According to nVidia nForce 680i-based motherboards are targeted to extreme overclocking, and on ECS PN2 SLI2+ we could find several overclocking options (BIOS P25):

• FSB clock: Can be adjusted from 400 to 2500 MHz in 1 MHz steps. As the clock here isn’t the real clock but the QDR clock (i.e. four times the real clock), this means that you can adjust the external clock rate from 100 MHz to 625 MHz in 0.25 MHz steps.
• Memory clock: Can be adjusted from 400 to 1400 MHz in 1 MHz. As the clock here isn’t the real clock but the DDR clock (i.e. two times the real clock), this means that you can adjust the memory clock rate from 200 MHz to 700 MHz in 0.5 MHz steps.
• PCI Express clock: Can be adjusted as auto or from 100 MHz to 200 MHz in 1 MHz steps. This motherboard offers separated clock adjustments for each one of the three PCI Express x16 slots.
• HyperTransport clock (SPP <-> MCP Ref Clock): This is the clock rate between north bridge and south bridge and can be adjusted as auto or from 200 MHz to 248 MHz in 2 MHz steps; from 249 MHz to 298 MHz in 1 MHz; and from 300 MHz to 500 MHz in 2 MHz steps.
• HyperTransport multiplier: From 1x to 5x.
• CPU voltage: auto or from 0.50000 V to 1.52500 V in 0.00625 V increments and from 1.52500 V to 1.80000 V in 0.0125 V increments.
• Memory voltage: auto or 1.8 V to 2.5 V in 0.025 V increments.
• FSB voltage: auto or 1.2 V to 1.5 V in 0.1 V increments.
• North bridge voltage (SPP): auto or 1.20 V to 1.55 V in 0.05 V increments.
• South bridge voltage (MCP): auto or 1.500 V to 1.750 V in 0.025 V increments.
• HyperTransport voltage: auto or 1.20 V to 1.55 V in 0.05 V increments.

HyperTransport is the name of the bus used to connect the north bridge chip to the south bridge chip.

This motherboard also provides several memory timings adjustments, as you can see on Figure 9.

click to enlarge
Figure 9: Memory timings adjustments on ECS PN2 SLI2+ (P25 BIOS).

This motherboard provides several important overclocking features not found on mainstream motherboards and even on some high-end models.

The most important one is the ability to lock and configure the memory clock independently from the CPU external clock. On almost all motherboards the memory clock is derived from the CPU external clock, so if when you overclock the CPU you automatically overclock the memory as well. Thus when you reach the maximum overclocking your system can take you will never know what is limiting your computer from reaching an even higher overclocking, the CPU or the memories. With this option you can lock your memory clock at their standard clock rate (e.g. 800 MHz or 1,066 MHz) and only after you found the maximum clock rate your CPU can take you may start increasing the memory clock rate in order to find the maximum clock rate your memories can achieve.

The PCI Express clock configuration is also very important, as you can lock the PCI Express clock at a given value (100 MHz, for example). Usually when you increase the FSB clock you will automatically increase the PCI Express clock as well, and sometimes your overclocking will be limited not by the CPU but by the devices connected to the PCI Express bus. Thus with this option you can increase the probability of setting a higher overclocking. Almost all high-end motherboards have this option, but on ECS PN2 SLI2+ you can configure the PCI Express clock rate individually to each one of the three x16 PCI Express slots available.

On this motherboard the external clock rate is configurable at 0.25 MHz steps and a lot of people may not notice this, because in the setup the FSB clock rate is referred by its quadruplicated value (QDR), not by the real clock rate like on almost all other motherboards.

For instance, the maximum external clock rate we could configure was 1,226 MHz (i.e. 306.5 MHz), with our memories locked at 1,066 MHz. With this overclocking our Core 2 Duo E6700, which normally runs at 2.66 MHz, was running at 3.06 GHz, a 15% increase on its internal clock rate. With this overclocking our system performance increased 9.58% on Quake 4 and 8.20% on PCMark05.

We could configure our external clock above that but the system was unstable. We only consider our overclocking to be successful after we can run at least four times Quake 4 and PCMark05 with no errors.

Even though 15% seems to be a good number, we achieved better overclock with other motherboards. For instance, on MSI P35 Neo Combo we could set our CPU running at 314 MHz, on ASUS P5B we could set our CPU running at 316 MHz, on ASUS P5B Premium we could set our CPU running at 323 MHz and on ASUS P5N-E we could set our CPU running at 327 MHz.

We, however, didn’t play with voltage settings or any other fancy adjustments, so you may achieve a better overclocking than we did with more time and patience.