Overclocking MSI R9 290 on air

[minecraft-rychle]

Hi, I would like to ask if I should push my GPU more than this

When I was running Heaven Benchmark with +20mV I got artifacts (small ones) so I kept it at +25mV but after I put +13mV when the benchmark wasn't running and then run it again there were no artifacts so I am a little bit confused.
Should I just keep the benchmark running longer? If so for how long?

 

[norcalsc]

if you are getting artifacts back it down that is not good

 

[minecraft-rychle]

So yesterday I kept the Heaven benchmark running at
Core Voltage +13mV
Power Limit +20%
Core Clock 1121Mhz
Memory Clock 1516Mhz
My temps were hitting 80-82 but stayed at that for the full duration of the benchmark. There were no artifacts whatsoever. Now should I push the clocks a little bit higher or just keep it as it is ?