How to overclock your new Haswell CPU like a pro

Speed freaks, take note: If you shelled out for an unlocked version of Intel's new Haswell processor and haven't overclocked your PC yet, you're missing out on some serious--and free--performance improvements.

Last month I spent $1000 to build the ultimate Haswell PC, a speedy machine powered by a Core i7-4770K, a fast solid-state drive, 8GB of RAM, and a discrete graphics card.

Since gaming performance wasn't the primary goal, I spent the majority of my budget on a premium processor and solid-state storage drive. A speedy CPU-SSD combo gives you the smoothest overall computing experience, especially with a streamlined operating system like Windows 8.

But I had an even better reason for buying a premium Haswell processor: overclocking.

Sure, I could have saved a few bucks building the new Haswell-powered rig by opting for a cheaper CPU, perhaps using the extra scratch to slap in a more powerful GPU, but I thought that springing for the top-of-the-line Core i7-4770K was ideal for a couple of reasons. Not only is the Core i7-4770K the fastest quad-core processor Intel has released to date, but its "K" designation means the chip is unlocked, which allows for easier and more flexible overclocking. I didn't do any overclocking for the original article I wrote, because budget constraints forced me to stick with stock cooling, but that didn't stop me from upgrading the rig after the fact to see just what it could do with a bit of tweaking.

I'm happy to report that the overclocking process was simple, safe, and rewarding. You can easily net a 15 to 25 percent improvement in PC performance tests by overclocking your Haswell CPU with the following process.

It's getting hot in here

First, a quick bit of background about the danger of overclocking: If you follow the desktop processor scene, you may remember reading about the thermal issues that cropped up while overclocking Intel's third-generation Ivy Bridge processors. Eager enthusiasts immediately started overclocking the chips and found that the processors seemed to run significantly hotter than the previous generation's Sandy Bridge-based products under certain overclocked conditions.

Two things were to blame for making Ivy Bridge chips hot under the collar: First, the 22nm tri-gate transistors used to manufacture the chips were more densely packed into a smaller area with Ivy Bridge, which in turn increased the chips' thermal density. Second, Intel replaced the flux-less solder used to bond the integrated heat spreader (or IHS) to the die on Sandy Bridge processors with a lower-performing, paste-like thermal interface material. The combination of increased heat density and a crappier TIM caused overclocked Ivy Bridge chips to heat up pretty quickly. The chips were still highly overclockable, but you had to take extra precautions because their temperatures would more quickly and more dramatically spike under load.

Unfortunately, Haswell chips suffer from the same thermal issues. The chips are manufactured using a similar 22nm tri-gate process, and the same thermal paste is used under their heat spreaders. As such, you need to be aggressive about keeping your Haswell CPU cool while overclocking to maintain stability and ensure peak performance.

The cooling conundrum

Overclocking an unlocked Haswell CPU is technically possible with any cooler, but the more significant the overclock, the more powerful the CPU's cooler must be. This is true of any processor, but since this is your brand-new CPU we're talking about, I have to warn you again to please, please use a high-quality cooler. The bigger and badder, the better.

If you really want to push the limits of your Haswell-based processor--using voltages above 1.25V or so, and clock speeds approaching 5GHz--I strongly recommend water or more-exotic cooling methods. A high-end air cooler, however, will still allow for plenty of fun. As part of my overclocking experiment, I enlisted the help of a gargantuan air cooler from Noctua, the NH-U14S, which dwarfs Intel's stock cooler.

The Noctua NH-U14S is big--roughly 6 by 4 inches, weighing just shy of 3 pounds with a single 140mm cooling fan installed. The heat-sink assembly is made up of a copper base and multiple copper heat pipes, linked to a large, dense array of aluminum cooling fins. All of the joints on the heat sink are soldered, and the entire thing is nickel-plated and polished to a mirror-like finish. With its increased mass and large surface area, the Noctua NH-U14S can dissipate much more heat than the paltry stock Intel cooler, which will ultimately result in lower operating temperatures and potentially higher overclock speeds.

Haswell Overclocking 101

The process of overclocking your new Haswell CPU is pretty similar to overclocking older Intel processors. With Haswell though, if you plan to overclock, you must have a CPU with the "K" designator--like the Core i7-4770K we used. Non-K-SKUs have very limited overclocking potential due to hardware locks (boo!) that Intel has built in to this processor generation.

There are two ways to overclock your CPU: by increasing its multiplier or by increasing its base clock (BCKL) frequency. For example, the Core i7-4770K's base 3.5GHz clock speed is achieved with a multiplier of 35 and a BCLK of 100MHz: 35 x 100MHz = 3500MHz, or 3.5GHz.

The Core i7-4770K's 3.9GHz max clock is achieved with a multiplier of 39 and the same 100MHz BCLK. Increase either the multiplier or the BCLK, and the processor's clock speed will ultimately increase as well. Because K-SKUs are unlocked, you can freely change the multipliers and set the BCLK to different ratios, or straps, of 100MHz, 125MHz, 167MHz, and 250MHz. You can then tweak the actual BCLK in small increments as well, as long as you're using an overclocker-friendly motherboard like my Gigabyte Z87-UD3H.

