6970's Mh/s drop ~25% when raising core from 900 to 905, advice needed.

[Tullphan]

Hi,

I seem to have encountered a strange problem. The core speed of my 6970 (crossfire) stock is 850. As I raise it in increments of 5-10 MHz, the Mh/s scales along with it (increases). @900 Mhz core speed my Mh/s is 800. When I bump the core up to 905 Mhz, It drops to ~600 Mh/s. What is causing this? I've increased voltage, decreased mem clock, and my temps are great (75C load)

I want to take it higher, but can't. Any help is appreciated.

[P4man]

What miner, what OS and what app are you overclocking with?
BTW, not sure Id call 75C that great, even if its not likely the cause.

[Tullphan]

cgminer, windows 7 x64, using most recent drivers.

[P4man]

Not sure, but Id have a look at your catalyst settings, and perhaps disable crossfire (remove the bridge) and/or disable ccc

[Tullphan]

Well, after significantly more googling, I found out that the 6970's downclock themselves whenever you raise the core over 900. The remedy to this is to go into the CCC Overdrive settings and set the "power control settings" to +20%

I am now getting ~425 Mh/s @ 950 core per card. Hope this helps other people in the future!

[P4man]

Ah gotcha. BTW, cgminer can set that value too. Its the "powertune" thing.

[Tullphan]

Ah gotcha. BTW, cgminer can set that value too. Its the "powertune" thing.

Thanks!