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Truth be told, overclocking most processors isn't exactly difficult to do these days. An unlocked K-series chip does confer certain advantages, though. You can raise the multiplier to increase the core frequency while keeping the base clock steady, which is handy since an overclocked base clock can cause other system components to run at funky speeds. The K series gives the user total control over Intel's Turbo Boost feature, too, so you can balance the CPU's peak clock speeds against power consumption and different workload types. Also, you get unlocked memory clock multipliers with the K series, so higher memory frequencies are possible without altering the base clock.

A good motherboard will shepherd you around many of these difficulties, by locking down the PCIe clock to the correct speed, for instance. An unlocked CPU just makes everything a little easier and, with the right BIOS, gives you near-total freedom.

For our K-series overclocking attempts, we chose the Asus P7H57D-V EVO motherboard and a Thermaltake Frio CPU cooler; the combo is pictured above. We chose the Frio because its five heatpipes, large surface area, and dual fans will dissipate a tremendous amount of heat. We chose the EVO board because Asus was able to provide us with a special BIOS that supports fine-grained tweaking on K-series processors. Have a look at the menu below for a sense of the control it offers.

The base multiplier, or "CPU ratio setting" goes as high as 63, ridiculously enough, so there's plenty of leeway for setting clock speeds there. The 875K's default ratio of 22 multiplied by the 133.33MHz base clock yields a 2.93GHz core speed. The BIOS also allows us to set the additional multiplier increments added by Turbo Boost, depending on how many CPU cores are busy—those go as high as 15 beyond the base multiplier.

Gigabyte's recent BIOSes do support the K-series chips' unlocked multipliers, but they don't give you control over individual Turbo multipliers.

The version of the Asus BIOS we used for the bulk of our testing exposed memory clocks up to 1600MHz for the 875K and up to 1333MHz for the 655K. A later revision added 1600, 1866, and 2133MHz memory speeds with the 655K, though we weren't able to get the system stable at memory frequencies beyond 1600MHz in our few attempts with a couple of different fancy DIMM sets, even with loose memory timings and memory voltages over 1.65V.

We decided to overclock the two K-series processors by turning up the Turbo Boost multipliers in the BIOS, mainly because we could. We initially set all four Turbo multiplier offsets the same, however, since we had good cooling and didn't see the need to limit performance. Although the CPU's use of Turbo Boost frequencies is typically capped by certain thermal limits, the EVO's BIOS appears to have removed them. During our stability tests with Prime95, our fully loaded CPUs didn't appear to waver from their Turbo Core peak multipliers, even at high voltages and frequencies, when temperatures crept up above 70° C.

In fairly short order, as these things go, we got the 875K up to 4.13GHz by raising the Turbo offset for all cores to +9 and the core voltage to 1.275V. CPU temperatures hovered around 64° C at those settings, well within hand.

Bumping the multiplier up another notch just wasn't entirely stable, even at 1.3V and beyond. The system would POST and boot into Windows, but it crashed during our stability tests. To compound the problem, at those settings, CPU temperatures climbed into the mid 70s and were on the rise. We were at the limits of both the chip and our cooling. Had we only been at the limits of our cooling, we could have used individual Turbo multipliers to get a little more frequency out of one or two cores. However, even just one core set to a +10 offset produced a system crash.

4.13GHz ain't too shabby, though, all things considered. Your mileage may vary, but I'd say these results are fairly typical for casual Lynnfield overclocking with air cooling.

The 655K upstaged its bigger brother by reaching 4.4GHz without a fuss. That's at a Turbo offset of +9 for both cores, adding to a base multiplier of 24. We had to raise the voltage to 1.38V to get there, but temperatures remained relatively low at around 61°C in our stress tests. An offset of +10 wasn't stable, even at over 1.4V.

We've hit similar speeds with non-unlocked Clarkdales in the past, so this result isn't unexpected. Seeing a CPU's ticker going that fast doesn't fail to impress, though.

I had an awfully good experience with overclocking the Phenom II X6 1090T Black Edition using AMD's Windows-based Overdrive utility. I've not been favorably disposed to Windows-based overclocking tools in the past, but Overdrive exposes all of the X6's knobs and dials exquisitely and is very easy to use. Given that, I figured I should try out Intel's Control Center app, as well.

Unfortunately, this app only works with Intel's own motherboards, and only certain ones. I installed the 875K into the supported DP55KG mobo and attempted to replicate our overclocking success from the Asus board.

Control Center will let you set all of the multipliers and raise the CPU voltage, and it even gives the user the ability to adjust Turbo Boost's wattage and amperage limits. However, you'll notice in the picture above that all of the items marked with a blue asterisk require a reboot in order to take effect. Many of the good ones, including CPU voltage, are subject to this limitation. In my view, that makes this application nearly useless. The whole point of Windows-based overclocking is to avoid the reboots required with BIOS-based tools

Worse yet, I found that voltage settings in the app didn't always persist. If the system crashed and rebooted, it would often come up at a lower voltage than the one I'd just been using. I'd have to select the voltage I wanted and, yes, reboot once again. Ugh.

We had some trouble replicating the results we got on the Asus for another reason: the CPU voltage on the Intel board tended to droop under load, and that often led to a lockup or crash. This happened even after we raised the Turbo wattage and amperage maximums well beyond their defaults. The solution was to crank up the CPU voltage to 1.3875V in Control Center; it would then droop to 1.29V under load, as monitored in Control Center, and the system would remain stable at 4.13GHz.

Obviously, the total package here isn't great. AMD's Overdrive is miles ahead of Intel's utility, and Asus' BIOS is faster and easier to use. If you are using an Intel motherboard and want to overclock a K-series processor, the BIOSes on Intel's enthusiast-class boards have allowed control over Turbo multipliers on unlocked chips for some time now. That capability has been limited to expensive Extreme Edition chips in the past, but no longer. You're probably better off just using the BIOS, provided your board has the option.