Overclock Your PC

Nathan Edwards

If you’re running your CPU at stock speeds, you’re missing out on your PC’s true potential, because processors often harbor power beyond their official specs. Your proc, for example, might be rated to run at 3GHz but is actually capable of operating reliably at 3.3GHz. There are myriad reasons for the hidden headroom, ranging from natural variance among parts (even those made from the same batch), to the manufacturers’ practice of underclocking parts to meet market needs, to the improved capabilities of a part over the lifetime of its production.

The point is, you’re not a true power user if you leave a CPU’s hidden performance potential untapped. And the only way to release your proc’s inner speed demon is to overclock it. This story will tell you how. Even if you’ve dabbled in the practice in the past, you’ll want to read on. Because just as CPUs have changed over the years, so has the art of pushing them to their limits. Over the following pages we’ll tell you everything you need to know about overclocking today’s CPUs, be they AMD- or Intel-branded. We’ll explain what’s involved, how to determine what your hardware is capable of, and how to achieve optimal results. Most importantly, we’ll tell you how to overclock safely. Indeed, overclocking is serious business and should never be taken lightly.

When you tamper with the internal workings of your computer’s parts, you do so at your own risk. Overclocking can damage, or even destroy, your CPU, motherboard, RAM, or other system components, and it can void the warranty on those parts. So consider yourself warned about the potential hazards! That said, you’re unlikely to harm your hardware if you overclock with extreme caution and care. And following the advice and instructions we lay out here will help you. So let’s get started!

The Basics of CPU Overclocking

Pushing your processor to new heights can be extremely rewarding—if all goes right. Take the time to understand the concepts of overclocking and the factors that can affect success.

Determining a CPU’s Speed

There’s simple math that determines the clock speed of any CPU. Each CPU has a fixed internal number called the clock multiplier. That number multiplied by the reference clock of the front-side bus determines the stated clock speed of the processor. For example, an Intel 2.66GHz Core 2 Quad Q6700 has a clock multiplier of 10. The stock system bus speed for this processor is 1,066MHz. But wait, 1,066MHz multiplied by 10 equals 10GHz. What gives? Intel’s front-side bus is quad-pumped, so its actual reference clock is 266MHz (1,066MHz divided by four). That makes the clock speed of a Core 2 Quad Q6700 10 times 266MHz for 2,660MHz, or 2.66GHz.

This same math applies to AMD’s Athlon 64 CPUs, although, technically, they have no front-side bus; instead, a HyperTransport link connects the CPU to the chipset. A 2.6GHz Athlon 64 X2 5000+, for example, operates on a 13x multiplier using a 200MHz link—the actual HyperTransport link connection runs at 1GHz, as it operates on a 5x multiplier.
You can overclock both Intel and AMD CPUs by increasing the multiplier setting, increasing the “front-side bus,” or both. By using a combination of a multiplier and FSB overclock, you may achieve higher speeds with more stability. Depending on your situation, a combination of both may give you the best overclock, as your motherboard may simply not be up to running at excessively high speeds.

Multiplier Locking

CPU manufacturers will take measures to ensure that a processor runs at its intended speed by locking the multiplier. This fixes the multiplier setting, so it cannot be changed in the BIOS. This is done primarily to keep CPU “re-markers” from selling cheaper parts as more expensive ones, but it also serves to thwart overclockers.

But not every chip is locked. Intel’s Extreme series of CPUs does not feature multiplier locking nor does AMD’s FX series or some of its new Black Edition CPUs. This gives overclockers who pay the extra price of admission more flexibility in their adventures. A 2.66GHz Core 2 Extreme QX6600 CPU, for example, can be overclocked to 2.93GHz simply by increasing the multiplier from 10 to 11 without having to resort to front-side bus overclocking.

