Ultimate Core i7 Overclocking Guide -- We Push Nehalem to its Limits


Here’s everything you need to know about overclocking Intel’s new CPU

Overclocking can kill your CPU. It can corrupt your OS, melt your motherboard, and cause you to lose a month’s work or more. Despite those dire orange-alert warnings, however, overclocking has moved on from the Nerd’s Only Club to become practically a mainstream hobby in the last few years.

So why overclock if the risks are so great? For some folks, it’s about bragging rights. Like drag-strip racers who burn up an engine just to set a quarter-mile record, there’s a small community who will overclock a CPU to the brink of destruction just to run a benchmark and take a screen shot of the result.

The bulk of overclockers, however, are more concerned with the cost dividends. If you can take a $300 CPU and make it as fast or faster than one that costs $1,000, the money you save can go toward other components in your system. For these folks, it’s like getting a free high-end videocard.

Whether you’re a cheapskate or a drag racer, you’ll find that Intel’s new Core i7 CPU is unlike any previous Intel CPU, and overclocking this beast requires more tinkering than you might expect. Follow along as we explore what it takes to push this chip hard.

A Brave New World -- The crumbling of the front-side-bus wall means a wholesale change in how you overclock Intel’s top parts

Veteran Intel overclockers know that little about the process changed since the beginning. Like the basics of flying (stick, rudder, ball), all you had to worry about with an Intel processor was the front-side bus, clock multiplier, and core voltage.
Not so with Core i7. With Intel retiring the front-side bus, you’ll need to brush up on your overclocking skills and concepts if you want to get the highest-performing overclock out of the Godzilla of CPUs.

The Base Clock

If you haven’t kept up on current events, you need to know that Intel eliminated the front-side-bus architecture that has connected the CPU to the core-logic chipset since 1978. The memory controller, which used to sit in the chipset, is now integrated directly into the CPU.

Go into the BIOS on a Core i7 and you won’t see any reference to the front-side bus. Instead, it’s now the base clock or bclock. Some BIOSes also refer to this as the host clock or reference clock. On the current i7 procs, the base clock is 133MHz. While it’s not a front-side bus, most overclocking methods will require that you tweak the base clock just as you did with older FSB-based Intel chips and push it beyond 133MHz. One important thing to remember about the base clock is that it is the main reference clock for other components in the CPU— goosing this one setting will also overclock the RAM as well as the “uncore” (i.e., the L3 cache, memory controller, and Quick Path Interconnect, or QPI). We’ll get to more on this shortly.

Doing the Math

One thing that has not changed at all is the multiplier. The Core i7-965 Extreme Edition features a multiplier of 24 and is unlocked so you can move it up or down. The budget Core i7-920 features a multiplier of 20 and is upwardly locked so it will not move past 20. Experienced overclockers can grab a hall pass and skip to the next section, while those who have never done this before will need to stay in the classroom. As it was with the Core 2 and Pentium 4, the overall clock speed of the CPU is derived by multiplying the base clock (formerly the FSB) by the multiplier. For the 965, take 24 and multiply it by 133 to get 3200MHz or 3.2GHz. For the 920, take your multiplier of 20 and multiply by 133 to get 2667MHz or 2.66GHz.

Turbo Talk

One wrinkle to the multiplier is the new Turbo Mode. This mode essentially automatically overclocks a single core of the CPU under certain loads. If you have overclocked a Core i7-920 to 3.66GHz and then you switch on Turbo Mode, the CPU will actually run at 4.03GHz in single-threaded apps. Is it worth it? Frankly, we’re not sure. We are getting to the point where it’s pretty rare to be running performance-intensive single-threaded applications, so the performance boost will be minimal. You do get a 1x multiplier boost in dual-threaded apps so you most games would run at 3.83GHz. Sounds good right?

Our engineering sample Core i7-920 gave us results in line with what others have achieved with engineering sample parts as well as retail parts.

