Ivy Bridge Overclocking: What Does It Entail?
Lower power consumption, purportedly less heat dissipation, a smaller die size, lower manufacturing costs for Intel...but does the 22 nm Ivy Bridge design also leave less room for mainstream overclocking? Our launch coverage (Intel Core i7-3770K Review: A Small Step Up For Ivy Bridge) revealed that overclocking the new processor design wasn't any more fruitful than the already-mature 32 nm Sandy Bridge-based Core i7-2700K flagship. Although stock temperatures were lower, they ramped up very quickly once we started applying the voltages thought necessary to approach 5 GHz on air.
Overclocking: What’s Required?
The length of time a transistor in a digital circuit delays an electronic signal depends on its size, fabrication technology, layout, temperature, and operating voltage. The highest achievable clock rate of a circuit depends on this delay and the number of logic levels that a signal has to traverse in a single clock period. The latter number is fixed (and dependent on the processor's architecture). So, for overclocking, we focus on how a transistor's latency is affected by its supply voltage. A higher supply voltage can shorten the delay, but will also raise the transistor’s power consumption. Cranking up the clock frequency also increases the dynamic power draw per time unit, and thus further raise the circuit’s power consumption, leading to a hotter chip.
Both effects, taken together, explain why overclocking at an elevated CPU voltage increases the power draw and heat output, and why cooling an overclocked CPU can quickly become challenging. As in sports or any engineering discipline, trying to eke out that last couple of percentage points is most difficult.
CPU manufacturers have put in some safeguards against reckless overocking by inexperienced users (and unscrupulous system builders); starting a few years ago, both AMD and Intel started shipping most of their models with locked multipliers, releasing more advanced models opened up with overclocking in mind. Of course, enthusiasts know that they can either tweak the multiplier through their BIOS or through a Windows-based utility provided by many vendors for easier access to those settings.
In case of Intel's unlocked Ivy Bridge-based K-series SKUs, the highest CPU multiplier was increased to 63x (from Sandy Bridge's 57x ceiling), translating to a theoretical 6.3 GHz limit if you don't touch the 100 MHz BCLK. Going higher requires changing the base clock, which is rather difficult. Above a 110 MHz threshold, very few systems are stable. Be that as it may, it's going to take more than conventional cooling to hit those clock rates. In reality, you'll only see the limits of these architectures pushed in overclocking contests and YouTube videos.
Overclocking: Expectations
In the past, shrinking gate lengths have been seen to increase overclocking headroom. Smaller transistors require less voltage and consume less power, generally leading to better overclocking margins. Intel's Sandy Bridge-based K-series models easily achieved 4.3 to 4.6 GHz using air coolers, sometimes scaling even higher. Thus, our expectation for Ivy Bridge (along with many other enthusiasts, we'd say), was closer to 5 GHz.
However, we failed to achieve that goal, despite multiple tests in multiple countries using multiple Ivy Bridge-based samples. But we also received reports that Intel's 22 nm chips can break through speed records if you overcome their rapid heat-ramp using extreme measures like liquid nitrogen.
Knowing that LN2 is impractical in a production environment, we set out to achieve the highest overclock possible using conventional air cooling, discussing the causes of Ivy Bridge's limitations along the way.
