How to overclock your Intel CPU

You went for it. You shelled out for a top-shelf, unlocked processor, and now it sits at the heart of your new system, loafing at stock clock speeds. It’s surrounded by a motherboard to match, with a Z-rated chipset designed to let your cores run wild and free. The BIOS beckons with unknown performance potential. Who knows how far that new processor will go?

Fortunately, finding out isn’t as frustrating as it used to be. In fact, it’s surprisingly safe to perform a basic overclock when all the right parts are in place, and with some simple best practices, you can have your system just a few clicks shy of a pro-level performance makeover.

Before your rig hits the track, give it a check-up.

There are a plenty of ways to push a processor, but before unleashing your CPU’s inner beast, it’s best to give the system a quick look-over to make sure everything under the hood is up to the challenge. Note: We did our overclocking with an Intel Core i7-5930K Haswell-E CPU, but the instructions here will mostly apply across modern Intel CPUs and various motherboard BIOS’.

If you’re upgrading a previous build, clear any chassis screens, fans, vents and heat spreaders from dust or other obstructions. Use compressed air and soft, long-bristle house-paint brushes for hard-to-reach areas. Intake fans should have dust screens over them for easy cleaning. These help avoid internal buildup too. By the time you’re done overclocking, that system is going to need all the heat dissipation it can get.

While you’ve got your system open take note of the CMOS reset button or jumper and make sure you can reach it. If any of the new settings hangs the system before BIOS on reboot, you’ll be using this to return things to normal.

Practice makes perfect
Next, get a tube of high quality non-conductive thermal paste, like Arctic MX-4. Why use non-conductive paste? It won’t short out sockets or motherboard components if some gets off the chip. Remember you need just a small dab in the center of the CPU: the cooler will spread it around when you push down. Test your success by comparing idle temps via the BIOS. Poorly seated coolers or improper heat paste applications show up as abnormal idle temperatures or cores that run much hotter than the others. Some core variation is normal, however, so don’t worry about 3-5°C differences.

You can also do a post mortem on your technique by checking how evenly the compound is spread on the CPU’s lid and the cooler’s thermal plate when you separate them. Uneven distribution or worse, bare areas, mean the two surfaces aren’t meeting properly. Check to make sure the cooler’s screw-in posts are all threaded cleanly, and try reseating the cooler.

One trick is to have a friend hold the cooler plate even and still, applying gentle force while you tighten opposing corner screws simultaneously to keep the pressure even. Don’t overtighten; you can damage the socket. Just turn manually until you aren’t able to twist any more, and then give a last half turn with a screwdriver to finish. Well-engineered coolers compensate for this with built-in spring loaded screws that stop turning at an ideal thread height, taking some risk out of the process.

It also might be time to reevaluate your cooler if you’re still stuck with a stock unit. Go with at least a $35 Cooler Master Hyper 212 EVO, and consider water cooling with a closed-loop system like Corsair’s H100i GTX if possible. In addition to offering better performance, water-cooled systems are easy to attach to the CPU and allow better motherboard visibility and case airflow.

Base Clock vs Multiplier, where to start?
Now that the system is ready for track time, let’s limber up with some fundamentals. Your CPU’s clock speed is determined by two numbers: the base clock speed and the multiplier. The base clock speed, or BCLK, affects more than just the CPU and influences the speed of DRAM, storage controllers and other integrated components to varying degrees.

Usually set to 100 Mhz, most overclockers avoid changing this number initially since system instability is the usual result even with small increases. The vast number of motherboard subsystems tied to the BCLK speed means the chance that all of them will work beyond spec are slim. There are gains to be made and secrets to be discovered here for sure, but they are best left for later exploration, once the top clock speed of the processor is established by more stable means, namely changing the multiplier.

Happy times-tables
The multiplier affects CPU speed alone, so it’s the perfect place to start probing performance potential. This allows much higher clock speeds to be reached.

Intel’s Sandy Bridge, Ivy Bridge and Haswell series processors all have plenty of headroom in unlocked form, boasting multiplier-derived overclock speeds between 4.3 and 4.8 GHz when properly cooled. Determining where your CPU falls in this spectrum is straightforward.

Boot your system into a stable state by going into the BIOS and loading the motherboard defaults. Set DRAM speeds to AUTO or the recommended spec for the chipset, for example 1600 MHz for Z97 or 2133 MHz for X99. If there are previous settings you wish to retain for later use, write them down, save a screenshot or take a photo of the BIOS screens. Most motherboards provide savable BIOS profiles for this purpose as well.

Next, set a safe manual CPU voltage—1.25v is a good start—and link the cores so the multiplier change affects them all. Avoid using adaptive or offset voltage while setting up a system for overclocking. Stress tests performed while using adaptive settings can cause voltage spikes well beyond the listed numbers and can trigger crashes or even processor damage.

Now you’re ready to start adjusting your multiplier. Begin with 42 and keep raising the number until the system starts to show signs of instability, such as blue screen crashes, boot failures or application freeze-ups. Most chips manage 4.5 GHz or higher; the Haswell-E sample for this test maxed out at 4.6 GHz via simple multiplier changes. The number you reach is the basic maximum speed for your chip. That’s far from the end of the road when it comes to overclocking, however.

Testing for stability
Stability spot checking after each clock increase is a must, and several tools exist for this purpose. AIDA64 is a good choice: it combines system information, synthetic benchmarking, monitoring and stress testing. Run its benchmark before you begin overclocking as a baseline to compare performance against.

