Overclocking, the K-Series and What You’ll Want to Buy

If you haven’t noticed, the computing world is becoming more integrated. We review highly integrated SoCs in our smartphone coverage, and even on the desktop we’re seeing movement towards beefy SoCs. AMD pioneered the integrated memory controller on desktop PCs, Intel followed suit and with Lynnfield brought a PCIe controller on-die as well. Sandy Bridge takes the next logical step and brings a GPU on-die, a move matched by AMD with Brazos and Llano this year.

In the spirit of integration, Intel made one more change this round: the 6-series chipsets integrate the clock generator. What once was a component on the motherboard, the PLL is now on the 6-series chipset die. The integrated PLL feeds a source clock to everything from the SATA and PCIe controllers to the SNB CPU itself. With many components driven off of this one clock, Intel has locked it down pretty tight.

With Nehalem and Westmere, to overclock you simply adjusted the BCLK from 133MHz to whatever speed you wanted and sometimes toyed with multipliers to arrive at a happy end result. With Sandy Bridge, the BCLK generated on the 6-series PCH is at 100MHz by default and honestly won’t go much higher than that.

While I’ve heard reports of getting as high as 115MHz, I’d view 103—105MHz as the upper limit for what you’re going to get out of BCLK overclocking. In other words: next to nothing. A 105MHz BCLK overclock on a Core i7-2600 will take you from a stock speed of 3.4GHz to a whopping 3.57GHz. The form of overclocking we’ve been using for the past decade is effectively dead on Sandy Bridge.

Years ago, before the Pentium II, we didn’t rely on BCLK (or back then it was just FSB or bus overclocking) to overclock. Back then, if we wanted a faster CPU we’d just increase the clock multiplier. Intel has dabbled in offering multiplier unlocked parts for overclockers, we saw this last year with the Core i7 875K for example. With Sandy Bridge, those unlocked parts are going to be a lot more important to overclockers.

It works like this. If you have a part that does not support Turbo (e.g. Core i3-2100 series), then your CPU is completely clock locked. You can’t overclock it at all, have fun at your stock frequency. This is good news for AMD as it makes AMD even more attractive at those price points.

If you have a part that does support turbo (e.g. Core i5-2400), then you have what’s called a “limited unlocked” core—in other words you can overclock a little bit. These parts are limited to an overclock of 4 processor bins above and beyond the highest turbo frequency. Confused yet? This chart may help:

In this case we’re looking at a Core i5-2500, which runs at 3.3GHz by default. When a single core is active, the chip can turbo up to 3.7GHz. If you want, you can change that turbo state to go as high as 4.1GHz (if your CPU and cooling can keep up).

Overclocking these limited unlocked chips relies entirely on turbo however. In the case above, the fastest your chip will run is 4.1GHz but with only one core active. If you have four cores active the fastest your chip can run is 3.8GHz. While Intel didn’t sample any limited unlocked parts, from what I’ve heard you shouldn’t have any problems hitting these multiplier limits.

There’s a third class of part: a fully unlocked K-series chip. At launch there are only two of these processors: the Core i5-2500K and the Core i7-2600K. Anything with a K at the end of it means you get all multipliers from 16x all the way up to 57x at your disposal. It’s effectively fully unlocked.

These chips overclock very well. Both my Core i5-2500K and Core i7-2600K hit ~4.4GHz, fully stable, using the stock low-profile cooler.

This is all you need for 4.4GHz

With a bit more effort and a better cooler, you can get anywhere in the 4.6-5.0GHz range:

It's a bit too early to tell how solid these near-5GHz overclocks will be, but I'm confident in the sub-4.5GHz overclocks we were able to sustain.

You do pay a price premium for these K-series SKUs. The 2500K will cost you another $11 over a stock 2500 and the 2600K costs an extra $23. In the case of the 2500K, that’s a small enough premium that it’s honestly worth it. You pay $11 extra for a chip that is very conservatively clocked and just begging for you to overclock it. Even the 2600K’s premium isn’t bad at all.

