First of all we decided to see how well the new CPU with D0 core stepping can overclock without any voltage increase past the default value. The Core i7 CPUs with C0 processor stepping that have been in the market so far, have worked stably at up to 3.6GHz frequency in the same conditions. Further overclocking required higher Vcore.
D0 processor stepping proved suitable for work at higher frequencies. Our test Core i7-975 XE processor remained absolutely stable at 3.73GHz with Vcore set at 1.2V, which was the nominal setting for our particular sample.
As you can see from the screenshot, we used the opportunity to increase the clock frequency multiplier over its nominal value, which is available by Extreme Edition processors. However, it doesn’t diminish the importance of the obtained result: it is evident that processors with D0 stepping will be extremely demanded among computer enthusiasts who not only want to achieve maximum performance, but also care about power consumption and heat dissipation levels of their systems.
However, it is too early to wind up our discussion just yet. Let’s see what results we could achieve during Core i7-975 XE overclocking with the help of existing voltage adjustment options. The lower heat dissipation of CPUs with D0 processor stepping should be very helpful for overclocking success. As you should remember, it was increasing heat dissipation that limited the overclocking of the previous-generation Core i7 processors. It called for very advanced cooling solutions to fight that heat. Therefore, 4GHz frequency achieved with air cooling alone has been considered a very good result for the Core i7 CPUs. Our tests showed that the new Nehalem processor stepping allows you to easily pass this threshold.
As we continue working with an unlocked clock frequency multiplier, we could get our system to work stably at 4.13GHz. Vcore was increased to 1.36V. Note that with these settings during the stress-test the processor core temperature didn’t exceed 92°C, which suggests that we can hope for even better overclocking results. Unfortunately, further increase in the clock frequency multiplier caused system instability.
When we overclocked our CPU by changing the BCLK base clock generator frequency, the results again didn't improve significantly. With the multiplier at 21x (supported by any Core i7 CPUs) our processor reached 4.16GHz frequency:
The screenshot shows that the Vcore was set at 1.39V and the resulting frequency of 4.16GHz was obtained as 21 x 198MHz. We selected these particular settings trying to show what the Core i7 processors base don new stepping are capable of without the unlocked clock frequency multiplier. Also note that by raising the BCLK frequency to 198MHz we had to increase the Uncore CPU voltage to 1.255V. During the stress-test of an overclocked processor we saw that the temperatures were approaching critical heights. Maximum temperature during our test session reached 96°C. So, overclocking can only be improved with more advanced cooling solutions.
Although the frequency gain during overclocking was pretty significant, it is still too early to claim that the new D0 processor stepping will be a breakthrough overclocker solution. Yes, CPUs based on the new core version overclock better than their predecessors. And most importantly, they dissipate less heat during overclocking than the CPUs on C0 processor stepping. However, we do not see any overwhelming advantage. Of course, we can’t make any general conclusions after testing only one single CPU sample. However, the preliminary results reveal no significant difference between overclocker potential of Core i7 CPUs based on new and old processor stepping: only around 200MHz.