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Overclocking
I'm going to start by talking about my overclocking attempt for the Core i5-661, since I didn't have time to include overclocking results in our initial review of that product. The i5-661 was the first of this bunch I attempted to overclock, followed by the i3-530 and the Athlon II X4 635.

As usual, our overclocking efforts involved some quick-and-dirty assessments of stability and a pretty vanilla suite of tweaks—changing the frequency by modifying the base clock speed, raising the core CPU voltage and some related values, lowering the memory multiplier to keep DIMM speeds sane, and keeping an eye on cooling. We conducted some quick stability tests with each new clock rate using Prime95, and we then benchmarked the best stable speed for each processor.

Using those simple methods, I was able to coax the Core i5-661 up to a frequency that makes my head spin: 4.5GHz at a very healthy 1.4V. That's nearly a 50% overclock on the Core i5-661's 3.3GHz stock speed (although the i5-661 can step up to 3.6GHz via Turbo Boost.)

Getting to this speed using our Asus P7H57D-V EVO motherboard was alarmingly easy. I just turned up the base clock from 133MHz, dropped the memory multiplier, and everything seemed to work right. Then I stepped up through various clock speeds waiting for the i5-661 to show signs of a problem. At 4.15GHz, I noticed the CPU was generating heat faster than Intel's puny stock cooler for the Clarkdale processors could take it away. The i5-661 was stable during our stress test, but CPU temps climbed to 74° C before I pulled the plug. Next, I swapped in a Thermalright tower cooler, which kept temperatures comfortably in the 50s, and kept on pushing.

The reason the i5-661 overheated was the same reason overclocking seemed so strangely effortless: turns out the Asus mobo automatically adjusts several key voltage values, including the CPU voltage, when you're overclocking. By the time the i5-661 hit 4.5GHz, the core was at 1.4V, well up from the 1.16V stock value. Having played with it some, I think the Asus BIOS is pretty intelligently tuned, and I'm convinced you'll want a better cooler than the stock Intel one if you want to extract the most from a Clarkdale.

My success with the i5-661 made me excited to see what the i3-530 could do—and it did not disappoint.

The i3-530 was stable at 4.4GHz with a 200MHz base clock, again a nearly 50% overclock. For those of you who don't know this fact, overclocking itself is magic, and when you reach a mark like 50%, your body releases endorphins into your brain, triggering a feeling of happiness and well-being. I was pleased to experience this sensation, which brought back fond memories of doing this a decade ago at frequencies an order of magnitude lower, when I overclocked a Celeron 300A to 450MHz.

The Asus board's auto-voltage feature had taken the i3-530 up to 1.4V at 4.4GHz, but I was curious to see whether that much juice was really necessary. I started down at 1.2375V and tried to get the i3-530 to POST and boot at 4.4GHz, but I wound up stepping clear up to 1.3875V before the system was stable again. Obviously, Asus has done its homework on Clarkdale voltages, though you may want to tweak things yourself to keep power consumption in check.

The Athlon II X4 635 wasn't quite as willing to rev, but it did make it up to 3.48GHz at 1.45V (stock is 2.9GHz at 1.4V). We were using a much beefier cooler on the X4 635, from a Phenom II X4 955, and it kept temperatures reasonable without us having to dig out a massive tower.

Here's how the overclocked processors performed in a couple of benchmarks.

The Clarkdales are screamers at those clock speeds, matching up well against the fastest CPUs around. Heck, the overclocked i5-661 nearly ties the quad-core Core i5-750 and Phenom II X4 965 in Cinebench's multithreaded test. Good grief.

Another question we wanted to investigate is what overclocking does to the power consumption of these processors. One reason today's CPUs have so much clock speed headroom in them is that frequencies are usually limited primarily by power and thermal envelopes. Does overclocking a Clarkdale to ridiculous heights make any sense, or will you be left with a room-heating monster?

To find out, I stuck the overclocked systems on our power meter and ran our Cinebench power test. I was using the Asus H57 board at this point, remember, which draws a little more power than the Gigabyte P55 board we used for our main power consumption tests, so I've included and marked the H57 results in the graphs.

The Athlon II and its motherboard seem to do a better job of dropping voltage levels at idle while overclocked than the Core i3-530 on the Asus H57. The Asus simply holds the voltage steady at 1.3875V, regardless. That explains the high idle power draw for the overclocked Core i3-530.

At any rate, the overclocked systems are entirely reasonable, despite some big gains in power consumption. The overclocked Core i3-530 system's peak power draw is a couple of watts lower than our Core i7-870 test system's, and the overclocked Athlon II X4 635 is a few watts below our Phenom II X4 965. If you pay extra to Intel and AMD to get a faster CPU, you'll still be in the same thermal territory as our overclocked specimens. You will need a decent air cooler in order to overclock them, but nothing too elaborate.