Turbo Boost, BCLK, QPI: Exploring Intel's Core i7, i5, i3 CPUs

Intel made a few important changes when they switched to the new Core i7, i5, and i3 architecture, and you'll need to know about these changes to effectively overclock your system.

So, in this guide, we'll explore all of the important ones, such as:

  1. What determines the CPU frequency in the new system.
  2. How the new base clock rate (BCLK) and quick path interconnect (QPI) work.
  3. And how the new turbo boost functionality can dynamically alter your speed based on demand.

What Determines the CPU's Speed?

The formula for calculating the CPU's speed is actually pretty simple:

CPU Speed = Base Clock Rate * Multiplier

So all we need to do to increase the CPU speed is to increase the base clock rate (BCLK) or multiplier.

If you take a look at the picture below, you'll see the overclocking section of the BIOS for the ASUS P7P55D-E Pro motherboard that we used for this guide.

asus p7p55d-e pro bios ai tweaker frequencies

Overclocking Section of the BIOS on an Asus P7P55D-E Motherboard.

We'll now go over each of these important frequencies, so you'll know how to manipulate them when you're overclocking.

Base Clock Rate (BCLK)

If you're familiar with overclocking earlier Intel CPUs, the equation in the previous section probably looks familiar--except the Front Side Bus (FSB) has been replaced with a Base Clock Rate (BCLK).

The FSB used to be the main method for communicating between all of the different parts of the system, but this has now been replaced by a new Quick Path Interconnect (QPI) system.

In the new QPI system, BCLK is used to determine the frequencies of all of the other major components, such as the CPU, memory, and QPI. So, whenever you increase BCLK, all of these speeds will also get overclocked by default.

The default BCLK for 1st generation i7, i5, and i3 systems is 133 MHz, and for the newer Sandy Bridge and Ivy Bridge, it's 100 MHz.

Quick Path Interconnect (QPI)

The FSB was like any highway in your city: one road with a bunch of lanes going in both directions. However, as we all know, the main highway can get seriously congested during rush hour because everyone's trying to use the same road.

Well, the FSB was no different. It could also bottleneck the system when there was a lot going on because multiple components were trying to use the same bus to communicate at the same time.

QPI was meant to fix this problem by providing a direct connection between each of the important parts of the system, so they could communicate without using a shared bus, therefore reducing the bottleneck.

CPU Multiplier

Intel has been using a multiplier to determine the CPU speed for years now, so this is nothing new if you've overclocked previous processor generations.

Since the default BCLK is the same for every new Intel processor, Intel simply changes the default multiplier for each CPU to arrive at its advertised speed. If we could simply change this value, we could easily change the speed of the processor, without having to mess with the rest of the system.

But this usually isn't possible because Intel locks the maximum multiplier on most of its CPUs, so you're usually forced to overclock by increasing BCLK, which (by default) directly affects the speed of the rest of the components in the system.

If you're using an extreme edition or unlocked processor, such as the i7-875K or i5-655K, your multiplier is unlocked, so you actually can overclock by just increasing the multiplier. However, you may still want to also overclock your BCLK because it will lead to higher system performance than just increasing your CPU multiplier. Then, if you do get limited by BCLK, you always have the processor multiplier to fall back on.

Turbo Boost

Turbo boost has added a slight twist to the way things worked before. It allows the CPU to run faster than its advertised speed if some of its cores aren't busy and the maximum power level hasn't been reached. It does this by using different multipliers depending on how many cores are busy.

The table below shows the maximum turbo multiplier (by processor) depending on how many cores are active.

Maximum Multiplier Default 4 Cores Active 3 Cores Active 2 Cores Active 1 Core Active
i7-870/875K 22 24 24 26 27
i7-860 21 22 22 25 26
i5-760 21 22 22 25 25
i5-750 20 21 21 24 24
i5-680 27 NA NA 28 29
i5-670 26 NA NA 27 28
i5-660/661 25 NA NA 26 27
i5-650/655K 24 NA NA 25 26

So, if you're using an i5-760 and turbo boost is enabled, below are the maximum speeds depending on how many cores are busy.

Maximum Speed with:

  • 1 or 2 Cores Active: 133 * 25 = 3.325 GHz
  • 3 or 4 Cores Active: 133 * 22 = 2.926 GHz
  • Default (without turbo boost): 133 * 21 = 2.793 GHz

What Next?

This article is part of our ultimate overclocking guide: How to Overclock: Intel Core i7, i5, and i3 CPU Overclocking Guide.

If you're following along in the guide, you can get back to where you just were by clicking the link that follows: .