Clock Frequencies at Partial and Full Load
PowerColor's HD7870 PCS+ Myst Edition is pegged at its 925 MHz base clock rate when you push the Tahiti LE GPU really hard using general-purpose computing workloads. Games tend to be less taxing, giving the board some headroom to spend time at its 975 MHz Boost frequency. The higher clock rate isn't very consistent, though, particularly as you play through more demanding titles at higher detail settings.
Depending on your card and the rest of your system components, overclocking PowerColor's HD7870 PCS+ Myst Edition can increase its performance by 15% or more. We pushed the card's core clock all the way up to 1.2 GHz without touching its voltage. Setting PowerTune to +20% is all that it took to achieve a stable overclock.
This is pretty consistent with what we've seen from Radeon HD 7900-series cards. Our Pitcairn-based Radeon HD 7870 only makes it to 1,150 MHz.
There’s not much room to overclock the HD7870 PCS+ Myst Edition’s memory, though. We pushed it to exactly 1,600 MHz. Increasing the voltage is not a good idea, since the un-cooled RAM gets too hot. Besides, a 1,500 MHz factory setting is enough to prevent bandwidth bottlenecks from negatively influencing frame rates. If you care about the card's longevity, it’s better to just leave the memory frequency be.
PowerColor's HD7870 PCS+ Myst Edition shows its Tahiti roots when it comes to CrossFire configurations; you cannot pair it up to a normal Radeon HD 7870. Matching it up to any Radeon HD 7900-series card isn't a problem, though. In fact, we installed our review sample next to a HIS 7950 X², and we matched the performance level of the two cards by overclocking the PowerColor HD7870 PCS+ Myst Edition to 1.15 GHz. This gets us close to the same single-frame rendering times. However, if you leave both cards at their factory-set frequencies, you'll see a lot of micro-stuttering. Even RadeonPro couldn’t alleviate the issue, leaving us with dropped frames.
At $240 per card, two PowerColor HD7870 PCS+ Myst Edition boards make for a very competitive high-performance setup for less than $500. Now, let's take a look at power consumption.