The Role of Core Voltage

When you overclock, you essentially run the CPU out of spec. Upping a CPU’s core voltage allows you to run a CPU way out of spec by further increasing your overclocking headroom. For example, a stock Intel Core 2 Duo E6600 running at 2.4GHz eats about 1.2 volts. To get the same CPU up past 5.6GHz, one overclocker increased the core voltage to 1.9 volts. As you can imagine, if AMD and Intel designed a CPU to operate at a certain voltage, running it higher will greatly decrease the life expectancy of the CPU. This is the most dangerous element of overclocking. The worst we’ve personally seen from overclocking a CPU via its multiplier or front-side bus is instability or a corrupted OS. But by adding a ton of voltage to a processor, you risk nuking it. Proceed with caution!

How Do I Know if My CPU is Overclockable?

As a rule of thumb, a very mature CPU production line will yield parts that are capable of running at much higher than rated speeds. So, while it’s not a guarantee, overclockers are generally better off with later-stepped CPUs.

An even better way to determine your processor’s overclocking credentials is to download CPU-Z (www.cpuid.com). This freeware utility will identify your Intel or AMD CPU and tell you such nitty-gritty details as the stepping and revision of the proc. Steppings and revisions are internal labels that Intel and AMD use to denote versions. A step denotes larger changes while a revision indicates fairly minor tweaks.

Once you find out that your retail 2.4GHz Core 2 Quad Q6600 is a revision G0 you can rejoice in knowing that it runs cooler and can withstand more heat than the previous B3 step version. You learn those particular CPU qualities only by doing research, and the best resources are online overclocking databases. Almost every enthusiast PC site has a section devoted to overclocking, where users post details of their own experiences with various CPUs. MaximumPC.com, ExtremeSystems.org, and FiringSquad.com all include areas for users to discuss overclocking exploits.

Next: Have the Right Hardware!

Have the Right Hardware

Your CPU isn’t the only part that matters in your quest for more speed. Here are the other components to care about.


You get what you pay for and, generally, a cheap-ass motherboard will yield marginal overclocking results. The more expensive the motherboard, the more likely it is to have better components and better overclocking capabilities. That doesn’t mean all sub-$100 mobos are overclocking duds, but you’ll have to troll the forums and customer reviews on enthusiast sites to determine if a cheapo mobo can OC.

One additional tip: If you’re buying a motherboard for overclocking, you’ll likely have the best success with the latest “spin.” Mobo vendors update their boards with fixes and more recently built boards will usually overclock better.

Power Supply

We’ve long said that the PSU doesn’t get the attention it’s due, and that’s especially true when it comes to overclocking. The fact is, the need for clean, reliable power is of utmost importance if you’re pushing a CPU, RAM, and motherboard to the edge.

You don’t need a 1,200-watt PSU like this PC Power & Cooling Turbo-Cool, but you do need a quality, name-brand unit.

If you have a high-compression engine in your street racer, are you going to fill it with 85 octane fuel? A cheap power supply is the equivalent of questionable Kwik-E-Mart gasoline. For overclocking, you don’t need a 1,200-watt PSU, but you do need a name-brand unit. Generally, it’s safer to have a PSU that delivers a bit more power than you need. While it may not be the most power-efficient scenario, a 750-watt PSU running at 450 watts will probably live longer than a 500-watt PSU running at 450 watts.


Excessive heat can cause system instability, so it’s essential to keep your overclocked CPU cool. To achieve extremely high overclocks, some hobbyists bathe their CPUs in liquid nitrogen. Others use phase-change units (essentially tiny freezers) to push 3GHz chips past the 5GHz mark. The point is, you can’t expect to push your 1.86GHz proc to a reliable 4GHz using a $12 heatsink. Know your overclocking goals and then choose your cooling accordingly. Air cooling is the most modest solution, followed by water cooling, peltier/liquid combinations, phase change, and exotic liquids, such as liquid nitrogen. Also remember that the extra heat produced by overclocking will warm up the rest of your machine, so you may have to upgrade your case’s cooling or the case itself if you experience overheating issues. For our CPU cooling recommendations, see page 34.