The system stability test on the tools menu is what we’re looking for. Start it up and select CPU, FPU, memory and cache to test general stability, but use FPU alone for temperature. Processors run hot when AIDA’s FPU test is executed by itself, so take solace that actual full-load temps will be a few degrees lower and form a safety margin against throttling.

Once you’ve got a stable system, run the benchmark suite and use the compare results with your scores before overclocking.

Power trip
Boosting voltage to the CPU is the next step, and this is one area where it pays to be very careful. The 1.25 starting default voltage is low enough that a boost to 1.275 or 1.28 should result in a few hundred additional MHz to the top overclocking speed. In the case of our test Haswell-E sample, that boost is enough to join the rarified 4.7 GHz CPU club.

Note that as voltages rise, so does the temperature, and the curve is not linear. After 1.3 volts or so, serious cooling solutions are recommended and the clockspeed benefits start to thin out. Stay under 1.3 with 24/7 overclocks for the sake of processor longevity. On Intel’s new Skylake processors, you can stray into 1.3 volt territory, but keep voltage under 1.4 to stay safe.

Haswell processors run hot out of the box, which brings us to how temperature can limit overclock potential. Idle temps will be higher than Sandy or Ivy Bridge, so just make sure they are consistent so you know the cooler is seated properly. Under load, peak temperatures shouldn’t exceed about 80°C. Much past that point and Haswell begins to throttle its speed to reduce heat.

Core and Uncore
When Intel absorbed the Northbridge aspect of motherboard chipset design into the CPU itself, it referred to the transplanted functions as the System Agent, but the quirky term uncore is the one that stuck with enthusiasts. The uncore takes care of all the system processes not performed by the cores or the chipset itself, such as integrated memory controller functions.

While boosting the uncore frequency can result in minor performance increases, the main benefits to adjusting the uncore come from reducing it. If your CPU is having problems going past 4.3 or 4.4 GHz, try reducing the uncore speed to 3.4 or 3.5 GHz to see if that frees up additional headroom for the cores to use. Any performance lost with lower uncore speeds is more than returned via higher clockspeeds. If reducing uncore doesn’t help free up more GHz or results in instability, restore it to its former value.

Uncore is also referred to as the cache or ring ratio on some motherboards; the exact term depends on the manufacturer. Everybody seems to have a cute name for it.

DRAM dabbling, bringing the rest of the system up to speed
Going beyond simple multiplier tweaking is where BCLK tuning and DRAM speed come into the equation, and carefully tuning these separates the digital dilettantes from the hardcore hobbyists.

While the BCLK setting starts at a safe value of 100, several other special BCLK frequencies, called straps, provide higher numbers that only overclock the CPU and DRAM, allowing for a more flexible spread of frequencies and multipliers. The main additional BCLK straps are 125 and 166, and while they’ve been around since Sandy Bridge-E they’ve only become widely usable with more recent processor generations. Some manufacturers also provide 200, 233 and 250 BCLK strap options, but these are rarely stable.

Note: On the new Skylake CPUs, things are different—you can bypass the straps and jump to a 350 MHz BCLK.

Why change the BCLK instead of the multiplier? Well, extreme overclocking competitors can actually run of out multipliers when pushing unusual configurations or shooting for records, but there are more practical reasons to explore these settings.

The real reason to alter the strap is to get memory speeds to optimal values while retaining the optimal CPU overclock. Since Haswell’s memory controller is integrated into the processor, they share headroom, so pushing one can punish the other. Allowing adjustments via both BCLK strap and multiplier gives more granular control over these components and allows increases in increments less than 100 MHz for the CPU and the typical 200 MHz or more for DRAM.

For example, the Haswell-E test CPU for this guide was not stable in a 47×100 configuration while running 2800 MHz DRAM, but was able to run normally using a 37×125 configuration that produces a clock speed of 4.625 GHz and a DRAM frequency of 2751 MHz. Don’t forget, the BCLK adjustments go both ways, so you can always lower it a few clicks below the strap for added stability.

Selecting XMP presets provided by high-speed memory in the BIOS configuration, and leaving other settings on AUTO, will frequently result in a quick and dirty system overclock that optimizes around memory stability and performance.

If you find yourself at a crossroads between pushing the CPU or the RAM speed, always go with the processor. That’s where you’ll find the most benefits.

Shortcuts, software and satisfaction
Most motherboard makers andIntel themselves offer software that duplicates some of the overclock settings usually kept locked away in BIOS. This allows tweaking without constantly rebooting into Windows to check the stability and performance returns for each adjustment, which saves a lot of time and frustration.

One-step overclock solutions via desktop utility or motherboard switch are common on enthusiast motherboards, offering an automated version of the adjustments discussed here with varying levels of user input along the way. Results from these are mixed and usually on the conservative side, although all of them seem to apply too much voltage, so take note if you decide to use this feature as a starting point for your system tuning.

The truly dedicated can take the final step and buy a motherboard made especially for overclocking, such as GIGABYTE’s SOC-Force series. In addition to sporting beefy components built to withstand the strain of hardcore overclocking, these boards feature actual buttons that affect the multiplier, BCLK and other settings directly, without requiring software or special modes, for the ultimate in tweaking flexibility. Some even support pro-level overclocking competitions via features such as provisions for LN2 cooling pots.

You don’t need liquid nitrogen or high-tech trophies to enjoy the speed increases or sense of satisfaction a healthy overclock brings, however. All you need to do is use your rig to feel the difference. With 30-40% clockspeed bumps possible on Haswell and Skylake, the golden era of overclocking is back again.