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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  • Taft12 - Tuesday, January 4, 2011 - link

    You first.
  • ReaM - Tuesday, January 4, 2011 - link

    the six core 980x still owns them in all tests where all cores are used.

    I dont know 22k in cinebench is really not a reason to buy the new i7, I reach 24k on air with i7 860 and my i5 runs on 20k on air.

    Short term performance is real good, but I dont care if I wait for a package to unpack for 7 seconds or 8, for long term like rendering, neither there is a reason to upgrade.

    I recommend you get the older 1156 off ebay and save a ton of money.

    I have the i5 on hackintosh, I am wondering if 1155 will be hackintoshable
  • Spivonious - Tuesday, January 4, 2011 - link

    I have to disagree with Anand; I feel the QuickSync image is the best of the four in all cases. Yes, there is some edge-softening going on, so you lose some of the finer detail that ATi and SNB gives you, but when viewing on a small screen such as one on an iPhone/iPod, I'd rather have the smoothed-out shapes than pixel-perfect detail.
  • wutsurstyle - Tuesday, January 4, 2011 - link

    I started my computing days with Intel but I'm so put off by the way Intel is marketing their new toys. Get this but you can't have that...buy that, but your purchase must include other things. And even after I throw my wallet to Intel, I still would not have a OC'd Sandy Bridge with useful IGP and Quicksync. But wait, throw more money on a Z68 a little later. Oh...and there's a shiny new LGA2011 in the works. Anyone worried that they started naming sockets after the year it comes out? Yay for spending!

    AMD..please save us!
  • MrCrispy - Tuesday, January 4, 2011 - link

    Why the bloody hell don't the K parts support VT-d ?! I can only imagine it will be introduced at a price premium in a later part.
  • slick121 - Tuesday, January 4, 2011 - link

    Wow I just realized this. I really hate this type of market segmentation.
  • Navier - Tuesday, January 4, 2011 - link

    I'm a little confused why Quick Sync needs to have a monitor connected to the MB to work. I'm trying to understand why having a monitor connected is so important for video transcoding, vs. playback etc.

    Is this a software limitation? Either in the UEFI (BIOS) or drivers? Or something more systemic in the hardware.

    What happens on a P67 motherboard? Does the P67 board disable the on die GPU? Effectively disabling Quick Sync support? This seems a very unfortunate over-site for such a promising feature. Will a future driver/firmware update resolve this limitation?

  • NUSNA_moebius - Tuesday, January 4, 2011 - link

    Intel HD 3000 - ~115 Million transistors
    AMD Radeon HD 3450 - 181 Million transistors - 8 SIMDs
    AMD Radeon HD 4550 - 242 Million transistors - 16 SIMDs
    AMD Radeon HD 5450 - 292 Million transistors - 16 SIMDs
    AMD Xenos (Xbox 360 GPU) - 232 Million transistors + 105 Million (eDRAM daughter die) = 337 Million transistors - 48 SIMDs

    Xenos I think in the end is still a good two, two and a half times more powerful than the Radeon 5450. Xenos does not have to be OpenCL, Direct Compute, DX11 nor fully DX10 compliant (a 50 million jump from the 4550 going from DX10.1 to 11), nor contains hardware video decode, integrated HDMI output with 5.1 audio controller (even the old Radeon 3200 clocks in at 150 million + transistors). What I would like some clarification on is if the transistor count for the Xenos includes Northbridge functions..............

    Clearly PC GPUs have insane transistor counts in order to be highly compatible. It is commendable how well the Intel HD 3000 does with only 115 Million, but it's important to note that older products like the X1900 had 384 Million transistors, back when DX9.0c was the aim and in pure throughput, it should match or closely trail Xenos at 500 MHz. Going from the 3450 to 4550 GPUs, we go up another 60 million for 8 more SIMDs of a similar DX10.1 compatible nature, as well as the probable increases for hardware video decode, etc. So basically, to come into similar order as the Xenos in terms of SIMD counts (of which Xenos is 48 of it's own type I must emphasize), we would need 60 million transistors per 8 SIMDs, which would put us at about 360 million transistors for a 48 SIMD (240 SP) AMD part that is DX 10.1 compatible and not equipped with anything unrelated to graphics processing.

    Yes, it's a most basic comparison (and probably fundamentally wrong in some regards), but I think it sheds some light on the idea that the Radeon HD 5450 really still pales in comparison to the Xenos. We have much better GPUs like Redwood that are twice as powerful with their higher clock speeds + 400 SPs (627 Million transistors total) and consume less energy than Xenos ever did. Of course, this isn't taking memory bandwidth or framebuffer size into account, nor the added benefits of console optimization.
  • frankanderson - Tuesday, January 4, 2011 - link

    I'm still rocking my Q6600 + Gigabyte X38 DS5 board, upgraded to a GTX580 and been waiting for Sandy, definitely looking forward to this once the dust settles..

    Thanks Anand...
  • Spivonious - Wednesday, January 5, 2011 - link

    I'm still on E6600 + P965 board. Honestly, I would upgrade my video card (HD3850) before doing a complete system upgrade, even with Sandy Bridge being so much faster than my old Conroe. I have yet to run a game that wasn't playable at full detail. Maybe my standards are just lower than others.

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