Overclocking the Core i5-661

While evaluating this processor, I ran all tests both at the processor's stock clock speed of 3.33GHz, and the highest stable overclock I was able to reach, 4.64GHz. Since the BX80616I5661 is not an Intel "Extreme Edition" processor, the only way to overclock it is to raise the base clock (BCLK) from its stock 133MHz. Starting with the base clock, I raised the frequency in increments and used Prime95 to test the stability of the resulting overclock. When Prime95 started reporting errors (or simply crashed), I backed the clock down or raised the processor voltage. SpeedStep and Turbo Boost were disabled for the overclocking attempts.

The iGPU proved to be a problem: Intel states that the iGPU runs at 900MHz, but there seems to be some sort of link between it and the CPU. When I selected "Manual" in the ASUS motherboard's "AI Tweaker" section, a new item labelled "BCLK/iGPU Frequency Sync Mode" appeared, but it's not mentioned in the motherboard manual, its on-screen descriptions in the BIOS are contradictory and truncated (as in "ending in the middle of a sentence"), and its effects are less intuitive than its name suggests. I had my best results leaving it set to "Auto" and adjusting the iGPU frequency separately.

Once I raised the BCLK past 170MHz, visual artifacts and flickering would appear on the screen, and the screen would go black, then re-appear with a Windows message that the graphics driver had "stopped working and recovered." I'd guess that increasing the base clock frequency increases the iGPU frequency as well, but Intel's mute on the subject, so this is just a guess. I suspect the iGPU speed is linked to the BCLK via a multiplier of some sort. Reducing the specified iGPU speed to 833MHz, I was able to reach a maximum BCLK of 185 at a rather high 1.4 volts. These are the settings I used for all the tests in the previous sections. Given the unresolved nature of the BCLK/iGPU link, though, we don't actually know what frequency the iGPU was running at.

iGPU mysteries aside, this is a pretty decent overclock: going from 3.33GHz to 4.64GHz is a solid 40% improvement, and this is reflected in the higher scores in the CPU and memory throughput tests. As the 32nm Westmere core (hopefully) spreads to other members of the Core family, we'll see overclocks like this becoming common on higher-end processors. Although the processor was stable under stress testing at these settings, attempting to stress the iGPU simultaneously in the power consumption test resulted in display corruption and crashes until I dropped the specified iGPU frequency from 833MHz to 800Mhz.

But although this overclock's impressive, it can't fully compensate for the sluggish memory subsystem, as the stock-clocked Core i5-750, even with a much lower clock speed, maintains a solid lead in memory throughput testing and loses only slightly in some of the CPU testing. And bear in mind that this is the i5-750 at its stock clock speeds. Benchmark Reviews tested the Core i5-750 and found that it's a pretty good overclocker in its own right, with a 3.6GHz overclock. Given the scores I recorded with this processor at its stock 2.67GHz, it's a safe bet that an overclocked i5-750 would easily win any performance test against an overclocked i5-661.

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