Actually, it was the maximum FSB frequency that determined how far we could overclock our Yorkfield processors. As we know from the Core 2 Extreme QX9650 tests, quad-core processors manufactured with 45nm process can work stably at over 4GHz frequencies. However, low frequency multipliers of the junior models wouldn’t let us go that far.
The maximum frequency for Core 2 Quad Q9300 was 3.57GHz obtained as 7.5 x 475MHz.
Our processor requires only minimal voltage increase to work stably at this frequency and further voltage increase doesn’t help overclocking it any higher. In other words, the CPU itself doesn’t limit the overclocking.
Core 2 Quad Q9400 uses 8x multiplier, so we can overclock it 237MHz higher:
The maximum frequency in this case reached 3.8GHz at 475MHz bus speed.
However, during Core 2 Quad Q8200 overclocking, things didn’t follow the standard routine. This CPU turned out capable of pushing back the overclocking maximum for quad-core processors. Looks like smaller L2 cache lowers the FSB utilization because of lower traffic for saving cache coherence of the processor dies. As a result, Core 2 Quad Q8200 could work on DFI LANPARTY DK P45-T2RS mainboard with exact same settings, but at a slightly higher frequency of 485MHz FSB.
So, Core 2 Quad Q8200 overclocked to 3.4GHz. Just like with Core 2 Quad Q9300, the system remained perfectly stable even with moderately increased Vcore and a standard boxed CPU cooler.
Core 2 Quad Q6600 overclocking is a completely different story. This CPU is designed for 266MHz FSB that is why it sues a pretty high 9x clock multiplier. Moreover, it belongs to the Kentsfield family and uses old 65nm cores. So, on the one hand, it doesn’t have a lot of overclocking potential, but on the other it can be achieved at relatively low bus frequency. The main problem during Core 2 Quad Q6600 overclocking is not the search for optimal mainboard BIOS settings, but proper processor cooling. For example, with a powerful Scythe Mugen cooler we managed to get this CPU to work stably at 3.6GHz. The processor Vcore in this case was raised to 1.425V.
You can see from the screenshot that FSB frequency was increased only to 400MHz, so any contemporary mainboard can do this. However, the CPU temperature increased to 90°C in burn mode. So, Core 2 Quad Q6600 overclocking is only possible with an efficient cooler.
As you know, AMD Phenom X4 processor doesn’t overclock that well. Only CPUs on Intel Core microarchitecture can boast overclocking by 1.5 times. As for the Phenom X4 9950 that participated in our today’s tests session, we managed to overclock it only to 3.2GHz. We had to increase the core voltage to 1.45V to ensure the system stability in this case.
We have also taken advantages of its “Black Edition” origin and simply increased the multiplier beyond its nominal value. Note that overclocked Phenom X4 9950 heated up a lot, just like Core 2 Quad Q6600, so we used Scythe Mugen cooler on it, too.