers one might see in a system built around the SLI Plus. That said, it should do a good job of keeping our four Skylake cores from getting too toasty.
Our tour of the SLI Plus' overclocking features starts with the OC Genie option in the firmware.
After enabling MSI's OC Genie and restarting the system, we were greeted with a warning screen. This screen let us know that while OC Genie was enabled, we shouldn't make any modifications to settings under the BIOS OC Menu. It also warned against updating the BIOS or clearing the CMOS. So noted.
Upon booting into Windows, we were greeted with a 4.4GHz clock speed, as promised by the firmware's help dialog box. All Turbo multipliers were set to 44x with the base clock at its default 100MHz. To support these speeds, the CPU was being fed 1.312V. This configuration proved to be perfectly stable. Our Prime95 stress test ran with flying colors and no signs of thermal throttling. CPU temperatures peaked at 78°C.
Next, we tried enabling OC Genie from within Windows using the Command Center. Enabling it under Windows required a reboot, and the firmware displayed the same warning message as before. Unsurprisingly, enabling OC Genie this way resulted in exactly the same configuration as enabling it in the firmware.
Having exhausted the board's limited auto-overclocking options, it was time to turn off the autopilot and see what we could do with manual tuning in the firmware. We started out, as we usually do, by tweaking the multiplier alone. We left all of the voltages at their "auto" defaults.
Beginning with a 42x multiplier, we started by testing stability with Prime95. Increasing just the multiplier, we made it all the way to a stable 4.6GHz. With this clock speed, the firmware's "auto" voltage setting was supplying 1.384V to the CPU. Under these conditions our Nepton was capably keeping temperatures under control, with a peak of 90°C while running Prime95.
Attempting a 47x multiplier led to thermal throttling almost immediately. The firmware fed our CPU with 1.472V at these settings. That approach seemed a little heavy-handed, so we switched over to manual voltage control in our quest for higher clock speeds.
Starting at 1.39V and a 47x multiplier, we attempted a Prime95 run. One of our worker threads instantly failed with an error. Raising the voltage in 0.01V increments, we were able to run the chip at 1.44V without any Prime95 errors or signs of thermal throttling. Even so, all the extra voltage was causing our Nepton cooler to strain—even with its fans spinning at full speed, CPU temperatures maxed out at 94°C.
Pushing clock speeds higher proved unsuccessful. We ran into Prime95 errors or thermal throttling. Still, 4.7GHz matches the highest overclock we've gotten from any other Z170 board with the same i7-6700K and Nepton 240M combo. This is a great result, even if it did take more voltage than I would be comfortable with for long-term use.
Z170-based boards feature a revised reference clock architecture that decouples the PCIe and DMI bus speeds from the base clock. This setup allows tweakers to increase the base clock without having to worry about running other system devices out of spec. While it's much easier to overclock using multipliers alone, we ran a quick test to see how the SLI Plus fared when overclocking with base clock tuning. We first tried 200MHz in the firmware, leaving everything else on "auto." The system booted perfectly, and our CPU was stable at 4.2GHz:
We then got a little greedier and tried a 250MHz base clock. With all other settings on "auto," the firmware selected an 8x multiplier for a final clock speed of 2GHz. Manually setting this to 16x brought us to a final stable clock speed of 4GHz:
Throwing caution to the wind, we dialed in a 300MHz base clock, at which point the motherboard failed to POST. Happily, the firmware detected that failure and gave us the option of loading a default configuration or entering the firmware to retract our hubris. Still, a 250MHz base clock maximum is an impressive result.
Overclocking our Skylake CPU was a very smooth process on the Z170A SLI Plus. Tweaking clock speeds and voltages was a breeze in MSI's firmware. Given the limited auto-overclocking functionality, the board is more suited to seasoned tweakers who know their way around the firmware than for newbies just starting out. That said, the pre-baked CPU overclocking profile provided by OC Genie was perfectly stable, and could prove to be a good starting point for manual tuning.
Now that we've found peak stable clock speeds, it's time to turn our attention towards the SLI Plus' performance.