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Step 2: Examining Power Consumption

Core i7-870 Overclocking And Fixing Blown P55-Based Boards
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Overclocking our Core i7 processor at moderate voltage levels was enough to blow the voltage regulators of three budget/performance motherboards, and the primary purpose of this article was to define the cause of that damage. How much of a power monster is an overclocked Core i7-870?

Two important numbers are needed to get a reasonably-accurate estimate of the CPU power draw when given the above global wattage numbers. The first is Intel’s 95W maximum TDP for the stock Core i7-870 processor and the second is power-supply efficiency, which has been independently rated at 90%. Assuming we really did reach full TDP for the stock processor by using eight Prime95 threads, the full system power (178W) minus power supply inefficiency (17.8W) and processor power leaves 65W remaining for “everything but the processor.” Because the GPU remained idle throughout this power-consumption test, this number is believable.

At 3.8 GHz and 1.25V, the system consumed 225W. If we subtract the 22.5W of power supply inefficiency and the 65W used by other components, we’re left with a processor that uses 137W of power. That’s getting pretty close to the 150W ASRock said its board was designed to provide.

At 4.0 GHz and 1.35V, the system consumed 274W. Subtracting the 27.4W of power supply waste and 65W used by other components leaves us with a processor power-consumption figure of 181W.

And now for the “big” number from our standard 1.45V test: at 4.3 GHz, our system consumed 339W, which, subtracting for 33.9W of power supply loss and 65W for other components, yields a CPU power consumption number of 240W. CPU power consumption exceeds the “sane” limit of 200W even if our “other component” estimate is off by as much as 40W.

Though we seriously doubt component power consumption estimates are off by so large an amount, CPU voltage regulator efficiency is one more variable that can’t be assessed because the motherboard itself stands in our way. Efficiency is usually inversely proportional to heat, and overclocking really pushed Asus’ voltage regulator hard.

With temperatures and stress changing the efficiency of other components, the best we can hope to do is to look at the numbers above and take an educated guess. Our data above proves that the processor consumes no more than 240W when fully overclocked, and our best estimate is that actual power consumption falls somewhere between 200W and 240W with the processor under full load at our maximum overclock.

If program performance were exactly proportional to clock speed, our system would lose 7%, 18%, and 30%, respectively, in full-load efficiency by overclocking at 1.25V, 1.35V, and 1.45V CPU core, respectively. On the other hand, idle efficiency only drops markedly in response to shutting off power-savings features.

To determine actual efficiency, we have to consider the actual performance benefit from overclocking. This is mitigated somewhat by graphics-centric benchmarks that did not benefit much, with our highest overclock showing a mere 18% performance boost.

The result of huge power losses with moderate performance gains is a decrease in efficiency of over one third at our highest settings.

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  • 13 Hide
    bucifer , November 3, 2009 11:50 AM
    I would be great to see how the more popular i7 860 or at least i5 750 scale with the voltage.
    I don't think i7 870 is a popular choice because of it's price (people would go for socket 1336)
Other Comments
  • 6 Hide
    cyberkuberiah , November 3, 2009 6:19 AM
    but some of us would rather give some extra beans and go 920 , and have dual pcie2.0 x16 . a few extra watts doesn't matter too .
  • 1 Hide
    Anonymous , November 3, 2009 7:02 AM
    FYI: Power consumption of switching cmos silicon increases with the square of voltage, and linear with frequency. The increases shown here seem to be in line with that, rather than the stated decrease in voltage regulator efficiency (which certainly does decrease, but probably much less).
  • -5 Hide
    Crashman , November 3, 2009 8:01 AM
    dan__gFYI: Power consumption of switching cmos silicon increases with the square of voltage, and linear with frequency. The increases shown here seem to be in line with that, rather than the stated decrease in voltage regulator efficiency (which certainly does decrease, but probably much less).


