Overclocking Ivy Bridge: Treating This Hot-Head Gingerly
Our Core i7-3770K and Core i7-2600K samples ship at very similar clock rates. The Ivy Bridge-based CPU runs at a nominal frequency of 3.5 GHz and can accelerate up to 3.9 GHz with a single core active by virtue of Turbo Boost, providing ample thermal headroom. These clock rates match Intel's Core i7-2700K, flagship of the mainstream Sandy Bridge family. Unfortunately, our only -2700K is in the U.S. So, our German team used a Core i7-2600K it had on hand, which is clocked at 3.4 GHz and capable of reaching 3.8 GHz thanks to Turbo Boost.
Initial Overclocking Successes
This hasn't been talked about a lot, but every overclocker has to appreciate the fact that Intel allows Ivy Bridge-based K-series parts to change their multipliers during operation. It's no longer necessary to reboot between modifications. We've been doing this with our AMD chips for a while now, so kudos to Intel for catching up. Using Intel’s Extreme Tuning Utility, a well-designed utility, we're able to tune our Core i7-3770K from within Windows.
We easily pushed the -3770K above 4 GHz without breaking a sweat. In fact, we hit 4.6 GHz quite easily. There, we encountered our first instabilities, which we tried to counter by raising the core voltage.
Frustration at High Clock Rates
As we proceeded in our overclocking efforts, regardless of whether we used a higher core voltage or not, we observed something frustrating: even below 4.5 GHz, our Ivy Bridge-based Core i7-3770K began thermal throttling. That is to say it reduced its clock rate in order to bring its temperature down. In other words, our overclocked -3770K was already running too hot, even at its default voltage setting.
Core Temp 1.0 RC3 reports that our Core i7-3770K reaches 90-100°C (194-212°F) internally when it's overclocked to 4.5 GHz. No wonder the chip's thermal monitor tripped, throttling the CPU. This phenomenon dropped the effective clock rate of our chip to approximately 3.5 GHz, corresponding to the CPU’s nominal frequency.
For comparison purposes, let’s look at the Core Temp readouts of Sandy Bridge- and Sandy Bridge-E-based processors:
Our 32 nm Sandy Bridge-based Core i7-2600K also got quite hot on the same test rig. However, each core stayed well below 90°C (194°F). As a result, the system maintained its overclocked frequency of almost 4.8 GHz. That's 300 MHz more than the new Ivy Bridge-based CPU!
Even our six-core Core i7-3960X (Sandy Bridge-E, the one with more than 2.2 billion transistors) posted better temperature readings. Each and every one of the six cores stayed at or below 81°C (176°F) at a very impressive clock rate of 4.7 GHz.
We want to share several observations that will help explain what’s going on.