Custom PC's Ultimate Core 2 Overclocking Guide

Posted at: Wednesday 31st October 2007 by Alex Watson

Not sure where to begin when overclocking? Alex Watson uncovers the dark art of coaxing more speed out of your PC's components for free.

As the FSB speed is constant between different models of CPU - for instance, both the Q6600 and the QX6800 have a 1,066MHz FSB - Intel uses the clock multiplier to fix a CPU's speed. The Q6600 runs at 2.4GHz, as it has a 266MHz FSB (1,066/4) and a multiplier of 9. For the QX6800, the equation is 266 x 12 = 2.93GHz.

The FSB also determines the speed of many other components inside the PC, including the PCI-E and PCI buses, and the memory. RAM speed is calculated according to an FSB:DRAM ratio. For example, with a 1:1 ratio (known as synchronous), the memory speed of a Core 2 Duo is 266MHz. There are two data transactions per clock (the 'Double Data Rate' in DDR2), meaning that the memory in effect runs at 533MHz. There are other ratios that allow you to set the memory speed asynchronously from the FSB.

The secret of overclocking lies in tweaking the FSB, RAM and multiplier settings. For example, if we increased the FSB of a Q6600 from 266MHz to 300MHz, the CPU frequency would rise to 2.7GHz (300MHz x 9).

Problem alert: The frequencies of the FSB, memory and other buses are linked, so increasing the FSB also increases their frequencies. With a 300MHz FSB, the DDR2 memory frequency would increase to 600MHz (provided you’re sticking with a 1:1 ratio), and the PCI and PCI-E frequencies would also rise. Many devices, particularly S-ATA hard disks, are highly sensitive to changes in frequency, so to keep them safe, you need to ‘lock’ the PCI and PCI-E buses at their default speeds, which are 33MHz and 100MHz respectively. Many modern motherboards will do this automatically, while others should provide an option in the BIOS.

The BIOS

The BIOS is a chip on the motherboard that stores a software program that initialises and configures the hardware in your PC. While it's full of mundane options (ooh, the clock!), its bare blue menus also conceal the levers of overclocking power.

You can also use Windows software applications to modify the key settings for overclocking, such as ClockGen (www.cpuid.com), but the BIOS is far more powerful and flexible, so it's essential that you become familiar with it. BIOSes differ considerably, but most Core 2 boards will have a similar range of key settings that can be tweaked and altered. We'll go into detail in the next section of the feature, but the first step is to access your board's BIOS. You'll need to refer to your motherboard manual to see how to do this, but it's usually just a case of pressing the 'Del' key when your PC POSTs (the first few lines of text that show up on screen, prior to Windows loading).