|Overclocking the Radeon HD 4890 and GeForce GTX 275|
|Putting the Pedal To The Metal|
After running our cards in two and three GPU combinations, we switched back to single card installations and checked to see how much more performance we could squeeze out of each of our contestants. We chose RivaTuner as our instrument of choice, which lets us control the GPU and Memory speeds, as well as the Shader clock speeds on the GeForce cards. RivaTuner also allows us to modify the fan speeds in case the cards started to get a bit too hot.
We would find out quickly that controlling the fan speeds become instrumental in overclocking our GTX 275s. The fan speeds seemed to be locked in at 40%, regardless of GPU temp, and as we attempted to raise the clock speeds even a little bit we were met with instant crashes. By cranking up the fan speeds in RivaTuner we were able to move on with our overclocking with the obvious price of much increased noise output. We found that the ASUS and Gigabyte boards, both of which touted higher quality components, had the tamest overclocks, EVGA came in the middle, and the MSI Twin Frozr OC model - which comes overclocked by default - achieved the highest speeds for all three areas. Obviously the enhanced cooler on MSI's card helped us in this regard.
As for the Radeons, each of our cards comes with higher clock speeds than reference values, ranging from XFX's 25MHz boost to the core speed to Diamond's heady 75MHz. Using 925MHz as the start off point, since that's where the Diamond HD 4890 XOC is already running, we pushed up the core and memory speeds 5MHz at a time until we reached a point where we could go no further. And unlike the GeForce cards, the fans on our Radeons dynamically changed speeds according to temps and load conditions. Once we reached our final stable overclocks, we noticed a strange irony: the lower the original clock speeds that a card had, the higher the final overclocked speed we achieved. Thus, the XFX HD 4890 which started out as the slowest card wound up with the fastest GPU at the end, nullifying the Diamond HD 4890 XOC's 50MHz advantage.
We ran through two of the games in our benchmark suite to see what the new clock, memory, and shader speeds would bring to the table. In ET: Quake Wars, overclocking the Radeons brought them level with the non-overclocked GTX 275s; however, those same GTX 275s were now operating at close to or at the same level of a GTX 285 at their highest speeds. Far Cry 2 originally had the cards at similar performance levels, but that's slightly skewed as all the Radeons were overclocked by default while only a single GTX 275 had higher than normal speeds. With the extra headroom afforded to the GTX 275s, they were able to pull away even further from the Radeons when all was said and done.