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From 2.66 To 4.0 GHz

Overclocking Core i7: Power Versus Performance
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Core i7 920 Default Clock Speed: 2.66 GHz

When idle, the Core i7 will reduce its clock speed to 1,600 MHz if SpeedStep is enabled. This equals the 133 MHz base clock times 12.

At peak, the maximum clock speed switches to 2.66 GHz.

If a single thread creates a 100% CPU load, the CPU can increase the multiplier from the x20 default (2.66 GHz) to x21, which results in an effective 2.8 GHz clock speed.

If the CPU load stays at 100% for only one core, while the operating system dispatcher has little work to do for the remaining cores, the Core i7 can increase the core clock speed to 2.93 GHz by incrementing the multiplier from x21 to x22. Keep in mind that Turbo Mode applies only to one core.

Core i7 920 at 3.33, 3.66, 3.8 and 4.0 GHz

At 3.33 GHz we could still work with the default voltage of 1.25 V.

The Turbo Mode pushed one core from 3.33 to 3.48 GHz.

A speed of 3.66 GHz required a slightly increased core voltage of 1.275 V.

At 3.8 GHz, we needed to increase the CPU core voltage to 1.345 V for stable operation.

Reaching 4.0 GHz core clock speed required a massive core voltage increase to 1.50 V.

SpeedStep still works even at a 4.0 GHz core speed and 200 MHz base clock, reducing the clock speed to 2.4 GHz (multiplier 12x) when idle. However, Turbo Mode doesn’t work reliably at more than 3.33 GHz anymore, for voltage/stability reasons.

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  • 6 Hide
    eximious , April 13, 2009 8:14 AM
    Interesting and insightful article. It might be interesting to do a similar analysis when the new D0 stepping is widely available. Thanks for including a variety of video encoding / editing benchmarks too.
  • 0 Hide
    zedx , April 13, 2009 8:14 AM
    No undervolting / underclocking? I'm sure you can greatly improve efficiency by doing something like a clock of around 2.4 ghz and a voltage of around 0.8 - 0.9 volts... Even in default you might be able to undervolt quite a bit...
  • -2 Hide
    tacoslave , April 13, 2009 8:15 AM
    i wish i had one
  • -6 Hide
    onerec , April 13, 2009 9:01 AM
    does this mean that AMD phenom2 is better than Intel when it comes to overclocking?
  • 7 Hide
    falchard , April 13, 2009 10:22 AM
    lol no, this one went from 2.66 to 4.0. In the other comparison with the Phenom II it went from 3.0 to 3.8.
    What I find interesting is that both articles found the magic number to be 3.6 ghz.
  • 5 Hide
    jonpaul37 , April 13, 2009 12:43 PM
    3.6 - 3.8 seems to be the universal sweet-spot for any CPU that allows for the OC. I've OC'd my E8400 to 3.6 and get great results, anything higher might yield an FPS or 2 more, but at the possible expense of more power.
  • 1 Hide
    Anonymous , April 13, 2009 12:46 PM
    Ever since the stock P4 3.6ghz came out, the magic number has been 3.6, every new generation of CPUs since then, people have expected to get somewhat further, but IMO it's safe to say that without some radically new technology, the magic number will always be about 3.6.
  • -4 Hide
    Anonymous , April 13, 2009 12:59 PM
    I will stick with my q9650 overclocked to 4.0Ghz at 1.24v ntm the 8gb ram at 1066 mhz. lolz
  • 4 Hide
    TripGun , April 13, 2009 1:39 PM
    Very good article. However , I do have a problem with the test being ran with an "engineering sample" chip. A lot of boxed I7's won't hit 3.66ghz without at least 1.35 volts. Good read though.
  • 0 Hide
    Anonymous , April 13, 2009 1:40 PM
    Nice article.

    Thanks.
  • 1 Hide
    Pei-chen , April 13, 2009 2:00 PM
    I always believe the sweet spot is half way of stock speed and max OC. In this case, 3.33GHz is the sweet spot while 3.66 means you got a good overclocker. Good read.
  • -2 Hide
    Anonymous , April 13, 2009 2:39 PM
    I second the need for a underclock/undervolt analysis. Also, how about seeing exactly how voltage affects the processor without changing the clock speed. Many people hit 3.8-4Ghz at lower voltages so a base comparison on how much voltage affects the power (theoretically squared relationship) would be helpful...
  • 0 Hide
    Anonymous , April 13, 2009 2:47 PM
    I agree from the results that the safest overclock is 3,33Ghz with turbo mode enabled, for the casual gamer.
    Besides, you will not notice much difference in games running them at 3,33 or 3,66.
    Also the life expectancy of overclocking the CPU @3,33, is longer than @3,66Ghz.

