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
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 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.

Conversely, when the system is making only light demands of a CPU, the multiplier can be automatically reduced. This reduces power draw and produces less heat – and, since the system fan can run more slowly, less noise.

Unfortunately, if you try increasing the multiplier yourself, you’ll probably find it goes only one way. You can freely turn it down to make your PC run more slowly, but you can’t increase it beyond the factory preset value. This is because on regular CPUs the multiplier is locked by the manufacturer to stop you buying a cheap model and clocking it up to match the top-end part.

Unlocked performance

So the easiest way to take advantage of overclocking is to buy a processor that isn’t locked. Such chips, as you’d expect, typically cost more than regular locked models, but in terms of performance per buck they can be much better value.

AMD describes its unlocked chips as Black Editions. On these chips you’re free to raise the multiplier as high as the motherboard can handle. Until recently, Intel did the same thing, offering Extreme Edition processors at the top of its various ranges.

In the past few years, however, Intel has switched mostly to “K” processors, such as last year’s Core i5-655K and the current Core i7-2600K. On these chips the standard multiplier is locked, but the Turbo multipliers aren’t, enabling you to tweak how the processor performs under load – which is, after all, when it counts.
We’ve found that unlocked chips normally run at least as fast as the most powerful chip in their range, and can often go considerably higher

The manufacturers don’t actually promise that unlocked chips will run any more quickly than their advertised stock speeds. And if you do manage to damage one through overclocking it, you won’t be covered by the warranty. In our tests, though, we’ve found that unlocked chips normally run at least as fast as the most powerful chip in their range, and can often go considerably higher.

For example, although AMD’s Phenom II X4 Black Edition chips start at 3.2GHz, we’ve yet to have problems running one at 3.7GHz. Intel’s Core i7-2600K ships at a stock speed of 3.4GHz, but it sailed through tests with its Turbo speed dialled up to 4.5GHz.

Unlocking AMD cores

Many AMD chips sold as dual and triple-core models actually have four working cores, some of which have simply been disabled. These cores can often be automatically re-enabled by a motherboard with a “Core Unlocker” feature. This doesn’t exactly count as overclocking, but it’s another way to get extra power from your processor for free.

The advantage of an extra core isn’t as dramatic as raising the frequency – you’ll only see a real benefit in highly parallel tasks such as 3D rendering. And, of course, there’s no guarantee that you’ll be able to unlock any particular chip – although we haven’t heard of any failures with AMD’s current CPU range.

Keeping your cool

Since the settings for overclocking a CPU are found in the BIOS, you can experiment without having to open up your computer case. But if you want to get serious about overclocking, there’s one hardware issue to consider: cooling. If your CPU has a small heatsink, it may not be able to manage much of a boost before your PC starts crashing. The additional heat is also likely to cause the fan to spin at full pelt all the time, increasing the noise of your system.

For dedicated enthusiasts, there’s a thriving market in high-efficiency heatsinks, covered with heatpipes and fins. But even cheap third-party models are typically more effective than AMD’s and Intel’s standard fans – and they’re almost invariably quieter.

It’s worth thinking about thermal paste, too. This gooey substance is sandwiched between your heatsink and CPU and helps conduct the heat away. Your CPU should already have a healthy dollop of paste in place, but an even re-application won’t hurt. If you buy a new heatsink, you’ll need to apply fresh paste to ensure proper contact is made.

Even cheap third-party models are typically more effective than AMD’s and Intel’s standard fans – and they’re almost invariably quieter

There are various types of thermal paste available, at prices ranging from a few quid to more than £20. The premium brands provide greater conductivity but, unless you’re a real enthusiast, a cheap tube should be fine. You can test its effectiveness with a CPU temperature-monitoring program such as the free Core Temp tool.

Overclocking Sandy Bridge processors:

Intel’s unlocked Core i5-2500K and Core i7-2600K CPUs are a tempting proposition, offering stunning performance for a modest premium over the regular models. However, there are a few points to bear in mind if you plan to overclock one of these chips.

The first is a chipset limitation. If you have a motherboard based on the latest Intel Z68 Express chipset, you’re good to go. But motherboards based on the H67 chipset don’t support overclocking. Boards based on the P67 can overclock, but they won’t let you use the processor’s onboard graphics.

If your motherboard does support overclocking, just look in your BIOS for “CPU settings” or something similar (check your motherboard manual if you can’t find it). Here you’ll find all the multiplier settings you need to increase Turbo Boost performance.

On a quad-core processor you’ll see four settings, enabling you to set different multipliers for when four, three, two and one cores are in use. This allows you to balance power consumption and heat against multicore performance. A model with two physical cores will offer only two settings, even if it has Hyper-Threading, since it would make no sense to try to run the virtual core at a different speed to the real one.

On some motherboards, the Turbo multipliers are expressed as absolute values: for example, if the base multiplier is 20, you may have the option to turn single-core Turbo Boost up to 28. On other boards, the figure is shown as an offset: in this case, you’d want to set the single-core setting to eight.

If you’re feeling brave, you can also adjust the current limit, a figure in amps that determines how much power the CPU will draw before automatically reducing its own speed (to ensure it doesn’t overheat). We don’t recommend this, for obvious reasons.

Once you’ve set your multipliers, simply boot into Windows to enjoy your new speed boost. If you want to see the effect of your changes, Intel offers a free gadget for Windows 7 that lets you keep track of your CPU frequency and see the effect of Turbo Boost. It’s called the Intel Turbo Boost Technology Monitor.