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Power, Heat, And Efficiency

System Builder Marathon, March 2010: $3,000 Extreme PC

Efficiency is a comparison of work completed to energy used. Overclocking forces higher power consumption, but can still result in improved efficiency whenever performance is increased by a greater amount than power consumption. We start our analysis by checking the amount of power used at the source.

At 160W, idle power consumption was already high before we overclocked. Pushing our processor 61% above stock had only a moderate impact on idle power use, yet its full-load power went up by 64%.

A drastic increase in full-load CPU temperature could have a greater effect on power consumption than the moderate increase in voltage, since semiconductors become less resistive as they heat up. Once again, we remind readers that this temperature was reached by running eight threads of Prime95 simultaneously over the period of several hours and that operational temperatures were much cooler.

The liquid-cooled graphics card amazingly operates at only 25.6 degrees Celsius over ambient at loads and settings, a full 66% lower than temperatures typically achieved with air-cooled cards. The drastic difference in CPU and GPU temperatures is probably due to a flow restriction at the GPU water block, where the CPU block relies on high volume, while the GPU block relies on high pressure. Were we to add a second graphics card, we’d probably connect its cooling lines parallel to the first card for increased flow through the water block and radiator.

Because some of our games showed CPU limits at default settings, a gaming performance increase of 31% at a 31% GPU overclock appears to have occurred by happenstance. In other words, the same GPU clock probably would have provided far lower performance gains had it not been accompanied by a significant CPU overclock, though many hours of additional testing would be required to show an exact numeric correlation.

A 40% increase in performance corresponds to a 53% increase in power consumption, dropping the overclocked configuration’s efficiency by 8%. While certainly no “green” PC, we’re sure there are many users who would gladly pay the higher power bill for this performance improvement.

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