Crank up an Intel K series Core CPU to 4GHz and beyond

Last month we showed you the necessary ingredients to upgrade your PC to a 4.5GHz Ninja for less than $500 with the Intel 2nd Gen Core i5 2500K at its beating heart. Now that you’ve gone and maxed your credit card buying these components and have assembled everything inside your case it’s time to crank up the default speed. Follow our guide and this zippy 3.3GHz CPU should hit somewhere between 4GHz and 4.5GHz (see above) possibly even higher if you’ve got a good chip. Note that our guide is applicable for any of the Intel 2nd Gen Core CPUs that have a K at the end of their model number denoting the fact that they’re multiplier unlocked. 

This series of CPUs is far simpler to overclock than anything we’ve ever encountered but you’ll still need to familiarise yourself with a few key BIOS settings as you’ll be adjusting them. Different motherboards have different names for these settings sowe’ve tried to incorporate the most commonly used terms:

CPU Ratio: Also known as the multiplier this is the key setting for changing your CPU’s speed. In the i5 2500K it’s set to 33 which is then multiplied by the base clock (100MHz) to reach the final CPU frequency of 3300MHz (or 3.3GHz). We’re going to increase this value to hit 4.5GHz but not just yet.

CPU Voltage/CPU Vcore: This is the main voltage of your CPU which fluctuates due to the CPU’s power saving features. When overclocking we must disable these features to ensure the voltage stays steady.

VCC/VCCIO /CPUIO: Separate to the Vcore this is the voltage of the I/O terminals of the CPU. Again we’ll need to give this a slight increase to hit 4.5GHz.

VCCSA: This refers to the CPU’s System Agent voltage and again it’ll need a very slight boost to stabilise an overclock over 4GHz.

LOAD LINE CALIBRATION/VDROP: Despite what you set your CPU Voltage to the CPU usually receives slightly less than you’ve asked for especially when under load. Enabling LLC or Vdrop ensures that the voltage stays as close to your desired level as possible.

C1E SUPPORT: This lowers the CPU ratio of your CPU when it’s not under load.

OVERSPEED PROTECTION: If the CPU sees that it’s drawing more current than expected OverSpeed protection kicks in. However this is a barrier to overclocking as we deliberately require the CPU to draw more current than the default value.

SPREAD SPECTRUM: This setting is used to lower the electro magnetic interference of your board but it can play havoc with overclocking. Now that we’ve shown you the main settings let’s get tweaking!

WARNING! It’s necessary to slightly increase the voltage of your CPU to get the best possible overclock. If you crank up the juice too high you’ll turn your shiny new chip into a hunk of useless silicon so the voltages we recommend in this article are very conservative. They should be fine and we’ve built countless PCs without ever damaging a CPU but you overclock at your own risk. APC is not responsible for any damage to your PC and overclocking voids your CPU’s warranty… though it’s basically impossible for the retailer to tell if the CPU is damaged via overclocking. Use that handy nugget of information as you will.


It’s best to ensure your CPU is already 100% stable. For this we recommend the free program SiSoftware Sandra 2011 Lite available from Install this small app then head to the tools section and select Burn-in mode. When it prompts you to select which tests to run select the following two modes – Processor Arithmetic and Processor Multi-media. Leave it to run for a couple of hours and if it doesn’t crash you’re good to go. Back everything up on your PC as in rare circumstances we’ve had a bad overclock corrupt our Windows install requiring a full reinstall. You have been warned!


Just as it starts booting up hit delete or F2. This will take you into the BIOS where the fun settings are. You can overclock most boards from within Windows these days but the BIOS is the most reliable method of tweaking. Firstly we want to disable all the CPU settings that could prevent the overclock:

Limit CPUID Maximum – disabled

C1E SUPPORT – disabled




If you can see any other settings that refer to saving power or alternating the phase to the CPU disable them. Google the meaning of each just in case – we can’t cover them all here as different motherboards have different features. Reboot your PC.


Head back into the BIOS again as soon as it boots – now it’s time to increase the voltages. Don’t screw around with these and favour conservative values where possible.

CPU Voltage/Vcore – set to 1.35V. This is as high as we recommend going on air cooling. If you’ve got a water cooling kit or a deathwish 1.4V might be acceptable but we wouldn’t risk it with our own hardware.

VCC/VCCIO /CPUIO – set to 1.1V

VCCSA – set to 1.1V

That’s it for voltages – time to reboot to implement them.


In the old days we had to stuff around with lots of different ratios to overclock but now it’s easy – just increase the CPU Ratio/ multiplier. The default value is Auto or 33. Set this to manual control and then change the value to 40 giving you a CPU speed of 4GHz (40 x base clock of 100MHz). Reboot and let Windows load.


If Windows loads successfully reboot your PC and head back into the BIOS increasing your ratio by x1 each time i.e. 41 42 43 etc. Keep doing this until you can no longer boot into Windows. On our CPU the highest we could boot into Windows was a ratio of 46 giving us a bootable CPU frequency of 4.6GHz.


It’s time to load up SiSoft Sandra 2011 again and run the burn-in tool once more. Do exactly as described in step 1. You’ll probably find that this will crash despite your PC booting into Windows. If it does crash reboot and drop your CPU ratio by 1. If not you’ve hit the ceiling of your CPU already. While running the Burn-in mode fire up a temperature monitoring program – your motherboard will include this software on the installation CD. Keep an eye on CPU temperatures as the burn-in test runs and make sure it doesn’t get over 75C. With our Noctua NH-D14 in place our CPU peaks at around 60C while under load which is fine for a 4.5GHz monster.


It’s never been an easier time to overclock your CPU since Intel made the K version of the 2nd Generation Core multiplier unlocked. As you’ve seen with our guide it’s possible to turn this $250 CPU into a beast that will run rings around Intel’s $1000 CPUs easily hitting the 4GHz mark and probably making it all the way to 4.5GHz. Take your time and play it safe with voltages and you’ll have a stable safe overclock – we’ve followed the same mantra for the last decade and have never damaged a CPU in that time. Finally if you get stuck at all head to your motherboard manufacturer’s forums as there may be an obscure setting on your motherboard that is stopping it from reaching its maximum potential. Most of all have fun – there’s nothing quite like running a CPU that is faster than anything in stores!