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CPU Overclocking

The information we have just given you in the first part of our article suggests the best ways of overclocking Core i7 processors. The main idea behind it is to increase the base BCLK frequency that results into CPU clock frequency increase. However, since BCLK is also connected to a few other system frequencies, such as North Bridge built into the processor, memory and QPI bus, you should better use lower multipliers for Uncore, DDR3 SDRAM and QPI frequencies during overclocking. This way you will be able to better uncover the potential of your processor and will prevent your overclocking experience from stalling because some other system frequencies get overboard. As usual, you can certainly improve your overclocking results even more by raising the major voltages above their nominal settings, but make sure not to get too excited about it, at least without efficient CPU cooling in place.

We decided to find the maximum frequency our Core i7-920 processor will be able to hit without any additional voltage increase. For this experiment we locked the processor core voltage and Uncore voltage at their standard 1.2V in the ASUS P6T Deluxe mainboard BIOS. Note that using Auto setting is not recommended for these voltages, because in this case the mainboard will increase them on its own during overclocking without your knowledge or control.

To avoid any unexpected surprises during overclocking, we disabled EIST and Turbo Boost technologies and locked the CPU multiplier at 20x – the default setting for Core i7-920 with 2.66GHz nominal clock speed. We set 8x multiplier for the memory. Therefore, since Uncore frequency should be at least twice as high as the memory frequency, we used 16x multiplier for it. To get the desired QPI frequency setting, we used the lowest available multiplier of 18x.

With these settings our BCLK frequency reached 175MHz without any stability issues. By the way, we tested our system stability using 64-bit Prime95 25.7 utility in Small FFTs and Blend modes. This program proved to be the best tool for detecting over-overclocking and reported errors when most other popular stability check tools (including OCCT, LinX and IntelBurnTest) showed a pass.

As a result, our processor overclocked to 3.5GHz, which is a pretty good result considering it worked at its default 1.2V Vcore. The maximum core temperature during the stability tests never exceeded 74°C.

Of course, we can improve this result by raising the processor core voltage. However, we have to warn you against overdoing it, because voltage increase leads to higher heat dissipation, which ends up being the primary obstacle to higher overclocking results. Namely, in most cases you shouldn’t push the processor core voltage over 1.35-1.4V if you only use air cooling, because the CPU may get overheated without reaching its maximum frequencies.

However, we still couldn’t push the frequencies that much higher only by setting the CPU Vcore at 1.35V. Even though the system passed CPU stability tests with BCLK at 180MHz, it failed the memory stability tests. We may have reached the maximum overclocking for the North Bridge built into our processor, as it works at its own frequency connected to the BCLK and uses its own voltage setting. By the time the base frequency hit 175MHz, Uncore frequency already reached 2800MHz, which is evidently the best L3 cache can do at its default voltage. Therefore, we increased the Uncore voltage to 1.35V for further experiments. We have also increased CPU PLL voltage to 1.88V just in case.

These measures made our CPU stable at 190MHz BCLK. It overclocked to 3.8GHz and the frequency of its integrated North Bridge reached 3040MHz.

In this mode system passed all stability tests without problems, but further frequency increase resulted into Prime95 failure, and even additional processor voltages increase didn’t help. Looks like 3.8GHz frequency is the maximum our test Core i7-920 processor can do, even though the temperature of the hottest CPU core during our stability tests reached only 86°C, while the critical temperature limit is set at 100°C.

In fact, frequencies around 3.8GHz are the most widely spread maximum for Core i7-920 CPUs overclocked with air coolers. This is the conclusion we can make judging by our test results and basing on the feedback from the first users who purchased these processors. By the way, the engineering sample of the Core i7-920 CPU that we also have in our lab demonstrated the same overclockability. Even with the Vcore increased to 1.4V it overclocked only to 3.8GHz.

In this case we are talking specifically about the cores potential and not about the integrated North Bridge. To confirm if this was true we lowered the processor clock multiplier to 19x and got its running stably with the same voltage settings but higher 200MHz BCLK frequency.

Let’s sum things up now. We managed to overclock our Core i7-920 processor with the nominal 2.66GHz frequency to 3.8GHz. In other words, we got more than 40% clock frequency gain by using air cooling and potentially safe voltage settings for our processor. The recommended settings that were checked out on two different processors are given on the screenshot below (with an ASUS P6T Deluxe mainboard):

Note that among other things we owe our today’s success to high-quality memory that can work at 1600MHz frequency with 1.65V voltage setting. However, our memory could work at even higher speeds, up to 2000MHz. Unfortunately, our attempts to achieve higher speeds failed. When raising the memory frequency, we also had to increase the frequency of the North Bridge built into the processor. But to our great disappointment, it refused to work at 4GHz even after a serious increase of the Uncore voltage.

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