CPU Overclocking Peculiarities
First of all, let’s see which performance boosting methods the ASUS Z87-K offers to its user. Like other ASUS products, it has the ASUS MultiCore Enhancement which sets the CPU frequency multiplier at its maximum value at any load, although normally this value is set by Intel Turbo Boost for single-threaded loads only. Although this option is Enabled by default, the mainboard doesn’t change the CPU’s frequency multiplier at default settings. In fact, the MultiCore Enhancement feature only works when the Ai Overclock Tuner option is set at Manual or X.M.P. instead of Auto, so this may be a bit confusing. It would be far more logical and intuitive if the ASUS MultiCore Enhancement were turned on and off when you select Enabled and Disabled, respectively.
The OC Tuner option can be used to reach better results. This automatic overclocking feature can now be customized by the user. If you select Ratio Only, it overclocks by increasing the CPU frequency multiplier. The BCLK First option also increases the base clock rate. ASUS mainboards could raise the latter for automatic overclocking before, but only to 103 MHz since higher values might provoke instability. The new BIOS can increase that clock rate to 125, 166 and 250 MHz.
In the Ratio Only mode the OC Tuner technology would increase the frequency multiplier of our Intel Core i7-4770K processor to x41, x42 and x43 when four, three or fewer CPU cores were in use, respectively. The power-saving technologies were active at that, reducing the voltage and the frequency multiplier (to x8) at low loads.
However, we were alarmed to see the CPU voltage go up very high at peak CPU loads. The OC Tuner feature set it at 1.253 volts, which was too much.
The following screenshot illustrates the high load created by the LinX utility with AVX support. The CPU-integrated voltage regulator immediately increased the voltage from 1.253 to as much as 1.322 volts. This provoked a sudden growth of temperature and the ensuing throttling dropped the CPU frequency. So this overclocking method is no good if you want to get any performance benefits. It only leads to higher temperature and power consumption, not to higher speed.
In the BCLK First mode the OC Tuner feature increased the base clock rate to 125 MHz and lowered the CPU frequency multiplier to x34. The voltage was fixed at 1.2 volts, so it didn't go up at high loads and down at low loads. The memory frequency didn't change. It has to be increased manually if you want to use OC Tuner.
Overclocking the CPU to 4.25 GHz automatically was good enough but we hoped to get much better results through manual tweaking. We were disappointed, however. In our preliminary tests we only made our CPU stable at a clock rate of 4.3 GHz. And the problem was that the CPU-integrated voltage regulator would increase its voltage above 1.2 volts at high loads, leading to a high temperature and a power draw of over 190 watts. Even if this worked on an open testbed, the system might become unstable in a closed computer case.
Recalling that the base clock rate can now be changed in a wider range than before, we tried that method, too. You don’t have to set it at 125 MHz sharp because the mainboard remains stable even if the base clock rate is a few megahertz off that value. So we reduced the frequency multiplier to x35 and lower to disable Intel Turbo Boost and then tried to set it at x36 for Turbo Boost to work, but nothing worked. If we increased the voltage just a little, it wasn’t enough to ensure stability at low loads. And when we increased it more, we got high temperature and high power consumption at high loads. We might have achieved better results if the adaptive mode permitted to increase the CPU Core Voltage Offset to ensure stability at low loads and also reduce the Additional Turbo Mode CPU Core Voltage to make up for the voltage increase on the CPU-integrated regulator. Unfortunately, ASUS mainboards can only increase or decrease voltages, not both simultaneously. The Offset Mode Sign option sets the minus or plus sign for both parameters: you can’t increase one and decrease another.
Thus, our Core i7-4770K processor can only be overclocked to 4.2 or 4.3 GHz and not more. Although all power-saving technologies are active, such overclocking is not energy efficient because the CPU-integrated regulator increases voltage too much at high loads, leading to high power consumption. However, you can reach higher frequencies and lower power consumption with Haswell CPUs. Paradoxically enough, you have to give up some of the power-saving features for that. Instead of increasing the CPU voltage in the offset or adaptive mode, you need to fix it at a certain constant level. In this case, the CPU-integrated regulator doesn't act up but maintains the required voltage with the promised precision: the voltage doesn’t drop at low loads, but also doesn’t get unnecessarily high at peak loads.
We quickly found out that overclocking our CPU to 4.3 GHz was quite possible then. The power consumption of our testbed lowered from 190 to 170 watts and the temperature got down to an acceptable 85°C. Inspired by this initial success, we tried to reach 4.5 or even 4.6 GHz but the reality cooled our enthusiasm. To make our CPU stable at 4.4 GHz we had to increase its voltage to 1.180 volts, so the power consumption again rose to 190 watts. The CPU temperature was not higher than 90°, though, so such overclocking should be possible not only on an open testbed but even in a closed computer case. Besides increasing the CPU frequency, we also stepped up the memory frequency and adjusted memory timings.
We wouldn’t bother so much about such a small increase in CPU clock rate if we overclocked it for practical purposes, of course. We'd have stopped at 4.3 or even 4.2 GHz, but in our tests we want to check out the best that the mainboard can do, so we try to reach the best results possible.
We had started our tests with the advanced CPU cooler Noctua NH-D14, but the cooler’s efficiency turned out to have little effect on overclocking Haswell CPUs, just like with Ivy Bridge ones. The problem is that the cooler, even though efficient in itself, cannot get the heat off the CPU effectively due to the small CPU die and the inefficient thermal interface underneath the CPU cap. Even though the CPU temperature gets very high, the cooler itself is not very hot. We replaced the cooler with a Scythe Mugen 3 and got the same 90°C at peak load. The temperature was 91°C to be exact, but we can explain this small difference.
The ASUS Z87-K mainboard, like other ASUS products, cannot regulate the speed of 3-pin CPU fans, so both fans of our Noctua NH-D14 cooler worked at their maximum speed during our overclocking experiments, producing a lot of noise. The 4-pin fan of the Scythe Mugen 3 was rather quiet under the same conditions, not even reaching its full speed, despite the high CPU temperature. The difference is that the additional heatsink on the mainboard’s hot power components was cooled by the Noctua cooler’s fans, so its temperature was up to 60°C. With the Scythe cooler, the heatsink was as hot as 80°C. So, the slightly higher temperature of the CPU at continuous loads with the Scythe Mugen 3 may be due to the influence of the mainboard’s voltage regulator which is located nearby. That’s another factor that makes the test harder for the mainboard, so we decided to use the Scythe Mugen 3, especially as its fan could be regulated automatically, as opposed to the Noctua NH-D14.
So, overclocking the Haswell at constant voltage really works, but the voltage remains the same at low loads when the CPU clock rate is low and the CPU doesn’t really need such high voltage to be stable.
We will see in the next section of our review how this affects the power consumption of the overclocked system.