Undervolting a GTX 660

First things first, I know people will want to give me a whole host of reasons why I shouldn't want to do this but I'm really curious to see whether it can be done. So here goes...

I'm building a custom case SSF computer over the next month and I have a 100W limit for the graphics card. Unfortunately I'm also restricted to an nVIDIA card since I'm dual booting OSX and have already purchased an i5 3450 with an HD 2500 GPU (Not ideal I know). I was initially planning to use a GTX 650 in the build since the power draw is perfect but as we've all seen its hardly a powerful card and I'm used to the high end (times are hard).

So I have two options; wait and hope that the Ti comes out next month and is a worthwhile card, or, get a 660 and undervolt/clock it down to 100W draw. Now I'm impatient and also prefer to make the things I want work for me instead of wanting the things that already work. Plus the idea that I'll be able to overclock it back into a powerful card once I'm over this SFF obsession I have at the moment is also nice.

What kind of reduction in clocks/voltage am I looking at to knock 30W or more off the 660 and can this be done through software? Whether this would present issues in OSX is also a concern but I can't imagine why it would (I'm yet to explore whether the 660 is compatible at all but Kepler is doing pretty well so far).

I suppose a massive GTX 650 overclock is also an option but I assume there are less hurdles with an underclock. I guess nobody quite has these cards yet but I'd appreciate the experiment if anyone shares my curiosity.

AFAIK,Kepler cards lack proper voltage controls last time I checked unless you go for specific models. It might be worth also asking over on OcUK as there are threads on Kepler overclocking,which might be useful.

Why do you have a 100W limit for the graphics card?? Are you using a picoPSU?? Even with a 200W one with a suitable power block such as a modded Dell DA2,I would be a bit careful overloading them. The DC-DC PSUs can run very hot under excessive load and you need a good quality power block.

You cannot be TDP or space limited if you intend to use a GTX660 and a Core i5 3450. Would this PSU fit:

http://www.amazon.co.uk/dp/B0047DFX7...SIN=B0047DFX7Q

I use one in a modded Shuttle G2 case. The 300W version is also capable.

Unless you are undervolting and underclocking the Core i5 3450, I would have gone for one for the Core i5 3470S or even one of the 35W Core i3 dual cores.

Edit!!

Here are some details of the GTX650TI and GTX655:

http://www.3dcenter.org/news/zwei-we...karten-geplant

I am undervolting the CPU, it worked out cheaper that way and I'll hopefully get to keep the stock clock. I'm using a flexATX PSU, the Seasonic SS250SU. I started off trying to make the smallest system I could with a uATX motherboard since miniITX boards are too expensive and wouldn't allow much in the future when I inevitably switch back to a normal case.

I'd already bought the PSU after finding it cheap on eBay before I realised that my aluminium supplier won't cut the panel sizes I want and that the minimum on any dimension is 100mm as opposed to 80mm in my design. I'm keeping the construction ridiculously easy since I need the project finished by the time I start back at uni, so it's all precut alumnium sheet, angle and pop rivets. The extra 2cm height has opened up the door to dual slot cards but it will still be a squeeze.

250W probably seems underpowered but I don't need it to last more than a year and it's more of an experiment. I figured everything but the GPU wouldn't get too close 150W so there's my headroom. There's no optical drive and I'm using an SSD and hopefully a NAS.

The Seasonic is not a bad PSU TBH. However,you do realise the 250W rating includes both the 3.3V and 5.0V lines too,and the PSU should not be run at 100% anyway,ideally not more than 70% to 80% of the 12V line(s) rating,as after all these are SFF PSUs.

Also,a mATX motherboard will consume more power than a mini-ITX motherboard.

However,TPU measures actually power consumption of the cards:

http://www.techpowerup.com/reviews/A..._Cu_II/25.html

It is quite possible the GTX650TI and GTX655 will consume around 100W or slightly less.

When I was modding my Shuttle G2 case,I chose the PC63J since it was easily the most powerful PSU in its form factor.

Edit!!

What I would do is get the rest of your build done, and check your power consumption at the wall using the IGP when the CPU is running a stress test.