In the old days the front-side bus’s speed was tied to the speed of the system RAM, so you had to overclock both. That’s no longer the case, but some folks still prefer to give their RAM some extra juice. This is the purpose of pricey, high-performance RAM. It’s been certified by the RAM manufacturer to operate at a given “overclocked” speed. We say overclocked because RAM speeds and timings are actually spec’d by an organization called JEDEC. The top standard speed of DDR2 today is 800MHz. Overclockable DDR2 RAM generally runs in the 1,066MHz range, with some pricier modules pushing 1,250MHz. While it’s not necessary to overclock your RAM to overclock your CPU, there are some instances when you will get improved performance if the FSB and RAM run at similar speeds that are closely synced. Some applications will also favor the increased bandwidth of overclocked RAM. Your research will help you determine if this applies to your parts.

Overclocking an OEM Machine

So, you’re ready to crack open your business-class Dell, HP, or Gateway and give it some gusto, eh? Fugetabout it. The overwhelming majority of OEM machines and notebook PCs prevent overclocking to reduce complaints from the chumps who OC recklessly and ruin their machines. Even motherboard brands known for overclocking may be neutered in an OEM machine. Got it? OK, now we’re going to contradict ourselves. Some OEM boxes do overclock. Dell’s XPS and Hewlett-Packard’s Blackbird PCs are designed to overclock. Still, for the most part, overclocking and OEM machines don’t mix.

Next: Overclocking Intel!
Or, skip to Overclocking AMD!

Overclocking Intel

The steps we take to push our Core 2 Extreme QX6700 beyond its 2.66GHz stock speed can be applied to any
modern Intel processor.

Step 1: Back Up Your Data

While the risk of hardware loss is generally very low, there’s always the possibility of OS corruption or data loss.

Step 2: Enter Your BIOS

Get into your BIOS by hitting the Del, F1, or F2 key during boot. The key will vary by motherboard, so check your documentation if you’re not sure what to press. Once in the BIOS, you will need to find the appropriate configuration screens for overclocking. The screens we refer to in our examples are specific to the EVGA 680i SLI motherboard—they will differ from BIOS to BIOS. Your mobo manual or an online search can provide guidance, but often you just need to dig around.

Step 3: Goose Your CPU's Multiplier

One way to overclock your Intel CPU is to increase its multiplier—if it’s unlocked, which is true for any Extreme-class Intel processor. The downside to doing a multiplier-only overclock is that there is very little granularity. Taking a 2.66GHz Core 2 Extreme QX6700 from its stock 10x multiplier to 12x jumps you all the way to 3.2GHz. If you want to hit 3.1GHz, a multiplier overclock won’t let you do it. Try increasing your CPU’s multiplier just a notch or two (in our BIOS, the multiplier setting is in Advanced Chipset Features, System Clocks). Then reboot your system and see how it runs. If your system crashes or won’t start, see Step 7.

Step 4: Increase your Front-Side Bus Speed

The other, more likely, way to overclock your Intel CPU is through the front-side bus. By bumping the FSB beyond its stock 800MHz or 1,066MHz, you increase your CPU’s clock speed. On the majority of CPUs, this will be the sole overclocking option, as only the most expensive Intel chips are unlocked. On our EVGA 680i board, we went into Advanced Chipset Features, FSB & Memory Config.

Here, we set the FSB Memory Clock Mode to Unlinked. This effectively separates the RAM clocks from the front-side bus. (If your chipset doesn’t allow you to unlink the RAM, you will need to choose an FSB-to-RAM speed ratio; make sure your choice keeps you within your RAM’s spec. See Step 6 for more info.) Increase your FSB by just 20MHz increments. Reboot with each increase to see if your machine will boot (if your system crashes or fails to reboot, see Step 7). With the multiplier set at its stock 10x, we pushed our 2.66GHz Core 2 to 3GHz by increasing the FSB speed from its stock 1,066MHz to 1,200MHz.

Step 5: Add Some Voltage

We wanted to go beyond the 3GHz we achieved, but our attempts at pushing the FSB further made our system unstable. There’s still hope for more speed if we increase our CPU’s voltage. In our BIOS’s Advanced Chipset Features, System Voltages screen, we can increase the CPU voltage, the chipset voltage, and the memory voltage, in addition to the voltage of a few other parts. By pushing the CPU voltage of our early-rev 2.66GHz Core 2 Extreme QX6700 from 1.11 volts to 1.39 volts, we’re able to push the FSB up to 1,333MHz and achieve a stable 3.2GHz CPU speed. How much voltage is safe? It’s difficult to say, as the number differs among CPUs and motherboards. We recommend that you troll forums and overclocking databases to see how far people are going with individual chips. We can’t give general recommendations on voltage as each CPU has different specs and anything over stock could nuke your chip.

