Third Step: 4.2 GHz (FSB1772)
Next stop: 4200 MHz. Please hold on to your keyboard.
We decided to move in 200 MHz increments, which is why 4.2 GHz was the next step. As we saw stability issues and system crashes, we decided to further increase the processor voltage from 1.345 V to as much as 1.45 V. This helped to bring back stability for the overclocked system, now running at a FSB base clock speed of 443 MHz (FSB1772). Although we still did not relax the timings, the memory still worked fine at a +0.3V increase (1.5 V default).
The memory was also slightly overclocked at a 4200 MHz core clock speed, as the system speed, which runs at the same base clock speed as the memory, increased to 443 MHz.
Final Step: 4.3 GHz (FSB1812)
A 4.3 GHz clock speed was the limit. We couldn't get the sample to run faster, regardless of what we tried. Since the processors did not run hot at all during the overclocking, we assume that the system bus might be the bottleneck.
A 453 MHz Front Side Bus base clock speed (FSB1812) was the limit of the test sample. We tried increasing the voltage up to 1.5 V, and also increased the voltage of the other components up to +0.25 V for the Front Side Bus and +0.4 V for the memory. The "overvoltaging" also involved charging the chipset up to +0.35 V. We also tried different combinations of the above. Most likely the CPU sample just isn't capable of supporting much higher FSB speeds, which we have observed for Core 2 Duo E6x50 processors as well. Some of them could reach a 500 MHz base clock, while others do not.
Since this is only the beginning of the Core 2 Duo Wolfdale generation, we expect better clock speed margins to be available with upcoming processor steppings. The first 65 nm Core 2 Duo Conroe samples we had received in mid-2006 weren't able to reach 4 GHz. But today, a majority of the E6750 and E6850 processors can be overclocked to this speed with little effort.
Again, SpeedStep worked well even at 4.2+ GHz.