When you want to overclock a graphics card typically you need to verify and stabilize your tested settings. First off let us remind you that we recommend increments of 25 MHz on core and memory (from the default base clock frequency upwards). In the first stage you overclock your GPU, in the second the memory. Once you get artifacts or a lockup, back down 25 or preferably 50 MHz and typically that is your stable result in it's highest threshold -- under the condition that the room temperature is the same (yes heat and surrounding heat have an influence on your overclock).
Related to heat is your graphics card cooling system. If you are willing to allow for more noise, increase the fan RPM on the GPU towards a higher setting that you find comfortable. Cooling helps a lot for overclocking, it's as simple as that. That backside of your graphics card however gets hot as well, give it some airflow as well. So make sure your PC casing is well ventilated with decent airflow.
Overclocking tools - now you can use a 3rd party tool like AfterBurner, however without voltage tweaking we recommend you to stick to the AMD Overdrive functionality from within the AMD Catalyst drivers. The Radeon HD 7870 has a huige amount of freedom to overclock and the AMD overdrive settings are not at all too limited (opposed to the R7850).
The three variables you need to coop with are GPU clock, memory clock and power control settings.
But without a voltage tweak, our card is stable up-to roughly 1100 MHz. To emulate our setting:
Set the GPU clock at 1100 MHz on the core (1000 = default)
Set 1450 MHz on the memory (1200 =default). Memory is quad data rate hence you get an effective 5800 Gbps effective.
Set the Power control slider to 20%
The lowest slider 'power control' slider is important, your 7870 is limited and tied to the cards maximum board power. The maximum board power roughly 75 Watt however the 7870 is configured at a 140W TDP. By increasing the power slider towards 20% you increase the board TDP 140(W) x 1.20(MP) towards 175 Watt, this gives you more overclocking headroom as the card may now deliver more power throughout the board.
For the default overclock we ended at 1100 MHz on the GPU core and 1450 (x4 quad data rate)=5800 MHz on the memory.
If you like to go a little more extreme, you can opt to voltage tweak the graphics processor. The latest beta of AfterBurner supports voltages up-to 1.3 Volts on the GPU, that's enough to fool and fiddle with.
We do need to warn you, increasing GPU voltages remains more dangerous. You need to know what you are doing, apply more cooling and during the initial tress tests you really need to monitor for weird behavior and high temperatures.
Basically we'll be altering a few things. First off, and I mentioned cooling already, voltage tweaking will increase GPU temperatures, so we'll force our cooler towards a fan RPM of 40%, now trust me that's more noisy. Typically we only recommend voltage tweaking with say a liquid cooling solution.
In Settings increase the powertune slider towards +20%
Increase fan RPM to 40% or 50%
Increase Core voltage towards 1300 Mv
Increase core clock to desired and stable speed
Increase memory clock to desired and stable speed
Breaking free from limitations
Should you need more then AMD's restricted the clock frequencies, then you will need software that can deal with that. AfterBurner can do so, but you'll need to follow these instructions. Now this tweak disables AMD's Powertune. The results may be an instable graphics card. Really anything can happen. If you don't appreciate that, then overclocking is not for you.
The default limitations however should be more than plenty for overclocking, so we really do not advise disabling Powertune in the first place.
Anyway, we recommend you start off with a GPU core clock of 1100 MHz and work your way upwards. With each increase of say 25 MHz stress your GPU with a stringent application or game. A full 3Dmark 11 session would be a good indicator for stability.
We ended at 5800 MHz for the memory and a nice 1203 MHz on the GPU core (in combination with 40% fan RPM and thus a 1300 Mv voltage tweak).
Let's go check out what that does to the boards power draw, noise and heat levels.
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