This way you should be able to estimate what card you can use.

TPU was the review I based my 30W figure on. I'd like to believe that a PSU from a quality manufacturer that's rated for 250W can cope with being run at 80-90% for and hour or two at a time for a year. If you also check the power draw reading from sites that use a quad core testbed as opposed to the multitude of sites that use the hex core i7 you'll see that total system power is generally under 250W and that's with full ATX motherboards, higher power RAM and in many cases mechanical hard disks (plural). It should also be noted that these reading are taken at the wall whereas PSU ratings are what can be supplied, not what is drawn.

I understand that the different voltage line currents are very important but as far as I can judge the 12V line on the SS250SU is perfectly adequate as long as I don't over do it. I am confident that this PSU could struggle along with a stock clocked 660 but I'd rather give it a break. I really don't mind if this PSU gives up after a year and I trust Seasonic supplies not to take everything else with them when they die, but I'll put a protector on the 6 pin since I have a few lying around.

Due to funds I doubt that I'll have a card for the initial build so I can see how things go.

I think your underestimating how much wattage you have to play with. If your figuring on your 3450 with just an SSD will use 150w max I think your at least 50w over, especially as your undervolted! I'd be very suprised if you could even hit 100w with that. I think you have 150w to play with for the GPU and that includes some headroom. I guess you'll use the PSU at 80% in games.

Have fun. Also nowt wrong with undervolting if you wish to have low power, low temps. No one has any right to say to you you shouldnt do this :)

Butuz

Thank you! I was beginning to think I was crazy. I always try to use high estimates for these things since builds have been my side income for 6 years and I don't take risks.

The motherboard is the only uncertain quantity for power draw. I'd have thought by now with all the emphasis we're seeing on small form factors and low power systems that there would be more power analysis on motherboard reviews.

I think 50W is a reasonable estimate for a uATX motherboard, RAM and SSD. Aiming for 65W or less on the CPU which takes me to 115W and to keep 15% headroom I shouldn't go much over 210W which leaves me with just under 100W for the GPU.

A 7850 would be perfect but I'm yet to forgive the driver issues I had with a 4850 a few years ago and the HD 2500 IGP dictates nVIDIA. I wouldn't have this problem if I wasn't in America right now where I just got the 3450 for £110. I intended to buy the K originally so I could disable the graphics card in OSX and run on the HD 4000.

Thanks for you help guys. I'll check out the OCUK forums for undervolting info but I guess I'll just have to take the plunge. I've never failed to find exotic solutions to these kinds of problems.

Yeah it all sounds very viable to me. A good quality PSU (and Seasonic are about as good as they come) should be quite happy running at high loads for the odd gaming session.

Will be very interested to see what results you get from this.

Quote:

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Originally Posted by Butuz

No one has any right to say to you you shouldnt do this :)

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Rights?? What this got to do with rights?? :confused:

You have about 210w of 12v to work with on that PSU, that's MAX, not ideal load; ideally you'd want to keep it below 190 otherwise you risk crossing into overload at peak load.

It's reasonable to assume the rest of the platform, excluding GPU, will max around 100w max CPU load, possibly over 110 if you stress the IGP too, which would leave about 90w max for the GPU. You're *really* pushing it with that power supply; if you're very careful about what you run, try not to stress both GPU and CPU together you might get away with it, if you really can get the GPU down to 100w max.

I'm not that up to date on GPU underclocking, but since modern GPUs use power throttling to stop them frying the VRM circuitry when running apps like Furmark, there may be a way to take advantage of that. I wouldn't hold much hope for undervolting (by itself at least); last time I looked into it, it didn't make much difference to power consumption, but what I was just saying about the power throttling would have to taken into account too; if you lower power requirements for the GPU itself for a given load, the power circuitry would probably still max out at the same limits i.e. you may get slightly better performance, but power max power consumption would likely stay the same. Overclockers take advantage of this feature by increasing max power to allow greater overclocks if their specific card supports it and if the card's power circuitry can handle it without frying.