Unfortunately you can’t set your individual Turbo Mode settings on the cheap chips. Intel limits fine-grain Turbo Mode control to the Core i7-965 Extreme Edition. The pedestrian Core i7-920 and Core i7-940 are limited to a single multiplier increase for single-threaded apps, which is of marginal usefulness. It’s also clear that not all motherboard vendors think Turbo Mode is worthwhile. We’ve tested two different Asus boards that don’t implement Turbo Mode the same way Intel does. Instead of letting the user set the individual Turbo Mode settings on an Extreme Edition chip, your only option is to overclock all cores simultaneously.

Turbo Mode is something that should be evaluated based on your needs and the specifics of your overclock. For example, our case study actually found that a moderate overclock with Turbo Mode gave us better benchmark results than a higher-speed overclock without Turbo Mode.

Uncore: Separate but Connected

The Core i7 is a modular design with two main areas, the “core” and the “uncore”. Inside the chip, the actual execution cores that do the heavy lifting are treated as the “core.” The other parts, such as the integrated memory controller, L3 cache, and the Quick Path Interconnect are treated as the “uncore.” Since they’re separate entities, you can overclock the execution cores without overclocking the uncore to the same degree, in theory. This should let you hit higher speeds, since you wouldn’t be running the QPI, memory controller, or L3 quite as hard. In reality, however, it doesn’t work that way. Intel’s non-Extreme Edition Core i7 CPUs offer limited control over the uncore multiplier, so a boost to the base clock boosts the uncore speeds as well.

You need to set your Uncore multiplier to at least twice the multiplier for the system RAM. Also pay attention to the memory voltage. We had to run 1.66 volts for stable performance even at low RAM clock speeds. QPI also needed to be nudged up to 1.3 volts.

One thing to remember as you fumble around the BIOS is that the uncore must run at twice the speed of the system RAM. Here’s where it gets a little confusing. The speed of the uncore is determined by multiplying the uncore multiplier by the base clock. On a Core i7-920 chip, for example, the uncore defaults to 16. The uncore thus is 16 times 133 for a total uncore speed of 2,133MHz or 2.1GHz.

To figure out the RAM speed, you have to take the memory multiplier and multiply it by the base clock. In the case of a Core i7-920 chip, the default memory multiplier is 8. So to determine the main memory speed, multiply 8 by 133 for 1,066MHz. Why aren’t higher DDR3 speeds available? The highest official memory speed of the Core i7 is DDR3/1066. You can overclock your RAM to higher speeds, but depending on the motherboard, the only way to accomplish a memory overclock will be to crank up the base clock for the CPU—unless you own an Extreme Edition CPU.

The take away here is to remember to keep the uncore speed at twice the speed the RAM runs. If you plan to run DDR3/1600, you’ll need to run the uncore at 3,200MHz. On a Core i7-965, you can run that speed without overclocking. On a Core i7-920, you’ll have to overclock the base clock to get the RAM at that speed.

More about QPI

Another new element to the Core i7 is the Quick Path Interconnect that we mentioned earlier. This high-speed interface connects the processor (or processors, in a multi-proc machine) to the chipset. Intel currently has two QPI speed iterations. The Core i7-965 Extreme Edition runs at 6.4 gigatransfers per second and the non-Extreme chips, such as the Core i7-920 and Core-i7-940, run at 4.8GT/s.

The QPI is important to watch because ramping it up too far can kill your overclock. For example, pushing the base clock from its stock 133MHz to 200MHz on a Core i7-920 means that the QPI will default to 7.2GT/s. That’s quite bit more speed than the stock 4.8GT/s; however, we successfully tested a Core i7-920 running at 7.2GT/s and believe that’s still within the realm of viability.

The QPI speed of the Core i7-920 and Core i7-940 is derived by multiplying the base clock (133) by 36, which equals 4788 or 4.8GT/s. The Core i7-965 uses a default QPI multiplier of 48, but unlike the non-Extreme chips, the 965’s multiplier is not locked. If you believe your overclock is failing because you’ve cranked the QPI too far, you can try dropping the speed by changing the QPI multiplier. On some overclocking runs with a Core i7-965 Extreme Edition, we had to drop the QPI back from 7.68GT/s to 7.04GT/s to increase reliability. The bad news is that you can’t do this with the budget chips.