Step 6: To Overclock RAM or Not?

So you’re satisfied that the CPU is running far above its rated speed, but now you want to overclock the RAM. As we noted above, our nForce 680i board offers the option to run the RAM linked or unlinked. Linking the RAM sets the RAM speed as a ratio of the front-side bus’s clocked speed. The ratios are determined by the chipset, and in our case, we could choose between FSB: memclock ratios of 1:1, 5:4, 3:2 or Sync mode, which is fractionally equivalent to running at a 2:1 ratio.

Picking any of the settings will change the RAM speed. For example, if you push your FSB to 1,066MHz and choose a 1:1 ratio, your RAM speed will hit 1,066MHz—if you’re using overclockable memory (see RAM section on page 24). If you’re not using overclockable RAM, your box will probably just hard lock. A 5:4 ratio would give you 853MHz, 3:2 generates 711MHz, and Sync gives you 533MHz. Which is better? Some overclockers report that linked RAM gives better performance than unlinked. But you’ll have to test your system by running apps you typically use to determine which setting is the most stable and provides the best performance for your needs.

Step 7: Beep! Beep!

No, your system isn’t asking you where the Dagobah system is. That constant beeping means your overclock failed. With some motherboards, simply powering down by unplugging the system from the wall or switching off the PSU for a few seconds will get you back into the BIOS. In some cases, you’ll need to reset the system’s CMOS by cutting power and then throwing the CMOS-clear jumper or removing and then reinserting the coin-cell battery.

Step 8: Test It!

Just because you booted into the OS doesn’t put you in the clear. You should now stress-test the system using Prime95 or another application that really stresses the CPU. You might be tempted to use 3DMark06, but it’s primarily a GPU test, and many overclocked systems that pass 3DMark06 burn-ins will actually fail under heavy CPU loads.

Overclocking Results For Other Intel CPUs
CPU/Core Stock Speed
Overclocked Speed
Voltage / FSB X Multiplier
Rev / Step
Core 2 Quad Q6700 / Kentsfield 2.66GHz
1.40 / 343 x 10 B / G0
Core 2 Duo E6300 / Conroe 1.86GHz 2.88GHz 1.50 / 412 x 7 F / B1
Core 2 Duo E4500 / Allendale 2.20GHz
3.30GHz 1.40 / 300 x 11 D / M0
Pentium E2160 / Allendale 1.80GHz 3.37GHz 1.56 / 375 x 9 2 / L2

Performed on an EVGA 680i SLI mobo, 2GB Corsair Dominator 8500 RAM, a PC Power and Cooling 1KW PSU, and standard air cooling.

Next: Overclocking an AMD Chip !
Or, skip to Cooling Your CPU!

Overclocking AMD

While we wait for AMD to launch its new high-end part, we can push its low- and midrange procs, such as the
Athlon 64 X2 6000+ we use here, to new heights.

Step 1: Back Up Your Data

Overclocking is inherently risky, so back up your data. We mean it.

Step 2: Enter your BIOS

Get into your BIOS by hitting the Del, F1, or F2 key during boot. The key will vary by motherboard, so check your documentation if you’re not sure what to press. Once in the BIOS, you will need to find the appropriate configuration screens for overclocking. The screens we refer to in our examples are specific to the Asus M2N32-SLI motherboard, but they will differ from BIOS to BIOS. Your mobo manual or an online search can provide guidance, but often you just need to dig around.

