The G80 GPU, the heart of the GeForce 8800 series, is divided into domains that are clocked at different frequencies. The main GPU domain can change its frequency with a variable step of 9, 18 or 27MHz, but the shader processor domain can change its frequency with a step of 54MHz only. The problem is that many overclocking programs can work with the main domain frequency only. In the standard GeForce 8800 GTX the frequency ratio is 1 to ~2.31, but pre-overclocked versions of the card may have a different ratio and their frequencies are going to change in a different way at overclocking. Particularly, the frequency delta for the Foxconn and XFX cards is 2.06, for the Gainward Bliss 2.2, for the EVGA 2.44. This should be taken into account when overclocking. Fortunately, some overclocking tools, e.g. the latest version of RivaTuner, can report the real frequencies of the GPU domains to make the process of overclocking easier.
There is also a simple formula to calculate the frequency of the GeForce 8800’s shader processors during overclocking with a high enough precision:
OC shader clk = Default shader clk / Default core clk * OC core clk,
where OC shader clk is the (approximate) resulting frequency, Default shader clk is the initial shader processor frequency, Default core clk is the initial core frequency, and OC core clk is the frequency of the overclocked core.
Basing on this information and on the frequency monitoring data reported by RivaTuner, we attempted to overclock the graphics cards included into this review. Here are the results:
Surprisingly, each of the six cards could overclock its main GPU frequency to the same point, to 621MHz. A frequency range of 621-648MHz seems to be the limit for the revision A2 G80 chip. This time neither card could work at a frequency of 648MHz for more than a couple of hours.
The shader domain frequency achieved by the cards varied from a modest 1296MHz to a considerable 1533MHz. The biggest frequency growth of 270MHz was actually produced by the cards with reference frequencies, Asus EN8800GTS and Gigabyte GV-NX88S320H. The EVGA had the smallest frequency growth as it had started from a higher shader domain clock rate. The results obviously depend on the frequency delta. The smaller it is, the smaller the probability of reaching a higher shader domain frequency because the further overclocking is going to be limited by the main domain frequency which is usually not higher than 621-648MHz. You need to modify the BIOS to overclock such graphics cards more, but few users will dare do it, we guess.
The memory chips on all the cards behaved in the same manner at overclocking. Not surprising as every card in this review is equipped with Hynix HY5RS573225AFP-11 memory. We managed to reach a frequency of 1000 (2000) MHz on two cards and stopped just short of that mark in the other cases. It is also clear that the modification of the cooling system attempted by EVGA doesn’t affect the card’s overclockability much.
We want to warn you against perceiving the above results as a recommendation to choose the graphics card with the highest frequencies because overclocking depends on the capabilities of the particular sample of the card and a lot of other, often random, factors.