Sadly, Intel's Haswell chips, like their predecessors, offer limited BCLK adjustments. You can alter the BCLK by only a few MHz over or under the specific strap to fine-tune the processor's frequency. BCLK adjustments larger than 4MHz or 5MHz are very uncommon.

I should also mention that you can also modify multipliers for the memory and uncore speeds--even the iGPU frequency--if you want to tweak other aspects of performance. For now, I'll be focusing solely on CPU performance.

Talking volts and temperatures

Processors require a certain amount of voltage and must operate within a certain temperature range to remain stable at a given frequency. To hit higher frequencies, the processor may need more voltage. In addition, increasing the processor's voltage will result in higher power consumption and more heat output, which necessitates additional cooling.

That's overclocking in a nutshell: altering voltages and managing temperatures to maintain stability at higher frequencies.

Now let's talk hard numbers. With my system running in its stock configuration using the Noctua cooler I mentioned earlier, the Core i7-4770K idled at an average of about 32 degrees Celsius. On the low end of the scale, due to Intel's SpeedStep technology's dynamically throttling frequency and voltage, the chip ran at 800MHz with a voltage of 0.7V. Under a full load, with all cores pegged and Turbo'd to 3.7GHz, 1.076V was supplied to the chip.  And at the Core i7-4770K's peak Turbo speed of 3.9GHz, with only a single core loaded up, its voltage peaked at 1.104V.  The load temperature on the "hottest" core maxed out at about 68 degrees Celsius.

Keep in mind that these numbers are specific to this particular setup. Differences in your ambient temperature, motherboard, power supply, and other variables will change the data. I monitored PC temperatures and voltages in real time using a couple of free programs: Real Temp and CPU-Z. Real Temp reports the temperature of each individual core, and CPU-Z presents info on voltages, multipliers, and frequencies, along with a multitude of other data.

Before you attempt to overclock your processor, test your PC using these tools to determine baseline voltages and temperatures. The baseline information will help you determine if you need additional/more-powerful cooling and how much additional voltage is safe. Generally speaking, processors can safely run with higher-than-stock voltages (within reason) if temperatures are low. Or, they can safely run at higher-than-normal temperatures, if voltages are low. But if both the voltage and temperature are abnormally high, you can damage the chip.

Since I don't use any exotic cooling, bumping up the processor's peak voltage by about 10 percent is a safe bet, considering how cool the chip runs under load. The highest voltage I observed while running the system at its stock speed was 1.104V. Bump that up 10 percent, and you get 1.214V.

Tweaking the chip's voltage and multiplier

The Gigabyte motherboard I used for the build has a BIOS/UEFI with a full complement of overclocking tools built in, so I entered the BIOS and bumped up the processor's voltage to 1.21V. Then, while still in the BIOS, I changed the highest multiplier for each core from 39 to 42, which would result in a peak Turbo frequency of 4.2GHz (42 x 100MHz BCLK = 4200MHz) on all four cores.

After saving the changes I made, I booted into Windows and tested for stability using a combination of benchmarks and utilities that stress the processor. Specifically, I used Cinebench and PCMark 7. If the system is able to loop the Cinebench R11.5 multi-threaded test continually without crashing and PC Mark 7 successfully completed five consecutive runs, you can consider the system stable. At 4.2GHz, my PC didn't break a sweat. Everything ran perfectly and the hottest CPU core according to Real Temp never exceeded 73 degrees Celsius.

With 4.2GHz in the bag, I incrementally increased the processors' multiplier until the system was no longer stable. At 4.8GHz (a peak multiplier of 48), the PC couldn't reliably complete multiple Cinebench tests, although none of the processor's cores exceeded 82 degrees Celsius. If I'd been using more-exotic cooling, I could have tried applying more voltage to stabilize the system, but instead I simply backed down the multiplier a notch. At 4.7GHz (a multiplier of 47), the system was once again perfectly stable--and much faster.

Proof of peak performance

After all of that tweaking was done, I ran a few benchmarks to see just how much additional performance I gained by overclocking our Haswell-based system. When running it in stock configuration, the system put up a multi-threaded Cinebench R11.5 score of 8.09. In the multi-threaded POV-Ray benchmark, the system processed 1544.13 pps (pixels per second). And a low-res (1024 by 768), low-quality Crysis benchmark yielded 237.16 fps (frames per second). With the system's Core i7-4770K overclocked to 4.7GHz, however, performance increased dramatically.

While overclocked, this PC's multi-threaded Cinebench R11.5 score jumped all the way up to 10.26, an increase of over 27 percent. Performance in POV-Ray increased to 1959.65 pps (+26.9 percent). And in Crysis, frame rates jumped to 270.12 fps (+13.9 percent).

Those are some excellent performance improvements, and if you're willing to put the work in, they can be yours for what essentially amounts to free--provided, of course, that your system has ample cooling.


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