Core 2 Extreme QX6850 works internally at 3 GHz multiplying its external clock by 9 (333 MHz x 9 = 3 GHz).

The engineering sample we’ve got was with its clock multiplier unlocked and we could set it to x10, simulating a “QX6950” CPU, i.e. a Core 2 Extreme running internally at 3.33 GHz and externally at 333 MHz (i.e. 1,333 MHz). The results for this overclocking were added on all tables and charts of our review.

Then we left our sample with its default clock multiplier of x9 and increased the CPU external bus clock rate. We could increase the CPU external bus up to 378 MHz, what made our CPU to run internally at 3.4 GHz, a 13% increase on its internal clock rate.

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Figure 4: Core 2 Extreme QX6850 overclocking.

With this overclocking we saw a performance increase of 6.89% on PCMark05.

We could set the external clock above 378 MHz, but the system was unstable. We only consider an overclock to be successful when we are able to run PCMark05 and Quake 4 four times without crashing.

Overclocking success depends a lot on the motherboard and on the CPU used. Even between two identical CPUs one can reach a higher overclocking than the other, especially if they are from different production batches.

We didn’t play with any fancy configuration, like increasing the CPU voltage. Thus with more time and patience you will probably achieve a better overclocking with Core 2 Extreme QX6850 than we did.