We simply did not have time, in preparing our review of the new Radeon HD 6800 series GPUs and Nvidia's hyper-clocked GTX 460 response, to make any serious attempts at overclocking those cards. Happily, I've been able to take a little bit of time since then to see what I could squeeze out of a couple of the cards in question. What we found is tremendous headroom in of both of the major mid-range GPUs, especially when combined with custom coolers.
Our first victim was XFX's version of the Radeon HD 6850. This card is a little shorter than AMD's reference design, and it has a custom cooler that we've found to be tuned with a bias toward keeping temperatures low rather than noise levels. Generally, we'd prefer a less aggressive fan speed profile, but for this particular mission, the extra cooling was just what the doctor ordered. We chose to focus on the 6850 rather than the 6870 simply because the lower-end models based on a particular GPU tend to have more overclocking headroom.
After futzing with the Overdrive auto-tune tool in AMD's control panel, we quickly realized that software wasn't gonna cut the mustard. The core GPU clock slider in that utility stops at 850MHz with the 6850, and our card was perfectly stable at 850MHz at its stock voltages.
At the suggestion of Tarinder from Hexus, we decided to try Sapphire's TRIXX overclocking utility, instead, which offers both higher clock speeds and voltage control. Using TRIXX to tweak and MSI's Kombustor tool (a FurMark type GPU burn-in app) to confirm stability, we learned that the 6850 would do 900MHz quite happily at its stock voltage. Bumping up the GPU voltage allowed us to range even higher. The GPU would operate at 960MHz, but we saw visual corruption in Kombustor all the way up to the utility's peak of 1.3V. Ultimately, we settled on a max stable (and visually correct) GPU clock speed of 950MHz. We tested memory separately and determined that 1200MHz was its peak speed, and that memory speed worked well in conjunction with our 950MHz GPU clock.
At those frequencies, the card operated at a somewhat reasonable 84°C, with the fan at 90% of peak. We noticed that pushing up to 1.3V and higher GPU clocks caused temps to rise to near 90°C. I expect this card's overclocking headroom is somewhat limited by its cooling.
Still, 950/1200MHz is a huge step up from the 775/1000MHz default clocks on the 6850, enough to put the overclocked 6850 ahead of a stock Radeon HD 6870 in terms of theoretical ROP rate and memory bandwidth. (The 6870 still has more peak texture and shader throughput, though, thanks to its two additional active SIMDs. We've not yet explored the overclocking headroom on the Radeon HD 6870, but we'll try to squeeze it in when we can.)
Speaking of the 6870, our next victim, the MSI Hawk Talon Attack, is more of a competitor for it than for the lower-priced 6850. We chose to overclock this MSI card for a couple of reasons. First, because it's obviously built for overclocking, with a trick cooler and higher-grade GDDR5 DRAM rate for 0.4-ns operation. Like the XFX, this card's cooler runs loud but keeps GPU temperatures very low. Second, the GTX 460 is mostly a known quantity, but we wanted to see just how much headroom was left in one of these new, higher-speed versions of it. Heck, the stock GPU frequency on this thing is 810MHz, well above the GTX 460 1GB's original 675MHz base. Does the GF104 have anything left to give?
Turns out the answer is a resounding "yes." MSI's Afterburner utility was my weapon of choice here, because it allowed me to raise clock speeds and adjust three separate voltage values: the GPU, memory, and the PLL. After lots of iterations and testing, we worked out a best stable config of a 940MHz GPU core and 1175MHz memory (which shows as 2350MHz in Afterburner), with all of the voltage values turned up as much as possible, +100 mV for the core and memory and +30 mV for the PLL. We're showing 950MHz in the image above, but that speed wasn't 100% stable in our game test.
Interestingly, we didn't see any visual corruption in Kombustor with the GF104; it just crashed when it was unhappy. Also—and this may be related—this card's cooler proved to be much more effective at keeping temperatures in check. The picture above, from the end of a long load test, shows a 72°C reading for the GF104.
Both cards are pretty darned loud when overclocked like this, but those speeds were sustainable, at least in our limited experience with Kombustor's crazy-intensive load test.
You can see how the overclocked 6850 pretty much matches a 6870, performance-wise. That is a lot of power for 180 bucks. Meanwhile, the higher-priced (~$215) GTX 460 Hawk Talon Attack sinks its claws into a relative, the GTX 470, committing raptor fratricide. Both of our overclocked cards gain about 5 FPS on average here, or over 10% in each case.
Extensive overclocking has its downsides, too. That additional voltage translates into more power consumption at the wall socket. You're not really treading into difficult territory with the 6850, but our GTX 460's power draw is pretty considerable—something you may want to keep in mind if you're planning on trying a similar feat at home.