When we were briefed about the 7800 series last week, one AMD representative told us he pushed the Radeon HD 7870 from its stock 1000MHz clock speed to 1200MHz without so much as a voltage increase. We were curious to put that claim to the test, so we did.
We used MSI's excellent Afterburner tool to control clock speeds and GPU voltage, AMD's Catalyst Control Center to max out the PowerTune setting, and MSI's Kombustor application to test stability and get a quick sense of performance. Rather than relate my findings in prose, I'm going to paste in my notes. They should be fairly self-explanatory.
By the way, we found that the card's memory silently reverted to its default speed (1200MHz) when overclocked too high. The only way to tell was to run Kombustor's built-in benchmark at each setting and see where performance peaked.
1100MHz -- OK after 5 min burn-in
1200MHz -- OK after 5 min burn-in
1300MHz -- crash
1275MHz -- crash
1250MHz -- crash
1225MHz -- crash
1200MHz -- 6414 kombustor -- stock
1300MHz -- 6894 kombustor -- OK after 5-min burn-in
1400MHz -- 6423 kombustor -- (resets to stock)
1350MHz -- 7186 kombustor -- OK after 5-min burn-in
1375MHz - 7312 kombustor -- OK after 5-min burn-in
1300/1375MHz -- 1.250v -- crash
1300/1375MHz -- 1.275v -- crash
1300/1375MHz -- 1.300v -- crash
1275/1375MHz -- 1.300v -- OK after 5-min burn-in
Without overvolting, we succeeded in running the Radeon HD 7870 at 1200MHz with its memory chugging along at 1375MHz. Score one for AMD. We didn't stop there, of course. Once we raised the GPU voltage from 1218mV to 1300mV, we were able to squeeze an extra 75MHz out of the GPU. That left us with core and memory speeds 28% and 15% above stock, respectively. How did that translate in terms of gaming performance?
Yow! We're looking at a 20% increase in average frame rates, which is pretty spectacular. Did overclocking raise power consumption through the roof?
Apparently not. We recorded a 17W increase under load, which still puts the 7870 below even the old Radeon HD 6870.
Something tells me AMD's partners are going to be rushing to offer Radeon HD 7870 variants with higher-than-stock clock speeds. If vanilla retail cards are as overclockable as our sample, though, paying a premium for a superclocked card may not be necessary.
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