Overclocking 101: our step-by-step guide to maximising your PC’s speed

UPDATE: this article is about overclocking Intel processors. For overclocking AMD CPUs see here.

Did you know that your $250 Intel i5 760 CPU can be made to run faster than the $1500 i7 980x after just five minutes of tweaking? We’re not pulling your leg – it’s easy to do via the process of overclocking. All CPUs within a certain class (i5 i7 etc) are built identically before being artificially speed graded so they can be sold at various price points; overclocking gets around this and maximises the performance of your CPU. With today’s low-end Intel chips it’s easy to extract a 50% performance boost saving yourself $1250 in the process.

Once the black art of computing overclocking is now well and truly in the mainstream. Most mid-level motherboards include the necessary features for overclocking. Today we’ll show you how to overclock your Intel i5 or i7 chip in just five easy steps. If you’re looking for a cheap chip to overclock we highly recommend the i5 760.

Intel’s Core i5 760 CPU: recommended for overclocking.

[#PAGE-BREAK#Read this first!!!#]

Overclocking has the potential to damage your CPU. However the suggested settings in this article are extremely conservative and in over a decade of overclocking we haven’t damaged a single CPU. Having said that overclocking voids your warranty (though it’s impossible to tell if a CPU has been overclocked) and APC cannot be held responsible in the unlikely case of a CPU meltdown.

Look before you leap: in case things go bad
In the quest for a higher overclock it’s possible to push your settings so far that the computer will no longer boot. Your PC can get stuck in an endless series of reboots before it gets to the BIOS screen making it impossible to lower your overclock values. Most of today’s motherboards will automatically restore the default BIOS values after three crash-boots but in rare circumstances you’ll need to manually clear the BIOS. To do this look for a button or jumper on your motherboard called “Clear CMOS”. Press or bridge this to reset all of your BIOS settings to their default values. Bear in mind this may disable RAID settings and the like so before overclocking it’s always a good idea to take a look at the BIOS and record every setting. That way if you have to use the “Clear CMOS” button you know what values you’ll need to reconfigure.

If things go pear-shaped look for a button or jumper on your motherboard called “Clear CMOS”.

[#PAGE-BREAK#Step 1 – Taming the thermal beast#]

Step 1 – Taming the thermal beast
Running a CPU at speeds of up to 4GHz outputs enough energy to warm a small home. The default CPU cooler that comes with Intel CPUs isn’t good enough to handle this additional thermal load so you’ll need to upgrade the heatsink. While hardcore overclockers favour water or even refrigerated cooling a simple high-end air heatsink will do the job for moderate overclocks up to the  4GHz region.

Look for a cooler that is designed for overclocking and which is rated to handle the load of a 4GHz chip. This doesn’t necessarily mean it has to be loud though; our Thermaltake SpinQ VT is whisper-quiet while running the i5 750 at 3.8GHz and costs less than $75. Mount your HSF before starting your overclock and adding a case fan or two will help to supply your new cooler with a fresh supply of cool air.

Look for a cooler that is designed for overclocking and which is rated to handle the load of a 4GHz chip.

[#PAGE-BREAK#Step 2 – Unleash the beast#]

Step 2 – Unleash the beast
Certain features of your CPU are fantastic while it’s running at default speeds – but they can make overclocking impossible. You’ll need to disable these settings via your BIOS before you begin tinkering. To access your BIOS hit Delete as your PC is booting up and then head to the CPU settings area. Disable all of the following settings:

  • Speedstep (EIST)
  • Turbo Mode
  • C1E
  • C-states

While most of the better motherboard manufacturers use the same terminology these may be named slightly differently in your BIOS. If you can’t find the above terms consult your motherboard manual to see how they’ve been named differently. Disable these items and then reboot your PC.

You’ll need to disable some settings via your BIOS before you begin tinkering.

[#PAGE-BREAK#Step 3 – Increasing the juice#]

Step 3 – Increasing the juice
To run the CPU at higher speeds requires more voltage. This is the only risky part of the procedure – go crazy with your voltages and you can easily fry your CPU. Thankfully to get a safe and stable overclock requires just a slight bump in voltages. Head into the BIOS once again and look for a section containing various voltage settings; within this section should be manual voltage control for several subsystems of your motherboard.

For i5 users change your voltages to the following settings:
CPU Vcore (CPU Voltage) = 1.35V
CPU QPI/VTT Voltage = 1.3V

For i7 users change your voltages to the following settings:
CPU Vcore (CPU Voltage) = 1.4V
CPU QPI/VTT Voltage = 1.3V

Once you’ve adjusted all voltages reboot your PC.

[#PAGE-BREAK#Step 4 – Increasing the frequency#]

Step 4 – Increasing the frequency
Before we talk about increasing the values we need to do a brief explanation of how your CPU and system frequencies are calculated. Your CPU’s frequency is calculated by multiplying its CPU clock ratio (also known as a multiplier which is locked to artificially determine the speed of your CPU) by the base clock speed (or bclock). As the CPU clock ratio is fixed we must increase the base clock speed to increase the overall frequency of the CPU. However the baseclock also determines the speed of your memory and QPI (quick path interconnect which connects the CPU to the P55 chipset) so we need to look at those settings at the same time.

Your memory speed is determined by multiplying the base clock by the memory system multiplier while the QPI speed is similarly set by multiplying the QPI clock ratio by the base clock.
So as you increase your base clock to increase your CPU frequency you must also lower your QPI multiplier and memory multiplier to ensure your QPI and memory stay within their normal operating speeds:

Base clock
We’re going to aim for a conservative 3.6GHz overclock first. Find your CPU multiplier value and then divide 3600 by it. This should leave you with the target base clock. For example our i5 750 CPU has a CPU multiplier of 20x. Divide 3600 by 20 and we’re left with a value of 180 so we set our base clock value to 180MHz.
Set this to its lowest possible value which is usually 32x. Lowering this has no discernible impact on performance but having a higher QPI setting will stop your overclock from working.

Memory multiplier (also known as memory ratio memory frequency or DRAM frequency)
Drop this so that when it’s multiplied by your new base clock your final memory speed is equal to or lower than the speed your DDR memory is rated to. For example if you’re running DDR3 1333MHz memory you’d want your overall memory speed to be no higher than 1333MHz so you’d set your multiplier to 6.

Once you’ve adjust all of these settings reboot your PC for them to take effect. It should post with a CPU speed of 3600MHz.

[#PAGE-BREAK#Step 5 – Rock-solid stability#]

Step 5 – Rock-solid stability

Now you need to test how stable your CPU is at its new frequency. We recommend using IntelBurnTest run in maximum Stress Level. Keep an eye on your CPU temperatures – if they go above 90°C your cooler isn’t up to the job (some folks are happy with 100°C CPU temps but better to be safe than sorry).

Keep an eye on your CPU temperatures – if they go above 90°C your cooler isn’t up to the job.

If you can successfully run this for an hour without crashes it’s time to head back into the BIOS to slowly increase your bclock value by 10MHz increments all while adjusting the other multipliers to keep their frequencies within acceptable limits. Reboot the PC and run the stress test again. Keep doing this until the PC starts crashing then go into the BIOS and apply the last stable bclock setting.
Today’s Intel chips should happily run anywhere between the 3.6GHz and 4.2GHz region but your mileage can and will vary.

Remember to be conservative with your voltages to ensure there’s no damage and then enjoy the newly unlocked power of your overclocked CPU!