Probably around 1.40v, depends on a few factors though.
For full stability my DFI X48 needs 1.42v for 500 FSB, 600 MHz, PL 7, chipset optimizations set for max performance.
As far as I know, Seconds x MHZ = efficiency
So say you take 7.8 seconds at 6000 MHz for SPI 1M. That's 7.8 x 6000 = 46800 (or 46.8K you may see it expressed as).
From what I have noticed, 45 - 46K seems to be considered good efficiency. 48K is not so good.
I'm by no means an expert on this though so if I am wrong, please correct me.
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