Would I do it? Possibly, but do what CAT said and measure power consumption at the wall without the GPU first and see how much room you have to work with. Using at-the-wall figures actually isn't a bad way of testing since you don't really want to max the PSU; the ~80% efficiency approximately accounts for the ~80% max load you'll want to aim for.


On the subject of CPU undervolting, it's not necessarily completely straightforward:

If you get a board which allows you to set a specific CPU voltage, it will run at that voltage constantly, considerably increasing idle power consumption.
If you get a board which uses a voltage offset, it will also reduce the idle voltage by the same amount, which preserves the good idle power consumption but also carries a large risk of BSOD every time the CPU enters idle clocks.

Some people disable the CPU's idle features like Speedstep which generally negates to purpose of underclocking the CPU in the first place.

If it's only temporary why not just get a 650 then sell it when you upgrade? Seeing as it shouldn't be too far away and 660 (or comparable) prices will most likely have dropped enough you won't lose much especially as AMD seem to be starting a price war at about 660 level.

the gtx650 isn't that bad a card, it's between the 7770 and 7570 in performance around the same as the old gtx460 768mb, 4890, 5830 or gtx260 216core
which is pretty impressive for something that's around 65w max

It'll depend on the games and the resolution it should cope with most stuff at 1920x1080 at medium to high settings at 30fps
And you can sell it later if you want to upgrade.

I don't know how easy it would be to undervolt a graphics card or how much effect it'll have of power consumption.

As people have already said you need to pay attention to the 12v psu output not the total wattage a graphics card only uses 12v, both from the pci-e connector and pci-e solt
Also that psu doesn't include a pci-e connector so you'd have to use molex to pci-e adaptors and you might not have enough molex plugs (4) to run two molex to pci-e adaptors.

Hmmmm, matx system with 250W flexATX 1u psu? Sounds familiar ;)

First up, unless Intel massively dropped the ball with Ivy Bridge and power consumption, your base system is very unlikely to draw as much as 100W from the wall, let alone in DC power. Hitting my AMD A6-3670K as hard as I could I struggled to get it much above 80W, and that's with high GPU load as well as CPU load. With a discrete card you're not going to be loading the iGPU at all. 80W at the wall equates down to around 65W DC draw. That would mean your planned PSU has well over 100W DC draw available on the 12v line.

Looking around at a few reviews, whilst TPU managed to make the card peak at 129W on its own, the Hexus review showed an entire system draw of less than 200W from the wall whilst gaming. The Hexus test system is actually fairly representative of a decent gaming PC, comprising an i7-3770k and Z77 motherboard all at stock - be wary of reviews that use a massively power-hungry rig as the underlying system as these will give an unrealistic picture of the load power draw you can expect to see.

It's worth remembering that pretty much every reviewed GTX660 was a custom-design overclocked version, as nvidia didn't seed the press with reference designs at launch. That means they will have slightly higher power draw compared to stock clocked versions. Also, with nvidia's GPU boost targeting a power budget, it's possible that you could lower the target power in software and keep power draw in check that way. As others have mentioned, it's probably not worth the hassle of trying to undervolt/underclock a GPU due to the way power management is handled on modern processors of any kind - they simply won't draw any power they don't need!

But basically, I'd be surprised if you couldn't run that rig and a GTX660 off that PSU - so go for it :) Worst comes either your power supply will trip (it's a Seasonic, it'll have good OCP/OVP/UVP) or your system will BSOD, neither of which will damage the PSU or graphics card. If that happens you can sell on the GTX660 (which will still be nearly new) and buy something cheaper and, hopefully, less power hungry.

You can measure the power draw without the GPU, test the GPU in another machine to see what that takes and then only put the two together if they come in under 200W.

Or is there just no downside to just trying it altogether and relying on the OCP/OVP/UVP?

I would never rely on OCP TBH, it's rarely set properly to avoid trips at peak load, even on high end supplies, so you can easily overload the PSU without knowing it and be met with a ton of ripple.

Goto evga.com/precision/ and download the EVGA Precision X (you must register to the EVGA site to do this). It's a nice utility program for playing with nVidia GTX cards. Doesn't matter what company made your nVidia card. It will let you play with the GPU voltage, core frequency and other settings. Might help you. :)