How to overclock your CPU: an In Depth guide

Overclocking has always been popular among techie enthusiasts. But with both AMD and Intel offering unlocked processors at tempting prices, you don’t need to be an expert to get a performance boost.

How to overclock your CPU: an In Depth guide

There are risks to overclocking. Pushing a component too hard can damage it. Even if that doesn’t happen, the faster a chip runs, the hotter it gets, potentially making your system unstable or causing it to shut down completely.

So chip manufacturers don’t officially recommend overclocking your CPU, and it isn’t something we recommend in our reviews – except for unlocked chips, which we’ll discuss below.

Often cheap processors can be overclocked to match the speeds of their pricier brethren – or even exceed them

Let’s keep the risks in perspective, however. Most motherboards and processors, if pushed too hard, will automatically shut down long before permanent damage becomes a possibility. Plus, they’re designed with considerable electrical headroom, so they can normally be driven well above their “official” speeds before reliability starts to suffer.

Indeed, where a manufacturer offers several models of a processor running at different speeds, the chips are normally all functionally identical. The more expensive models have been tested and guaranteed by the manufacturer to run stably at higher speeds, while the cheaper ones are guaranteed to run only at lower frequencies, and are sold at a lower price to reflect that. But they can often be overclocked to match the speeds of their pricier brethren – or even exceed them.

Stop the clocks

Some motherboards allow you to overclock the CPU from within Windows, using custom software. Others offer handy buttons that you can press to nudge the speed up and down. But this is hardly the norm. In most cases, if you want to overclock your CPU, you’ll have to do it by changing the BIOS settings. Different boards present the relevant settings in different ways, but they’re normally easy enough to find: check your motherboard manual if you get lost, and click here for a step-by-step guide to changing your BIOS settings.

If you haven’t delved into your BIOS before, coming face to face with a page full of advanced settings can be daunting. Don’t worry. Almost all the options can be safely left at their default settings; you don’t need to know about idle states, C1E, north bridge frequencies or such like. If you accidentally change something you shouldn’t, simply press the appropriate key to restore the default settings. If you get into trouble, you can always exit the BIOS without saving your changes.

Should you somehow manage to make your PC unbootable, you can usually recover it by clearing the CMOS memory that stores your settings. This normally means setting a jumper or pressing a button on the motherboard; check your manual for details.

Such measures shouldn’t be necessary here, since we’re interested only in the two numbers that determine the speed of the CPU. These are the base clock, in megahertz, and the CPU multiplier, typically a number between ten and 40. You may see the base clock abbreviated to BCLK, or misleadingly referred to as the “CPU frequency”. Older boards may refer to the front side bus, or FSB, which for our purposes does the same job. The CPU multiplier is sometimes called the CPU ratio.

The speed at which your processor runs is established by multiplying the base clock by the CPU multiplier. For example, Intel’s Core i5-750 processor (at its stock speed) uses a base clock of 133MHz and a multiplier of 20 for an effective frequency of 2.66GHz.

You can overclock a CPU simply by raising either the base clock or the multiplier. Enthusiasts sometimes recommend increasing the voltage settings in your BIOS too, to provide the processor with a bit more juice to help it achieve its maximum possible speed. But that’s one tweak that, if inexpertly applied, can cause serious damage to your hardware. We suggest you play it safe and leave the voltage settings alone, even if that means missing out on a small amount of extra performance.

Base clock basics

The base clock doesn’t only determine the speed of the CPU. Several other components also derive their operating frequencies from it, including memory modules and PCI Express devices. These components are generally less tolerant of overclocking than a CPU, so if you start ramping up the base clock, your system may very quickly become unstable.

Some boards, however – especially ones sold as “enthusiast” models – are smart enough to decouple the memory and PCI Express buses from the base clock when it’s raised above its standard setting. With a board like this, you can increase the base clock without worrying about components other than the CPU.

If your board doesn’t do this, you can get the same effect by adjusting these components’ individual BIOS settings to compensate for the raised base clock. This requires an in-depth understanding of motherboard components and settings, though; we recommend most people leave well alone. If you do want to get into this level of technical detail, check out our sister title Custom PC and its website www.bit-tech.net – there you’ll find plenty of advanced resources for hard-core enthusiasts.

If you start ramping up the base clock, your system may very quickly become unstable

If you’re using a Sandy Bridge system, you can forget about tweaking the base clock at all. On these systems, this clock governs almost all onboard components, including USB and SATA controllers. If you speed them up by even a tiny amount, they’ll lose the ability to communicate with external devices running at standard speeds, making your computer functionally useless. If you want to overclock a Sandy Bridge PC, you’ll have to take a different tack.

A certain ratio

The other way to overclock a CPU is by adjusting the multiplier. This setting applies only to the CPU, so you can adjust it to your heart’s content without affecting any other part of your system.

This is how “self-overclocking” systems such as Intel’s Turbo Boost work. We mentioned above that the Core i5-750 uses a multiplier of 20, but when only one or two cores are under load, that multiplier is automatically increased to 24, delivering a speed boost of 4×133MHz, or 532MHz. AMD’s Turbo Core technology, used in its Phenom II X6 CPUs, works in much the same way.

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