4870x2 not stable with i7 920 overclock

darkblade · 7 Apr 2009 at 01:58

I'm having trouble getting by 4870x2 cards running stable under crossfire with the i7 920 overclocked. Main components: Asus P6T, i7 920, 6Gb Corsair 1600, 2x 4870x2, 1000w PSU. Vista 64 SP1, Cat 3.9. GPUs and CPU are watercooled

I can clock the CPU to 4Ghz (20x200) which is Prime95 stable with HT off (2 hours with HT on).
The problem comes when running 3dmark06. With Catalyst AI disabled (crossfire off) it will run fine with the above overclock and returns a score of 23,962

But with AI active, 3dMark06 crashes. With the above overclock, it always crashes on the loading screen of the first test. If I run the CPU at stock, the 3dMark benches run without issue. I can overclock to 3Ghz (20x150) and the tests complete (albeit with a score of 18,830). If I raise the BCLK to 170, I start getting crashes, errors, etc in various points with 3dMark - most notibly between the tests when returning to the desktop.

CPU Voltage: 1.4 (for 4Ghz)
QPI/DRAM Core Voltage: 1.35625

What BIOS settings should I experiment with to get the 4870x2 cards stable on crossfire with the CPU at 4Ghz? Any other thoughts/suggestions?

ViRuS2k · 7 Apr 2009 at 02:10

could be a PCI express voltage issue.

Bump up the voltage 2 notches on the PCI Express Lanes.

should cure it.

fornowagain · 7 Apr 2009 at 02:33

Regardless of the 1000w psu, could be a possible power issue. Any way of measuring actual rail voltages?

darkblade · 7 Apr 2009 at 17:52

Raised ICH PCIE Voltage from Auto (1.50v) to 1.52
ICH PCIE Voltage from Auto (1.50v) to 1.70

Still crashes on returning to the desktop after running the first test.
If it was a power issue - shouldn't I see 1 or more Red LEDs lit on the graphics card?

I've also disabled the second card and run 3dMark06 with Cat AI both on and disabled - this works as expected.

fornowagain · 7 Apr 2009 at 21:19

darkblade said:
If it was a power issue - shouldn't I see 1 or more Red LEDs lit on the graphics card?

Not if its dipping down too far under load. Have a look around, more than a few reports of 1kW+ psu's with rail issues. Try another if you can, just to rule it out.

darkblade · 8 Apr 2009 at 18:07

fornowagain said:
Not if its dipping down too far under load. Have a look around, more than a few reports of 1kW+ psu's with rail issues. Try another if you can, just to rule it out.

Excellent thinking - I'm using an Enermax Galaxy 1000w. Having given your advice some thought - I know that the main loom on the PSU has two PCIE connectors on it - possibly to the same rail. Having unplugged one of those and run a seperate PCIE cable from another socket on the PSU - it appears to have fixed the issue!

Thank you very much for your thoughts

Now to get back to overclocking!