Basic Concepts of Overclocking Intel Core i3/i5/i7 CPUs

Stabilize CPU Frequency

The last step in this guide is often the first step for users who run into problems and then troubleshoot for days afterward. Leaving it to the last step makes the task much simpler. You now have the following settings locked in; CPU VTT, IOH voltage, memory voltage, memory multiplier, and memory timings. That means when we are looking for our highest CPU frequency, there are only two variables we need to play with: bclock and CPU voltage.

Right now your CPU multiplier should be very low, and your bclock should be quite high. If we move the CPU multiplier up right now, we would undoubtedly become very unstable, and unlikely to post. The idea here is that if your bclock and memory are stable with the current settings, shifting the bclock down should not cause any instability. So, change you bclock to 140MHz, and switch your CPU multi up to its maximum. In our example i5 750, the normal maximum would be x20. Intel’s Turbo feature allows for extra multipliers, and some BIOS will even allow for the higher multipliers to be forced. It will not hurt to use this feature if you desire. So with my example of the i5 750, with some BIOS, I would be able to lock in a multi of x21.

“Load-line calibration”

This actually goes by a few different names, but they are all meant as a means to reduce or prevent v-droop. Most overclockers would advise you to enable this feature; I would only recommend it if you understand what it does. It does typically allow for measurably higher overclocking, but at the cost of violating Intel’s design specs, and putting more stress on the CPU. However, overclocking in its essence violates Intel’s design specs, so you’re not breaking any new ground with this feature. I do not enable load-line calibration on my daily/gaming system. But I always use it when I am trying to fine the absolute limit. For more insight on the matter, refer to this excellent explanation at

CPU Voltage

That brings us to the first thing that most users want to play with after powering up their new system for the first time: CPU voltage, aka “vcore”. As you can see, this is actually one of the last things you should be changing. I would recommend starting at a nice and easy 1.3V. Surprisingly enough, many users are able to achieve very good overclocks with this modest amount of CPU voltage.

Testing for your highest stable CPU frequency

Once the operating system has fully loaded, start up uXray. Now start up uXRay and verify that your overclocked settings have been properly applied, and that you are running at your desired CPU, bclock, and memory frequencies. Now start up you selected test program, for example Mprime. Run the test for five minutes. Then reboot and go back into the BIOS.

If the test ran without error, the raise the bclock by 10MHz, reboot into your OS and run the test again. If the test failed, raise the CPU voltage by 0.025V, reboot into your OS and run the test again. Continue to raise bclock or CPU voltage until you meet one of the following criteria:

  • You reach your desired bclock and successfully pass your test.
  • You reach your maximum safe CPU voltage.
  • Raising the CPU voltage does not allow for additional stability.
  • Maximum safe CPU voltage

For there is no maximum “safe” CPU voltage in my book. My recommendation is to determine your maximum safe voltage based on your temperatures while running your stability test. With stock air cooling this could be as low as 1.3V on some i7 CPUs while running MPrime. Or it could be as high as 2.2V when attempting Super PI 1M with an i5 670 on liquid nitrogen. Personally, I don’t like to see my load temperatures exceed 90C on air or water cooling, but it’s really up to you.

Is it stable?

So, once you find your highest CPU frequency by meeting one of the criteria above, lower the bclock by 5MHz, and run your selected stability test until you are satisfied. If you are looking for a stable system as a power user or gamer, Mprime for six hours is more than sufficient in my testing, but you may run longer if you desire. But for a true test of stability, I always like to play Crysis while encoding a Blu-Ray movie into an mpeg4 format.


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