Turning off the CPU VR Current Limit Override in the BIOS prevents the CPU from throttling back the multipliers under the Turbo Mode. Notice the lack of Turbo Mode tuning versus that with an Extreme Edition installed.

Va Va Voltage

You probably learned as a three-year-old not to mess with electricity after that incident with the wall socket. The dangers of electricity haven’t changed, but to wring the greatest clock speed out of your CPU, you’re going to need to overvolt the suckah. Actually, it won’t be just the CPU—our highest reliable overclock was only achieved by upping voltage to the CPU and parts of the chipset.

How much is too much? The default core voltage of the three current Core i7’s is 1.2 volts. We were able to push voltage to 1.5 on a budget Core i7-920 and successfully make it through some torture testing, but in our opinion, that’s probably too much juice (at least with air cooling). Sure, it ran our stress tests for a reasonable amount of time, but we don’t think the chip will live for long. A more reasonable voltage is probably 1.4 volts (just slightly more than the maximum allowable of 1.375), but take that with a grain of salt, too. Because your CPU, motherboard, cooling, and PSU will be different, the amount of voltage you can add will likely vary from our scenario.

Another area you’ll have to overvolt is the QPI interface. You’ll likely have to increase the voltage from its stock 1.1 to 1.3 to get a reliable overclock. Some folks recommend running QPI voltage at or above the CPU’s core voltage, but we didn’t have to on our budget chip as it was happy with 1.3 volts. Our Extreme Edition, however, needed 1.5 volts to the QPI, which was equal to the CPU voltage. Finally, you’ll have to add voltage to your RAM to get it to higher speeds. There has been chatter that a RAM voltage exceeding 1.65 can destroy a CPU— fortunately, most of the high-performance DDR3 binned for Core i7 doesn’t seem to need more than 1.66 volts to run, which is certainly within reasonably safe limits.

Sometimes the highest clock isn’t the best for performance with Core i7. Using Turbo Mode, we saw a 3.66GHz chip perform as well as one running at 3.80GHz, and without the excess heat and voltage.

Cool Running

Overclocking and cooling go hand in hand. If you can keep the CPU cool, you’ll increase your overclocking limit. For the majority of our testing, we used a beefy ThermalRight 120. As good a reputation as this cooler has, it’s still just an air cooler. Water cooling is an even better choice for overclocking as it’s more efficient at removing heat from the core. Better still would be an exotic phase-change or Peltier unit that could bring the CPU temps far below zero degrees Celsius.

What’s a safe temperature for Core i7? That’s also open to debate. We had to dig around the Internet to find that all three Core i7s have a maximum recommended temp of 67.9 Celsius. That, however, is the maximum temperature taken on the outside of the heat spreader with a calibrated thermistor. In English, that means that if you’re being told that all four cores are running 82 C under full load, you’re probably OK for the short term—but your chip probably won’t last five years. It would be wise to aim for 80 C or lower, and even better to run in the 70 C range. Just remember that you may have to crank it all back in the summer.

Breaking It Down

If your head isn’t spinning by now, you’re in good shape, but most people, even somewhat seasoned overclockers, will want a stiff drink after trying to absorb this information. So let’s review.

You buy a new 2.66GHz Core i7-920—you just couldn’t splurge on the Core i7-965 Extreme Edition. You’ve got a reasonably chunky air cooler, a good-quality PSU, and you want to overclock that proc. What should you do? First, you need to start goosing up the base clock until you get a clock speed that seems reasonable for the CPU. So, let’s say you want to aim for a nice conservative 3.5GHz. Start by setting your base clock to a speed that will get your CPU in the region. Take it from 133MHz to, say, 160MHz. The target clock speed you’ll want is actually 175MHz for 3.5GHz, but we’ll start with 160MHz.

We’ve read posts on the Internet of some people being able to reach a high clock speed without the need for additional voltage to the CPU, so you’ll want to see if your CPU is capable of it. If you want a good real-world test, run your favorite multithreaded encoder, such as HandBrake. Nero’s Recode is also multithreaded and will put a reasonable load on the CPU. If you want to really torture it, download Prime95 from www.mersenne.org . Unzip it and run Prime95.exe. Select In-place FFTs and make sure the number of threads is set to eight. By default, it should be eight for a Core i7. If it runs for, say, half an hour, you can aim higher. Add 5MHz to your overclock and try again. Go until it fails.