Step 3: Increase your CPU's Multiplier

Your choices for overclocking are determined by your proc. AMD’s FX-grade CPUs, like Intel’s Extreme chips, are unlocked and let you alter their multiplier settings. AMD recently began unlocking its Black Edition procs as a concession to overclockers who have stuck with the platform. Increasing the multiplier makes for a no muss, no fuss overclock. On our Asus M2N32-SLI board, we go into Advanced JumperFree Configuration and find CPU Multiplier. Our Athlon 64 X2 6000+ is locked, and thus can’t exceed its stock setting of 15x. If your chip is unlocked, you can select a higher multiplier. To get an Athlon 64 FX-60 from 2.6GHz to 2.8GHz, you would need to increase the multiplier from 13x to 14x. Next, test your system for stability. If it crashes or won’t boot, see Step 8.

Step 4: Meet the HyperTransport Link

There’s an overclocking alternative to altering a chip’s multiplier setting. If this were an Intel platform, we’d turn our efforts to the front-side bus and be instantly overclocking, but AMD’s design is a little more complicated. You’ll need to futz with the HyperTransport (HT) speed before you overclock. This interface between the CPU and chipset buzzes along at about 1GHz and doesn’t like to get too far out of spec. Often, people who overclock without reducing the HT speed confuse HT instability with CPU instability. To lower the HT link on our M2N32-SLI board, we go into the BIOS and drill down through the Advanced and Chipset menus. There we see a setting for CPU<->NB HT Speed. Our choices are 1 through 5 and Auto. The default is 5x 200, or 1,000MHz. Since this value will increase during the overclock, knocking it back to 4x (800MHz) or even 3x (600MHz) shouldn’t hurt performance. Keep in mind that when you overclock the CPU frequency, you overclock the HT as well. If, for example, you overclock your CPU frequency to 220MHz and are running a 4x multiplier on your HyperTransport link, you’ll actually be running an 880MHz HT. Set it at a lower speed and prepare to overclock.

Step 5: Boost your Frequencies

Now it’s time to overclock that sucker. On our M2N32-SLI board, we go to Advanced, JumperFree Configuration, and open CPU Frequency. There, we’re greeted by settings of 200MHz and up. We can bump the frequency up to 210MHz, which when multiplied by 15x (the CPU’s multiplier setting), gives us an overall speed of 3.1GHz. Another bump up to 220MHz gives us 3.3GHz. We recommend you increase speeds by 10MHz increments, testing for stability after each jump. If your machine crashes or fails to reboot, see Step 8.

Step 6: Add Voltage

In some cases, boosting the voltage to your CPU can help stabilize an overclock that’s crashing. Unfortunately, this is one of the more dangerous aspects of CPU overclocking as overvolting a chip could kill it. On our M2N32-SLI board, we went into Advanced, JumperFree Configuration and changed the CPU voltage from Auto to 1.5 volts. That’s about a tenth of a volt out of spec, but unfortunately for us, it didn’t help us sustain a 3.3GHz clock speed, so we’re stuck at 3.28GHz.

Step 7: RAM Divisors

With AMD CPUs, the RAM is linked to the clock setting, and the Athlon 64’s on-die memory controller supports only whole numbers for memory divisors. So a 3GHz Athlon 64 X2 6000+ can use either a 7 or 8 divisor to generate a signal for the RAM. Unfortunately, 3,000 divided by 7 works out to DDR2/857 and 3,000 divided by 8 works out to DDR2/750. AMD errs on the side of caution, so this processor actually runs the DDR2/800 at 750MHz. But when you overclock, you may inadvertently overclock the RAM further than you suspect. The 3.28GHz we achieved on our M2N32-SLI board, for example, runs the DDR2/800 slightly out of spec at 825MHz. That’s not something to worry about, but if you’re running your chip at much higher speeds than us, you’ll need to make sure the RAM isn’t running beyond what its maker guarantees. To do that, go into Advanced, CPU Configuration, DRAM configuration, and then Memory Clock Frequency. You should select a conservative low speed for now and clock it up after you’ve reached the CPU’s highest speed.

