Please remember that voltage increases in particular should be applied with care as they could have an adverse impact upon the longevity of your card. In many cases, applying additional voltage with a third party utility will void your warranty.
Before going into the actual clock speeds we achieved let’s talk about stability. To achieve true stability, a given graphics card has to complete ALL of our benchmarks without crashing or artifacting. In addition, it must be able to pass a 6-hour stability test with MSI’s Kombustor utility.
What we are showing below is two different overclocks for each product: one with its stock voltage and the other with a slightly increased core voltage. We kept the voltage tuning within some limits due to the impact it has on overall core temperatures. If at any time core temperatures reached above 90°C with the fan speed at 50% throughout testing, the voltage and clock speeds were scaled back until a reasonable sub-90°C peak temperature threshold was reached.
NOTE: GDDR5’s error correction features can constrain performance so in some cases, even though higher memory clocks are achieved, performance doesn’t necessarily increase. If potential errors (artifacts) are detected, GDDR5 has the capability to adjust latency, hold times, etc to correct performance before the errors can make their way to the display output. The result is software monitoring programs still reporting a higher memory speed but overall performance isn’t positively impacted in any way.
To avoid this, we made sure every memory clock speed increase we applied had a corresponding performance increase as well.
EVGA GTX 460 1GB FTW
EVGA’s GTX 460 FTW already comes with a slightly increase voltage which is likely necessary due to its already sky-high clock speeds. Unfortunately, this also meant that any additional overclocking was tightly constrained. Even the GDDR5 couldn’t really be pushed all that much. The reason we are not showing an increase in clock speeds at the card’s reference voltage is because the core couldn’t be pushed more than 10Mhz above it’s “stock” 850Mhz. This shows you how close our sample was to outright instability even at the clock speeds it ships with.
Maxing out the voltage in AfterBurner didn’t really result in all that much leeway either. The temperatures of the GTX 460 stayed well under the limits we set but stability above 900Mhz was impossible to achieve.
Sapphire HD 6870 1GB
Much like the EVGA card, the HD 6870 seems to be near its maximum allowable core clock speed at default voltage settings. However, there is some headroom to be gained if you are patient. The memory on the Barts XT on the other hand has the ability to overclock like a bat out of hell.
Increasing the voltage led to extremely good core speed scaling but also a rapid increase in core temperatures. In our opinion the default heatsink is well-suited for some minor overclocking but pushing past the 1Ghz mark results in high temperatures and fan speeds that can be annoying to you and anyone in the next room.
XFX HD 6850 1GB
The HD 6850 is an overclocking monster. Really, what more is there to say?
Even without a voltage increase, core speeds hovered around the 890Mhz mark and the GDDR5 found itself at a respectable 1092Mhz. Neither of these is quite as high as a stock HD 6870 but performance should nonetheless be interesting to see.
Adding a bit of voltage resulted in yet another bump of about 130Mhz which likely won’t affect performance by all that much but it still great to see an AMD HD 6800 series card matching its NVIDIA competitor in the overclocking department.
Unfortunately, we had to end our HD 6850 overclocking exercise due to heat issues with both the reference and XFX Vapor Chamber heatsinks.
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