    I wished sometimes 'underclocking' would be done at toms, to see how much power one can save when he's only writing documents, or browsing the web with such a powerful machine. I mean, unless you're a gamer, the computer stays most of the time in a passive mode (if not turned off).

    About the 'magical number',this probably changes with the die. When a processor is created @ 45nm the best results might be 3,6Ghz; but these results should be different on larger or smaller dies (eg: 65nm, 95nm, or the upcoming 40nm, or 32nm). It also has to do with the materials used. Current 45nM processes by intel are done with high metal K gates and stuff, they allow greater overclocking to the standard silicon processors...
  • 0 Hide
    Anonymous , April 13, 2009 2:51 PM
    If everything goes like now, and processors on a smaller die (32 nm, 28nm,...) will be identical copies of current processors, we might see a trend that the smaller the die, the smaller the possible overclocking.
    It all depends on how thick of a layer of insulation Intel uses between the transistors (in on-off switching by lack of words), and if they will invent or discover newer more efficient materials to develop processors or not.
  • 7 Hide
    theJ , April 13, 2009 2:59 PM
    I just ran some calculations on these power consumptions.

    Assuming you keep your computer on 365/24/7, at peak power all the time:
    -$104.49/year for a 2.66 Ghz
    -$204.56/year for 4.0 Ghz

    For a medium usage user: 365 days per year, 4 hours at peak, 20 hours at idle:
    -$61.97/year for 2.66 Ghz
    -$94.60/year for 4.0 Ghz

    For a more modest user: 365 days per year, 2 hours at peak per day, 4 hours at idle:
    -$17.62/year for 2.66 Ghz
    -$29.15/year for 4.0 Ghz

    This assumes 5.6 cents/kWh.

    This is just to give everyone a more convenient way to track power. I know i don't have a feel for 100 W compared to 200 W.
  • 0 Hide
    cadder , April 13, 2009 3:06 PM
    Stock cooling??? I'm not believing that.

    WRT undervolting, maybe we need new mobo features to allow custom "speedstep" features. Run the processor at max. OC speed when it needs it, then drop to stock speed or below when at idle or at low use.
  • 0 Hide
    optimus290 , April 13, 2009 5:05 PM
    these are good performance gains :) . but its not something ground breaking. :|
  • 0 Hide
    mcnuggetofdeath , April 13, 2009 5:07 PM
    cadderStock cooling??? I'm not believing that.WRT undervolting, maybe we need new mobo features to allow custom "speedstep" features. Run the processor at max. OC speed when it needs it, then drop to stock speed or below when at idle or at low use.

    Thats what got me too, with the abundance of good and cheap air cooling solutions negating the effects of Intel's Overspeed protection shouldnt be hard even w/o a board with a BIOS option to that effect.
  • 0 Hide
    funkjunky , April 13, 2009 5:40 PM
    I know I would rather have a bios option for better underclocking, so it is more transparent. Like super speedstep, where it drops the multiplier even more ;) .

    I can't see a reason for tom's to give underclocking numbers, until their are better native dynamic underclocking features on mobos. No one is going to manually under clock their computer when they go and change to writing documents... it needs to be automatic.

    Assuming Thej's numbers are even close to accurate, then having that extra bit of under clocking won't save you anymore than $5-$10, because most core components will still be running taking juice.

    This article was a convenient read, and even more convenient thanks to Thej =).
  • -5 Hide
    funkjunky , April 13, 2009 5:40 PM
    I know I would rather have a bios option for better underclocking, so it is more transparent. Like super speedstep, where it drops the multiplier even more ;) .

    I can't see a reason for tom's to give underclocking numbers, until their are better native dynamic underclocking features on mobos. No one is going to manually under clock their computer when they go and change to writing documents... it needs to be automatic.

    Assuming Thej's numbers are even close to accurate, then having that extra bit of under clocking won't save you anymore than $5-$10, because most core components will still be running taking juice.

    This article was a convenient read, and even more convenient thanks to Thej =).
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