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Overclocking Experiments

Although the new Core i7-875K and Core i5-655K processors with unlocked clock multipliers do not promise any overclocking break-through, it is still very interesting to check out their frequency potential. To test the new CPUs in real life we put together the following testbed:

  • ASUS P7P55D Premium mainboard (LGA1156, Intel P55 Express);
  • 2 x 2 GB, DDR3-1600 SDRAM, 9-9-9-24 (Kingston KHX1600C8D3K2/4GX);
  • ATI Radeon HD 5870 graphics card;
  • Western Digital VelociRaptor WD3000HLFS HDD;
  • Thermalright Ultra-120 eXtreme CPU cooler with Enermax Everest fan;
  • Tagan TG880-U33II PSU (880 W).

The primary goal of our today’s test session will be to determine the maximum frequency that can be achieved during overclocking of Core i7-875K and Core i5-655K processors by changing their frequency multiplier.

Core i7-875K

When we installed this processor into our testbed, we immediately noticed a few changed that occurred in the mainboard BIOS.

The “CPU Ratio Setting” parameter responsible for the clock frequency multiplier now allowed setting any value from 9x to 63x, but this was an anticipated consequence. It was much more interesting to see an additional “TurboMode x-Core Ratio Offset” parameter that offered complete control over Turbo Boost technology.

These settings allow managing the processor frequency intervals within Turbo Boost Technology. In other words, if you have a CPU with an unlocked clock frequency multiplier, you can manually set the scale with which its clock frequency will increase in Turbo mode when 1, 2, 3 or 4 cores are active.

Unfortunately, this was the last pleasant surprise. Core i7-875K doesn’t offer any additional multipliers for the DDR3 SDRAM frequency, or doesn’t allow changing the Uncore frequency of the processors. It means that the Uncore frequency is strictly connected with the base clock (BCLK) and equals 2.4 GHz at the nominal BCLK of 133 MHz. As for the memory frequency, you can only choose from 800, 1066, 1333 and 1600 MHz if you use default BCLK.

Now let’s go directly to overclocking. Core i7-875K offers total access to the clock multiplier. If you increase the multiplier, it only affects the work of the processor computational cores, all other sub-systems remain untouched. So, overclocking algorithms is extremely simple: it doesn’t require any changes in the memory frequency or any increase in the frequency of the processor Uncore part. Just raise the clock multiplier and increase the processor core voltage accordingly.

When the CPU Vcore increases to 1.35 V that is considered a pretty safe level when only air-cooling is used, we can get our CPU to run stably at 4.0 GHz.

This is a normal, but not outstanding result for a CPU on Lynnfield core. However, we didn’t expect anything else, because Core i7-875K is just another representative of a well-familiar processor family. So, there is only one remarkable thing about the obtained result: we didn’t have to increase the base clock (BCLK), which means there was no additional impact on the mainboard.

Core i5-655K

Just like Lynnfield, the unlocked dual-core Clarkdale also offers complete access not only to the “base” clock frequency multiplier, but also to Turbo Boost technology allowing the processor to pick any multiplier depending on the utilization of each CPU core. In other words, in this respect it has the same functionality as Core i7-875K. However, unlike the quad-core CPU, Core i5-655K also offers extended configuration options for the memory frequency.

When regular non-overclocker Clarkdale processors use nominal base clock (BCLK) of 133 MHz, they allow clocking the memory as DDR3-800, DDR3-1066 or DDR3-1333. Lynnfield processors, and Core i7-875K in particular, also add DDR3-1600 to this list. The multiplier used for the memory frequency in Core i5-655K is fully unlocked, so the memory controller in this processor can also clock the memory as DDR3-1866 or DDR3-2133 without increasing the BCLK any higher.

As for the actual overclocking, when we increase the CPU Vcore to 1.35 V, Core i5-655K processor can work with a 33x multiplier, i.e. at 4.4 GHz frequency. In his case the system remained fully stable, which we confirmed by running a complete LinX 0.6.3 stability test.

And one more time we see quite common overclocking result, although we did use a special overclocker CPU in our experiment. This proves once again that Intel doesn’t hand pick semiconductor dies in any specific way for their new unlocked processors. The frequency potential of Core i7-875K and Core i5-655K is perfectly comparable with that of other Lynnfield and Clarkdale CPUs. So, they can’t boast any other obvious advantages besides the unlocked multipliers.

Therefore, the use of new Core i7-875K and Core i5-655K processors in overclocker platforms can only be justified when overclocking by raising the multiplier doesn’t allow to fully uncover the frequency potential of the processor. And that may only occur in two cases. First if you have a “bad” mainboard that doesn’t have the necessary options for adjustment of BCLK frequency, and memory and Uncore voltages. And second if you perform extreme CPU overclocking, i.e. when the CPU frequency increases by more than 50%, which requires raising the BCLK clock far beyond 200 MHz barrier. In this case stability issues caused by the mainboard are inevitable.

 
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