Extreme Edition CPUs feature unlocked QPI and memory ratios and even let you override the individual Turbo Mode settings and change how the CPU throttles under power loads to either let you run higher overclocks or nuke your CPU faster.

Now it’s time to add voltage. A good starting point is 1.35 or 1.375 volts. Since 1.375 is the maximum allowable voltage under spec, you’re actually still playing it very safe. While you’re there, you may want to add voltage to the QPI. So nudge it up from 1.1 volts to 1.3 volts. Some say that the QPI voltage should at least equal the core voltage, but that’s up to you. You should also add voltage to the RAM to get it to spec. If you’re using RAM rated for higher speeds, give the RAM the amount the maker suggests. The safe limit seems to be 1.66 volts.

Intel says it locked the QPI and memory multipliers on its budget chips but some motherboards appear to circumvent this. If your board does this or you’re running an Extreme Edition, which has unlocked QPI and memory multipliers, you can try backing down your uncore and RAM multiplier. Most BIOSes should correct the ratios for you, but remember that the uncore multiplier should be twice that of the RAM. So if the RAM multiplier is set to 10, set the uncore multiplier to 20. If you back the RAM multiplier down to six, you can set your uncore multiplier to 12.

Now reboot and rerun your stress tests. Keep repeating the steps we’ve outlined until you hit your target or hit a wall. Once you’re at your target, do a longer stress test with Prime95 to see if it is truly stable. You may have to add cooling or voltage to get it to run for several hours.

Remember, cooling and quality power matter. If you think you can do all this with a cheap power supply and straw heatsink, think again. Want to know how far can you get Intel’s cheapest Core i7 to overclock with air cooling? Read our case study to find out.

Case Study: Core i7-920

Can you make Intel’s cheapest i7 outperform the vaunted Core i7-965 Extreme Edition?

For our case study, we decided to stick with what the overwhelming majority of overclockers will select: Intel’s cheapo $285 2.66GHz Core i7-920. We paired this chip with an Intel DX58SO “Smackover” board, 6GB of Corsair Dominator DDR3/1600 RAM, an EVGA GTX 280 videocard, a WD 150GB Raptor, and a PC Power and Cooling Turbo-Cool 1200.

For cooling, we used a ThermalRight 120 and ran the board on a lab bench with three small fans for auxiliary cooling: an 8cm on the hard drive, an 8cm USB fan blowing on the RAM, and a 12cm fan directed at the voltage regulators and the heatsink.

We used Windows Vista Home Premium in 64-bit and ran a few benchmarks at the CPU’s stock 2.66GHz clock speed with the default Turbo Mode on. We then used the board’s OS-based overclocking tool for the majority of our overclocking attempts. Why? Normally we prefer the BIOS as we don’t want to deal with yet another app starting in Windows, but the Intel Desktop Control Center let us turn knobs and tweak settings without having to boot into the BIOS. This saved us invaluable time.

For stress-testing, we used a custom blend of Prime95 that Velocity Micro developed for overclock testing. We’ve long used the utility and have found it to push machines harder than any other test.

We know from previous experience that the Core i7 seems to need a minimum QPI voltage of 1.3 volts, so that’s where we started. We intentionally kept our RAM clock speeds down since we just wanted to see how far we could push the CPU, but we were surprised that we had to move the voltage to 1.66 volts to reach stability. After a day of testing and one OS reinstall, we decided that the best performance we could squeeze out of the Core i7-920 was 3.66GHz with Turbo Mode on. This was done with a base clock of 183MHz, the core voltage at 1.4 volts, the RAM at 1.66 volts, and the QPI at 1.3 volts for a QPI speed of 6.59GT/s.

With Turbo Mode, the default 20 multiplier occasionally reached 21 or 22 for an effective clock of 3.83GHz to 4.03GHz, which allowed us to achieve benchmark results on par with our higher overclock attempt: We topped our chip out at 3.8GHz with 1.5 volts but the CPU’s thermals were unacceptable, running into the mid 80s C. In our Lab in the winter, that’s fine, but we knew this overclock would never survive a real summer in a real home.

