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Under Core i7's Hood: Comparing The C0 And D0 Steppings

Overclocking Core i7-965 (C0): 4.0 GHz

Our Core i7-965 Extreme sample is the same one we used in the initial Core i7 coverage article at the end of 2008. Thus, we’d expect newer samples to perform a bit better. We achieved 3.47 GHz by increasing the multiplier from the 24x default to 26x at a 133 MHz base clock.

The next step, 3.61 GHz, was reached through another multiplier increase to 27x. Still no need to adjust Vcore.

The move to 3.74 GHz required a slight increase in CPU voltage in the BIOS to 1.192 V. This resulted in an effective 1.280 V processor voltage, while the automatic voltage setting used 1.176 V and had the system run at an effective 1.264 V (see screenshots above).

In order to reach 3.87 GHz, we further increased the processor voltage to 1.216 V in the BIOS, resulting in an effective 1.296 V Vcore according to CPU-Z. However, this did not run reliably and we had to further increase the voltage to 1.232 V. At the end of the day, our maximum clock speed was a nice, even 4 GHz.

The maximum clock speed at which we could run our old Core i7-965 was 4.0 GHz at an effective 1.32 V, based on a 1.232 V BIOS voltage. According to findings across the Internet, it is safe to assume that some C0 processors can do another 200 to 300 MHz on air cooling with a little luck.

Settings and Results Table

Core i7-965 (CO) Overclocking3,334 MHz3,475 MHz3,609 MHz3,743 MHz3,876 MHz4,009 MHz
System Idle Power111 W111 W111 W112 W113 W113 W
System Peak Power232 W249 W251 W260 W267 W276 W
BIOS Vcore1.176 V1.176 V1.176 V1.192 V1.216 V1.232 V
CPU-Z VT1.232 V1.264 V1.264 V1.8 V1.8 V1.8 V
PLL1.8 V1.8 V1.8 V1.8 V1.8 V1.8 V
Fritz Chess Benchmark12126125171296713392-13832