What is overclocking?

An introductory guide to overclocking your PC

Overclocking involves increasing the speed (otherwise known as frequency) of your system components, beyond the manufacturer's specifications. In a modern PC the three major components which are commonly overclocked by enthusiasts are the CPU, the graphics card and the memory (RAM). Traditionally the focus of overclocking has centred on the CPU.

Caution: Overclocking can, in some cases, damage your PC's components. Proceed with caution.

There are several reasons why enthusiasts like to overclock their CPUs.

To save money.

Why spend $500 on a top-of-the-line CPU when I can purchase the entry-level $100 CPU and then overclock it to the same speed as the $500 part? If I am a keen gamer, the $400 saved could be put towards the purchase of a high-end graphics card instead.

To push the limits of performance.

Some overclockers like to take their components well beyond the speed of the manufacturer's fastest model. A great deal of satisfaction can be had from having a PC with a CPU running at 4.4Ghz when the fastest model available from the manufacturer runs at 3.3Ghz. It's a bit like performing aftermarket modifications to your car's engine to increase its horsepower beyond the factory specifications. I still fondly remember my old Celeron 600Mhz purchased late in the year 2000, which ran stably at 1080Mhz and formed the heart of my gaming PC at the time.

For bragging rights.

When I first became involved in overclocking PCs over 15 years ago it was the kind of thing you didn't really talk about, except with other enthusiasts. Many so called 'industry experts' at the time had a very dim view of the concept of overclocking, and tended to lump overclockers into the same category as Satan worshippers and conspiracy theorists.

These days, however, overclocking has become a mainstream activity and has inspired a multitude of informative websites and magazines. Many PC component manufacturers now sponsor international overclocking competitions and teams of expert overclockers strive to break records in a wide range of differing categories, sometimes using exotic techniques, such as supercooling components with liquid nitrogen. For example the fastest validated CPU frequency currently on the CPU-Z hall of fame board is an Intel Celeron 360 3.46GHz which reached the mind boggling speed of 8242.45Mhz!

OK — so how do I overclock my PC?

The principal ingredient in your overclocking success will be your motherboard. The motherboard in your PC connects all of the individual components including the CPU. The motherboard generates the final speed or frequency of the CPU by utilising a default system bus speed, and then applying a multiplication factor to it. For example the Intel Core i7 875K CPU in my own personal PC operates at a default frequency of 2.93GHz. This CPU has a multiplier of 22x, and runs on a default system bus speed of 133MHz, so 133MHz system bus multiplied by 22 = 2926 MHz ~ 2.93GHz.

In the good old days we had the ability to alter the clock multiplier by changing jumper caps or dip switches on our motherboards. My old Pentium 166 ran at 2.5x 66.6MHz = 166MHz. It was very common and very easy to change the multiplier to 3x 66.6MHz = 200MHz, or if you were lucky and had a really good chip you could push it to 3.5x 66.6MHz = 233MHz. As time went on the adjustment of the multiplier and the bus speed was transferred straight into the motherboard BIOS, and it was no longer necessary to manipulate physical jumper caps or dip switches on the motherboard itself.

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At a certain point in time the major CPU manufacturers — Intel and AMD — decided to produce their CPUs with locked multipliers. This meant that we were no longer able to manipulate our CPU frequency by simply increasing the multiplication factor of the system bus. But all was not lost. Motherboard manufacturers started to produce motherboards with incremental adjustments for the system bus itself, designed for overclocking. Once limited to a handful of standard bus speeds such as 60, 66, 100 and 133MHz, modern motherboards now have the capacity to generate any bus speed over a wide range in 1MHz increments. This gives us an alternative method of increasing the overall CPU frequency, but it also introduces another layer of complexity in understanding the process.

Increasing the system bus frequency also increases the system memory frequency, as the memory frequency is also generated from the system bus frequency. To overcome this limitation it was necessary to either use memory with a high frequency rating to cope with the speed increase, or select a motherboard with memory frequency multipliers. This gave a range of multipliers which could be utilised to bring the memory frequency back down to as close to its normal speed as possible. In some cases, to achieve a desirable CPU overclock it was necessary to run the memory at slightly below its rated frequency.

Armed with this knowledge of how our system frequencies are generated there are two more issues we need to address.


In order to generate more power from our motor car engine, we need to feed more fuel into it. Similarly, when we start pushing our CPU beyond its factory frequency setting, it will most likely require some added juice. Modern motherboards allow us to increase the voltage to the CPU. This will allow the CPU to operate at even higher frequencies than might otherwise be possible at the default voltage settings, and help to stabilise the CPU when we reach our final frequency during the stability testing phase. We may also need to add voltage to the Northbridge chipset on Intel based systems, particularly as the bus speed is pushed towards and past the 200MHz mark.

Please note, however, that increasing the voltage to any component in your PC increases the risk of that component failing. There are many forum groups and hardware review sites on the internet where you can research “safe” limits for voltage increase on specific CPU and Motherboard models, and if you are an inexperienced overclocker it is always wise to stay well within these limits.