Our take away is that it should be very easy to push the 2.66GHz Core i7-920 by 1GHz, and more conservative overclocks of 3.5GHz or 3.4GHz should be extremely easy. But to expect a truly reliable overclock over 3.6GHz will require good-quality water cooling.

Core i7-920 Benchmarks

(Turbo On)
(Turbo On)
(Turbo Off)
(Turbo Off)
Cinebench 64-bit 10 w/ 8-threads
16,097 21,474
Cinebench 64-bit 10 w/ thread
ProShow Producer
3DMark Vantage Overall 14,720
3DMark Vantage GPU 12,099
3DMark Vantage CPU
Valve Particle Simulation 140 190
Main Concept Reference AVC Pro
Best scores are bolded.

How Reliable is Your Overclock?

If you’re ever in an online game where some dude brags that he’s running his blah, blah rig at blah, blah speed using special blah, blah tricks, take it with a pound of salt. Like the great fish that got away, people tend to inflate their overclocking achievements.

We were very happy with Intel’s Desktop Control Center as an OS-based overclocking tool but this 4GHz overclock was just wishful thinking.

Sometimes they don’t even know they’re exaggerating. If you think pushing a machine to 5GHz and running a game marks an overclocked rig as bullet proof, think again. The majority of today’s games barely push two threads, and even when they do, the CPU doesn’t do much. The truth is that much of the heavy lifting in games is done on the GPU, so you can’t use gaming as a true judge of a stable overclock.

Instead, try a multi-hour encode or transcode of a video using a multithreaded encoder like HandBrake. Even better, try a serious ball-buster like Prime95. This math-heavy prime-number hunter features a built-in torture test that is truly torturous. For the majority of our tests here, we used a custom-blend of Prime95 that we’ve found to put the heaviest load on overclocked CPUs. In our experience, the benchmark manages to properly put our overclocking hopes in their place by blue screening an unstable machine in mere minutes.

If your machine will withstand a couple of hours of Prime95, you’re doing good. If it’ll run over night, it’s bullet resistant. If it’ll run overnight in the middle of the summer, in an 80 degree room, well, you really have something to brag about.

Core i7 Budget Buyers, Beware

It got to be so easy to overclock Intel’s Core 2 line that a Mac user could one-mouse button the cheapest 10MHz Core 2 chip to 12GHz. That pretty much killed demand for Intel’s 3.2GHz Core 2 Extreme QX9770 CPU since the only difference was an unlocked multiplier and higher front-side bus (Intel’s insane pricing didn’t help, either.)

With the Core i7, Intel is being a far more aggressive in differentiating the Extreme chip. No longer is it just a multiplier lock. With the Core i7-920 and Core i7-940, you cannot set the individual Turbo Mode ratios. The thermal override switch and amperage override are also disabled. Finally, QPI ratios and memory ratios are similarly limited.

Does this make the Extreme Edition a chip that you must have? It depends. Swapping the Core i7-920 with a Core i7-965 Extreme Edition is like moving from a budget car to a luxury car built on the same platform. You’ll reach for a knob in the budget car that was there in the luxury car and find that it’s not there.

In our experience, it was easier to get the Core i7-965 to the limits imposed by our air cooling than it was with the budget CPU. While it took us an afternoon to get the Core i7-920 to a reliable 3.80GHz, we had the Core i7-965 Extreme Edition at 3.83GHz in a few minutes. By adding the Turbo Mode, we had the latter chip effectively running at 4GHz, and we’re certain higher speeds were attainable with better cooling. One thing to keep in mind though: While the Core i7-965 will likely reach higher speeds than the Core i7-920, it won’t give you the same amount of headroom. That is, you can get a 1GHz overclock with the Core i7-920 on air but you won’t be able to do the same with the Core i7-965 without water or something more exotic.

Is the 965 Extreme Edition a better overclocker? Certainly. Is it worth the extra $700? That’s a tough call and ultimately something only you and your bank account can answer.

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