    Can you turn that into a more accurate estimate than 200W to 240W, where all that can be proven is that it's "high, but less than 240W"?
  • -8 Hide
    jeffunit , November 3, 2009 11:32 AM
    Are your power consumption measurements of the cpu, dc power or wall socket power? If they are the latter, which I suspect they are, then you have to factor in the power supply efficiency, as 150w socket, means 150w DC.
  • 13 Hide
    bucifer , November 3, 2009 11:50 AM
    I would be great to see how the more popular i7 860 or at least i5 750 scale with the voltage.
    I don't think i7 870 is a popular choice because of it's price (people would go for socket 1336)
  • 0 Hide
    ctbaars , November 3, 2009 12:32 PM
    Thanks for article.
    For me - This and previous articles have convinced me to game at stock, w/ tb+ settings on, and a high end GPU card and the i5 is most appropriate for my usage. I need to condition myself to turn off the computer esp. when noone is home.
  • 0 Hide
    avatar_raq , November 3, 2009 2:40 PM
    Although Thomas labels Asrock as "succeeds" I will not buy their motherboards, you'll never know what else this company ignores in the bios, and do you think they would fix that issue if it weren't for THG? After how many failing boards?
  • 2 Hide
    tecmo34 , November 3, 2009 2:52 PM
    cyberkuberiahbut some of us would rather give some extra beans and go 920 , and have dual pcie2.0 x16 . a few extra watts doesn't matter too .

    I agree with you 110%... :D 

    Also, I would like to see the voltage scaling using the i5 750, as mentioned by bucifer
  • 2 Hide
    Onus , November 3, 2009 3:10 PM
    A few extra watts being "used" is fine. A few extra watts being "wasted" is something else entirely.
    I don't see a howling difference on these overclocks either. If I bought an i7, that probably means I'd have little reason to OC it.

    While ASRock seems to be taking a "successive approximations" approach to improving their products, the ones I've bought so far have all been solid, but any OC has been mild.
    And, once again (even if it isn't quite epic), MSI = FAIL.
  • 0 Hide
    jerreece , November 3, 2009 3:49 PM
    I was glad to see this article. I was just thinking about this whole debacle this morning. :) 
  • -3 Hide
    Anonymous , November 3, 2009 3:54 PM
    "The result of huge power losses with moderate performance gains is a decrease in efficiency of over one third at our highest settings"

    The first thing i care about when over clocking is being "green"
    Why is this even in the report?
  • 5 Hide
    Crashman , November 3, 2009 4:02 PM
    Antigreen"The result of huge power losses with moderate performance gains is a decrease in efficiency of over one third at our highest settings"The first thing i care about when over clocking is being "green" Why is this even in the report?


    Sometimes you can acgtually gain efficiency when overclocking: This is especially true when voltage levels aren't altered.
  • 2 Hide
    cyberkuberiah , November 3, 2009 4:53 PM
    avatar_raqAlthough Thomas labels Asrock as "succeeds" I will not buy their motherboards, you'll never know what else this company ignores in the bios, and do you think they would fix that issue if it weren't for THG? After how many failing boards?


    i'd go with evga/asus ,and for amd , gigabyte or asus . the crosshair 3 formula is top end at just 200 dollars .
  • 2 Hide
    Shadow703793 , November 3, 2009 6:05 PM
    Can some one please make a list of what motherboards use the problamatic Foxconn socket?
  • 2 Hide
    warezme , November 3, 2009 6:13 PM
    Whats wrong with 3.8Ghz? Its good overclock, with minimal stress on all your junk. Why folks have to push their stuff to 4Ghz or higher stressing the hell out of the hardware just for a couple more lousy FPS. I have an X58 that can push voltage and run at 4.2Ghz but voltage and heat requirements go up way to much and only give me a few more FPS. Its not really worth it.
  • 0 Hide
    grimjester , November 3, 2009 6:55 PM
    CrashmanCan you turn that into a more accurate estimate than 200W to 240W, where all that can be proven is that it's "high, but less than 240W"?