Model Number Standard SKU K-Series SKU Price Premium
Intel Core i7-2600 $294 $317 +$23
Intel Core i5-2500 $205 $216 +$11

As an added bonus, both K-series SKUs get Intel’s HD Graphics 3000, while the non-K series SKUs are left with the lower HD Graphics 2000 GPU.

Compared to Lynnfield, you’re paying $11 more than a Core i5-760 and you’re getting around 10-45% more performance, even before you overclock. In a perfect world I’d want all chips to ship unlocked; in a less perfect world I’d want there to be no price premium for the K-series SKUs, but at the end of the day what Intel is asking for here isn’t absurd. On the bright side, it does vastly simplify Intel’s product stack when recommending to enthusiasts: just buy anything with a K at the end of it.

Since we’re relying on multiplier adjustment alone for overclocking, your motherboard and memory actually matter less for overclocking with Sandy Bridge than they did with P55. On both P67 and H67, memory ratios are fully unlocked so you can independently set memory speed and CPU speed. Even the GPU ratios are fully unlocked on all platforms and fully independent from everything else.

The Lineup The 6-series Platform & 6Gbps SATA Performance
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  • samrty22331 - Wednesday, June 1, 2011 - link

    visual studio 2010 professional not supported intel i5 2500k
    can you say
    how to install vs 2010 in this
  • Okurka - Saturday, August 13, 2011 - link

    The base clock of the HD 3000 GPU is 1100 MHz, not 850 MHz.
    That makes the 1550 MHz an overclock of 40,9 %, not 82,4 % as stated in the article.
  • khalnayak - Sunday, January 22, 2012 - link

    I am having Intel HD 3000 - Sandy Bridge in my system and i was willing to get the game called "oil rush" but then i have found weird response for the game here http://www.futurehardware.com/pc-gaming/288.htm , so i just wanted to know is there any one who have tested the Intel HD 3000 - Sandy Bridge for oil rush, any help for this will be highly appreciated.
  • thr0nez101 - Sunday, January 29, 2012 - link

    I've got intel hd graphics 3000 and according to this forum/review it has a prob running dawn of war 2 on low graphics... i have it set to max graphics and i runs a dream... same with a lot of games i play on it...
  • oliverr - Saturday, February 11, 2012 - link

    guys, is it safe to overclock the Intel HD 3000 GPU ? I own a 2500K CPU. I can overclock the GPU to 1450mhz and it looks stable . But i dont know how to read the temperature from the GPU unit, so iam afraid i could burn my GPU/CPU .
  • cerberaspeed12 - Thursday, February 16, 2012 - link

    hi.first off all sorry for my english.I have a doubt .I have seen the dells lap top.they are identical but one have the Intel Core i3-2350M 2.3GHz ,the odher is Intel Core i5-2450M 2.5GHz , and the third have Intel Core i7 -2670M 2.4GH

    the prices is 600 $,670$ and 800 $,I am working some live multi channel audio production and .net teh programing.So for wich one i soud go.Thanks
  • indyaah - Tuesday, February 21, 2012 - link

    any suggestions how can i??
  • weirdo2989 - Sunday, March 4, 2012 - link

    Hi Techies,

    I recently got this processor. It is ultimate for gaming.
    However in my windows CPU meter gadget, i can see only 2 cores functioning. Stock comes with unlocked multiplier afaik. But here in my system, it shows only 2 cores. Is there any way to activate all the cores for better performance?

    Any suggestions/tips would be highly appreciated.


  • 0121birmingham - Saturday, May 12, 2012 - link

    Just to say i wrote a small post on this issue at http://intel23976fpsproblem.blogspot.co.uk/
    It does not look like the problem has been fixed in the new z77 line up. DAM
  • milutzuk - Saturday, July 14, 2012 - link

    Beside VS2008 compiler performance I would like to see growing a database with some Java compiler performance, either under NetBeans or Eclipse. Thank you.

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