Hands on: The need for speed

Overclocking is a favourite pastime of PC hardware enthusiasts, including myself. However, most of us understand that overclocking isn’t without risks. So, as I attempt my highest overclock to date in this month’s column, I’ll also be reporting on the less desirable side effects.

Overclocking rig
There are two types of overclocker: those who make modest increases that they believe are still well within the part’s tolerances; and those that turn it into an extreme sport, pushing a component to the limits with exotic cooling hardware. I belong to the former camp. I’m happy to enjoy a small boost for free, but don’t want to bother with anything other than conventional air cooling.

For this month’s tests I used the same rig as last month, albeit with some Ram swapped out. So the test system consisted of an Intel QX9650 processor fitted into an Asus P5Q Deluxe motherboard with 2GB of Crucial DDR-2 800MHz memory and a Gigabyte NX66256DP graphics card. The system booted Windows Vista Ultimate 64-bit from a 500GB Samsung Serial ATA hard disk and was powered by a Corsair HX520W supply. Finally, a Zalman CNPS9700-NT Super Aero Flower kept processor temperatures under control with air cooling only, albeit with a fairly large 110mm fan.

As an Extreme model, the Intel QX9650 is one of the company’s few processors that features an unlocked clock multiplier. As such it allows us to compare the effects of overclocking by increasing it or the external bus.

Test results
To test the speed of each configuration with a real-life benchmark, I timed how long it took for Windows Movie Maker to convert a 2.09GB DV AVI file into a 77.9MB WMV file using the VHS Quality preset. This is a processor-intensive operation that exploits all four cores of the QX9650.

The QX9650’s official clock multiplier and external bus are 9x and 333MHz, respectively, resulting in a 3GHz clock speed under normal conditions. Here, the system encoded the file in 176 seconds.

Since the QX9650 allows its clock multiplier to be modified, I did that first as it would have no impact on the other components, such as memory. So I increased the multiplier from 9x to 10x, resulting in an overclocked speed of 3.33GHz. The system booted Windows without complaint, with the video encode taking 155 seconds – 21 seconds quicker.

Next the multiplier was increased to 11x, overclocking the processor to 3.66GHz. Again it booted without problems and shaved 14 seconds from the encode time, taking 141 seconds. This appeared to be too easy, but with the multiplier increased to 12x, the system wouldn’t boot Windows, failing with a blue screen. I tried gradually increasing the core voltage by five, 10 and even 20 per cent, but to no avail. The Asus Bios offered an 11.5x setting, but it too failed, so I returned the multiplier to the standard nine-times and the core voltage to its default.

Next I tried increasing the external bus speed, which in the absence of an unlocked multiplier is the only way most people can overclock their system. High-end motherboards offer controls to adjust the memory speed independently, but I prefer to keep the memory and external bus operating at the same speed for the best performance. Besides, with DDR2 800MHz memory, I could run the external bus at up to 400MHz without pushing the Ram beyond its specification.

So with the clock multiplier set to the default 9x, I increased the bus from 333MHz to 366MHz, overclocking the processor to 3.3GHz and driving the memory at 732MHz rather than the previous 666MHz. The video encode took exactly the same 155 seconds as the previous 10x/333MHz configuration – 21 seconds quicker than normal.