Overclocking your CPU generates extra heat. Overvolting your CPU generates extra heat. The fan and heatsink combination which ships with your CPU has been specifically engineered to cope with the thermal load your CPU will create while under normal conditions at its factory default settings. It won't be able to effectively dissipate the extra heat generated during any moderate to extreme overclocking attempts. This is why overclocking enthusiasts will always replace their CPU Fan and Heatsink with specially developed third party solutions. These heatsinks use fluid filled pipes which rapidly migrate heat away from the CPU surface and quite often dissipate more than twice the heat load of the original unit.

Some overclockers move beyond the realm of air cooling and use water cooling setups, which work in much the same way as the cooling system in your motor car. Beyond this there is an extreme fringe of overclockers who will introduce water chillers or phase change refrigeration into their cooling setup and push their CPU to the absolute limit.

I have been overclocking my own PCs since the mid-1990s using either high-end air cooling or water cooling, and still enjoy the feeling of knowing that I have a system capable of outperforming any default configuration on the market. Generally, however, I am willing to sacrifice a little bit of bleeding-edge speed if it allows me a significant reduction in voltage and heat output. For example, my current gaming rig has an Intel Core i7 875K 2.93GHz CPU running at 4.2GHz. This chip will run stably at just over 4.4GHz but requires quite an extra push in voltage to get there, with a significant rise in temperature being recorded when the chip is under heavy processing load. The loss of 200MHz is pretty much unnoticeable from a performance perspective and the chip is obviously a lot less stressed at this speed.

The final step in the overclocking experience is the testing phase. It is always wise to push your CPU speed up in small increments. Going to high too soon could result in damage to your CPU or motherboard. Nobody wants to blow up a $500 CPU or motherboard on their first day of testing. The way I approach my overclocking is to do some research first. I like to get an idea of the potential of a particular CPU model from the many hardware review sites on the internet and use that as a starting point for my testing. Every single site I checked was able to achieve at least 3.8GHz from the 875K CPUs they tested. All but two of the tests hit 4.2GHz or more, so I decided to begin my testing at 3.8GHz. If the PC posts, and boots successfully into windows, I then run a stress testing program called prime95. This runs all CPU cores at 100% whilst generating prime numbers and checking the results for accuracy. You can download the latest version of Prime95 from our Web site here: Prime95 download.

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If the Prime95 test fails on any of the cores, you know your CPU either requires a little more voltage, or has reached its speed limit. I also monitor my CPU temperature with a program called HW Monitor to make sure it is not getting too hot. You can download the latest version of HW Monitor from our website here: HW Monitor download.

Overheating CPUs will eventually fail. In this way you can incrementally keep pushing your CPU a little higher each time until you approach the point where it can no longer pass stress testing. You then back it off to a speed you feel happy with based on your testing results for voltage and temperature. I like to run Prime95 for a full 24hr test just to guarantee that the CPU is stable at this final speed. On occasion I have seen CPUs fail Prime95 after an hour or two of testing. This is not stable enough for my liking. Some overclockers would argue that Prime95 is no longer the stress test of choice for CPU stability, and are using a program called LinX. LinX is based on the Linpack CPU performance benchtest, but I must admit I have seen quite a lot of anomalous results from the use of this program, including brand new CPUs failing at default clock speeds and voltages. Some would argue that this indicated that the CPU is faulty straight out of the box, however, I am not convinced, and can say with assurance that any overclocked CPU I have stress tested for 24hrs with Prime95 has always performed faultlessly in any normal usage scenario including high end 3D gaming, decompressing of large files and video editing.

Before I conclude this post there are a couple of points I want to raise which should be of interest to any potential new overclockers. I stated earlier that the major CPU manufacturers have been multiplier locking their CPUs for many years now and generally speaking this is true, however there are some exceptions. AMD has been regarded highly by the overclocking community for their “Black Edition” product range.

Selected models of CPU have been released with unlocked multipliers and given the Black Edition moniker. These unlocked CPUs make overclocking a lot simpler for the novice in that you do not have to run your system bus or memory frequencies outside default specifications, and can ramp up the speed of the CPU by simply increasing the clock multiplier. Intel also started to ship unlocked CPU models. Originally restricted to their “Extreme Edition” flagship models, which were also very expensive, Intel recently introduced a “K” series, with just a few selected models having unlocked multipliers.

These models usually attract a price premium of $30-$50 compared to the locked versions. Intel on the other hand has also attracted the wrath of overclockers with their latest “Sandy Bridge” Core i7 CPUs. The motherboard chipset architecture which applies to these new CPUs has been changed. The system bus has been linked to other buses on the motherboard such as USB and SATA, all using a single clock generator. These other buses do not like being run out of specification at all. The end result? You pretty much can't overclock any of the new “Sandy Bridge” based CPUs, unless you pay the price premium and purchase an unlocked “K” series model.

The community will be watching with interest later in the year when AMD are set to launch their new CPU range based on the “Bulldozer” architecture, and no doubt hoping for unlocked multipliers and great overclockability.

Here at DCA Computers our experienced technicians can configure and build any type of overclocked system you can imagine, ranging from mildly overclocked air cooled systems, right up to high end water cooling or even phase change based systems. You can call us any time on (02) 9634 2555 to discuss your requirements.

Glenn Howlett is the general manager of DCA Computer Technologies a computer retailer and support provider. Read more articles at the DCA Computers blog, follow DCA Computers on Twitter and become a fan on Facebook.