    I'm not sure how to interpret the results, but the best fit I get for trying to get a constant W / (GHz * V^2) is a base load of only 7W plus a draw of 36.63-36.72W * frequency in GHz * voltage squared. The fit is fairly accurate; there's a 0,26% difference between the min and the max.

    Obviously stuff other than the CPU draws more than 7W, but I don't know enough about the hardware to give an explanation. I'd assume that you get fairly close to 7W + (voltage^2 * GHz * 36,7W) if you measure the draw at other speeds and voltages though.
  • 0 Hide
    Proximon , November 3, 2009 7:21 PM
    Thanks Crashman, this goes towards a resolution and at least we have a few lower budget boards now that look to be relatively safe.

    Didn't you use a different PSU last time? Playing it safe with the higher quality 850HX maybe?
  • -1 Hide
    Crashman , November 3, 2009 7:21 PM
    Shadow703793Can some one please make a list of what motherboards use the problamatic Foxconn socket?


    ASRock, Asus, Biostar, ECS, Foxconn, Gigabyte, and MSI use Foxconn sockets. Jetway and EVGA use the cheaper Lotes sockets.
  • 5 Hide
    Crashman , November 3, 2009 7:38 PM
    warezmeWhats wrong with 3.8Ghz? Its good overclock, with minimal stress on all your junk. Why folks have to push their stuff to 4Ghz or higher stressing the hell out of the hardware just for a couple more lousy FPS. I have an X58 that can push voltage and run at 4.2Ghz but voltage and heat requirements go up way to much and only give me a few more FPS. Its not really worth it.


    Uh, d00d, let me see if I can explain this in terms you can understand: 1.45V has been used for 45nm Intel processors long enough that it's now a standardized OC test voltage. There are many reasons for it having become this standardized test voltage, including the fact that it's considered the maximum safe voltage in some Intel documentation, that it's the maximum voltage most processors can run using above-ambient cooling, that it's the spot just before power consumption spikes, etc. It makes sense, and because it's NOT extreme, was never extreme, was never intended to be extreme, and is in no way extreme, it's something that any overclocking motherboard should tollerate.

    We understand that cheap boards exist. If you're going to market a cheap board towards low-cost overclocking, you need to put in over-current protection. If you're going to market an even cheaper board with no protection, you need to disable the overclocking features.

    It's one way or the other, when it comes to overclocking either do it right or don't do it at all. Half-fast solutions aren't acceptable in the overclocking market. It's a quality issue, and Tom's Hardware has tested MANY high-quality budget parts in the past.

    There's no excuse to cut quality when you can instead cut features to produce a cheap product. IE, if you really really really wanted to make a board that could only do 1.35V before blowing the VRM, and really wanted to sell it without overcurrent protection, you'd really really really want to limit the BIOS settings to 1.35V. Because when you didn't, you'd get caught with your pants down by a site such as this one.

    To not report such a finding would be proof of a lack of integrity. To give up testing at this setting would be to cave in for low-quality products at the expense of not revealing the superiority of high-quality products. The reader isn't served, the industry is disserviced, everyone loses.


    ProximonThanks Crashman, this goes towards a resolution and at least we have a few lower budget boards now that look to be relatively safe.Didn't you use a different PSU last time? Playing it safe with the higher quality 850HX maybe?


    Nah, same power supply since September, might have forgotten to change the model in the setup table.
  • -1 Hide
    JeanLuc , November 3, 2009 8:01 PM
    cyberkuberiahbut some of us would rather give some extra beans and go 920 , and have dual pcie2.0 x16 . a few extra watts doesn't matter too .


    Why do you want dual x16 slot when it offers no extra benefit? You might want to give this a read before you start clicking that thumbs down button, and if you do disagree please tell me why I'm wrong.

    Even the most powerful card in the world can't saturate an x8 slot according to that source.
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