Step 8: It Won't Boot

Don’t be bummed if your machine hard-locks—it’s the only way you’ll learn your CPU’s limits. To get out of the hole, shut off the PSU or pull the plug from the wall for five seconds. Plug it back in and power up the box. Some boards will automatically recover from a bad overclock and let you go back into the BIOS to aim a little lower. If this doesn’t work, you’ll have to power down again, unplug the PSU, and reset the CMOS via a jumper or button, or by pulling and reinserting the coin-cell battery. After five seconds, try booting it again—you should be able to access the BIOS.

Step 9: Test It!

Getting into the OS is about 65 percent of the challenge. You’ll now need to test the machine by pushing the CPU with an intensive workload. We don’t recommend gaming as a test since games are typically GPU-bound. Try a video encode or run Prime95. And if you have a multi- or dual-core processor, run a multithreaded app.

Overclocking Results For Other AMD CPUs
CPU/Core Stock Speed
Overclocked Speed
Voltage / FSB X Multiplier
Rev / Step
Athlon 64 X2 6400+ / Windsor 3.2GHz 3.43GHz 1.50 / 214 x 16 3 / JH-F3
Athlon 64 X2 6000+ / Windsor 3.0GHz 3.29GHz 1.525 / 218 x 15 3 / JH-F3
Athlon 64 X2 4200+ / Brisbane 2.2GHz 2.73GHz 1.524 / 249 x 11 1 / BH-G1
Athlon 64 X2 BE-2350+ / Brisbane 2.1GHz
2.61GHz 1.55 / 249 x 10.5 1 / BH-G1

Performed on an EVGA 680i SLI mobo, 2GB Corsair Dominator 8500 RAM, a PC Power and Cooling 1KW PSU, and standard air cooling.

Next: Keeping Your CPU Cool !

Keeping your CPU Cool!

Without adequate cooling, your overclocked rig is as good as toast.


It’s hard to get much worse than a stock air cooler for your CPU. That’s not to say there’s anything outright wrong with the fan/heatsink combo that comes with a new CPU—the little guy will likely keep your stock-clocked processor running well within safe operating temperatures.

The minute you start overclocking your processor, however, you’ll be jacking up your thermals to levels a stock cooler can’t handle. Granted, when overclocking, temps will go up with even premium air coolers, but a solid aftermarket device will give you more room to work with. Your initial temperatures will be lower, and they won’t rise as quickly as they would with a stock solution.

While other air coolers have matched the CNPS9700’s effectiveness, none has done so in this small a package.

Our current Lab champion, Zalman’s CNPS9700 ($80, www.zalmanusa.com), has maintained the throne for nearly a year. It uses a copper and aluminum framework to absorb the warmth produced by both Intel and AMD CPUs. The cooler’s 2,800rpm fan emits a tornado-like whoosh when it’s cranked to the max, but it also allows the device to reach epic levels of heat reduction. In fact, we now use the Zalman as a benchmark for other coolers. On the last test we ran, the device took our processor down to 37.5 C during our CPU burn-in test and 22.5 C when idle—a savings of 16.5 C and 9.5 C, respectively, over stock.


aftermarket air cooling is a fine way to manage CPU temperatures, but only to a point. Eventually, practicality and performance concerns render air coolers insufficient for OC’d machines. That’s why there’s liquid cooling. Not only can you reach lower temperatures when using a liquid-based setup as opposed to air, but you’ll also benefit from a lower sound profile.

Of course, there’s an obvious caveat: Liquids plus electronics can equal a serious monetary hit if you have to replace hardware that inadvertently gets wet. Installing a water-cooling kit in your rig is a delicate process, and the drama only increases if you’ve never done it before. Sure, you can go with a preassembled liquid-cooling kit, but in our experience, a majority of these units perform on par with—if not worse than—stock air coolers.

Hardcore cooling devices, like this CoolIt Boreas, come with a price—they require you to stuff more and more gear into your case.

The best liquid cooler we’ve found is CoolIT’s Boreas unit ($450, www.coolitsystems.com). A fancier, fatter version of the company’s Eliminator, the Boreas uses 12 thermoelectric modules to rip the heat from your molten tubing into a giant heatsink. Two 12cm fans take care of the rest, allowing the Boreas to beat our FX-60 test bed’s stock cooler by 20 C in idle and 32 C during our burn-in test.

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