We would like to take yet another approach: The second processors from the top are a bit too expensive for us to consider them practical for this build, and the least expensive processors usually have the lowest multipliers. Low multipliers can make overclocking difficult because some motherboards can’t hit FSB or HyperTransport frequencies high enough to excessively tweak the CPU and remain stable. So, for our no-nonsense Intel- and AMD-powered systems, we decided to go with middle-of-the-road processors and overclock them as far as possible. The chips we chose have higher multipliers and are only marginally more expensive than the most affordable processors in their respective lineups, yet they still only cost about a third of the price of Intel’s and AMD’s current flagships.


Intel Core 2 Duo E6400

There are currently two types of processors in Intel’s Core 2 Duo lineup, those with 4MB of L2 cache and those with 2MB. The processors with 4MB of L2 cache are the higher-clocked, more expensive offerings. When clocked at similar frequencies, however, there’s only a slight performance difference between the two processors. For our build, a Core 2 Duo with 2MB of cache would suffice, so we went with the Core 2 Duo E6400 ($250) and sprung for a high-performance CPU cooler: the Arctic Cooling Freezer 7 Pro.

In its stock configuration the Core 2 Duo E6400 runs at 2.13GHz. You can get that speed by multiplying the CPU’s respective stock multiplier (8) by its FSB frequency (266MHz). By raising either of those values, the processor would run at a higher frequency. The Core 2 Duo, however, isn’t unlocked like the Core 2 Extreme, so it’s not possible to overclock the chips by raising the multiplier. The only way to overclock a Core 2 Duo is by raising its FSB frequency.

Before we got to the business of overclocking our processor, however, we modded it slightly to help it run a little cooler. We followed essentially the same procedure you can find in the October issue of CPU on page 40. Because the CPU’s integrated heat spreader and our Freezer 7 Pro’s copper base were already nice and flat, all we did was use a wadding metal polish to eliminate small peaks and valleys from their surfaces. Doing so makes the surfaces mate better, which aids in heat transfer. This is a quick and easy mod that will usually shave a couple of degrees off a processor’s operating temperature, so there’s really no reason not to do it—well, except for that voided warranty thing.


Make The Grade

Once we sufficiently polished our CPU’s heat spreader and cooler’s base and installed everything, we began overclocking the system via the BIOS. We overclocked the system in multiple steps: First, we disabled whatever integrated peripherals we didn’t intend to use and then we locked the motherboard’s PCI Express frequency at 100MHz. Doing this will keep the PCI-E links from becoming unstable and potentially prevent an integrated peripheral’s failure from hindering our overclocking efforts.

Next we increased all pertinent voltages for the processor, memory, and motherboard chipset. At their default voltages, each of these system components would only overclock to a certain point, but with a little extra juice we could overclock them even further. Because we didn’t take any elaborate cooling measures, though, we only bumped up the CPU voltage from 1.35 to 1.41V, our memory voltage to 2.3V, and our MCH (northbridge) and ICH (southbridge) voltages by 0.1V.


Hit The Virtual Wall

After making the necessary voltage changes in the system BIOS, we then began overclocking our processor by increasing its FSB frequency. At first we raised the FSB frequency by about 10MHz at a time until the system was no longer stable and then backed the frequency down 1 to 2MHz until the system stabilized. When all was said and done, we settled on a 360MHz FSB frequency with a final CPU clock speed of 2.88GHz. Our CPU probably had some more in the tank, but the affordable Abit P965 motherboard we used for this experiment got flaky once the FSB hit speeds above 3.45MHz or so. There have been reports of the Core 2 Duo E6400 processors hitting above 3GHz, so we may revisit this CPU in the future and couple it with a more overclocker-friendly motherboard.

AMD Athlon 64 X2 4600+

As we already explained in the first half of this article, the components we chose for our practical power user systems had to meet certain criteria. Like our Intel processor, the CPU for our AMD build needed to be significantly more affordable than the company’s current flagship Athlon 64 FX-62 processor, but it also had be a good candidate for overclocking. After a bit of searching, we decided to go with AMD’s Athlon 64 X2 4600+. The price cuts AMD made prior to Intel’s introduction of the Core 2 Duo put the standard edition Athlon 64 X2 4600+ into sub-$250 territory, which made this 2.4GHz CPU a good candidate. We should note that AMD also offers an energy-efficient version of the X2 4600+ that would have worked just as well for this build, but it’s about 20% more expensive. (You can tell the difference between AMD’s energy-efficient processors and their “normal” counterparts by their model numbers. The third digit of the processor’s part number details the maximum power consumption of that model: A denotes “normal” power, O indicates a 65W energy-efficient processor, and D signifies a 35W energy-efficient, SFF processor.)


Prep The Hardware

To get our Athlon 64 X2 4600+ ready for tweaking, we gave it the same treatment as our Core 2 Duo E6400. Its integrated heat spreader was nice and flat, so we simply used a wadding metal polish to buff it to a mirror-like shine. The Thermaltake Mini Typhoon that we used to keep the processor cool already had a perfectly polished base, however, so we didn’t have to touch it. Although it works well, we decided to forego AMD’s stock PIB (processor-in-a-box) cooler in favor of the Mini Typhoon for a couple of reasons. For one, the Mini Typhoon performs better than AMD’s PIB. The Mini Typhoon’s also quieter than the stock cooler and its design helps blow plenty of air over our motherboard’s VRM and the system RAM—two things that should help stabilize the system while overclocking.

The Overclock

Unlike the Core 2 Duo E6400, the Athlon 64 X2 4600+’s multiplier is slightly unlocked. You can specify multipliers lower than stock; AMD does not allow higher multipliers, though. This opens up some interesting possibilities for tweaking overall system performance by utilizing a lower multiplier with higher HyperTransport and memory frequencies, but that’s a discussion we’ll have to cover someday in another article. Our goal for this build was to hit the highest, fully-stable processor frequency possible. That meant sticking with the X2 4600+ processor’s 12X multiplier and raising the HT reference clock as high as possible.

To help us achieve our goal, we had to alter a couple of BIOS settings before tweaking the CPU. As you’d expect, raising the HT reference clock results in a higher-clocked HyperTransport link. Some processors and motherboards will no longer be stable at a certain point, though, so before taking the processor any higher, we decided to drop the HT multiplier from 5X to 4X. Changing the HT multiplier to 4X set the link speed to 800MHz, giving us some room to work. We also dialed the PCI-E clock to 100MHz to prevent pushing the PCI-E interface out of spec. And finally, we cranked the processor’s voltage up from 1.35 to 1.5V and set the memory, HyperTransport, and chipset voltages to 2.3, 1.3, and 1.3V, respectively.

All She Wrote

With all of the preparation done, we slowly began raising the 200MHz default HT reference clock speed until the system became unstable. We increased the speed a few megahertz at a time and ultimately settled upon a 237MHz reference clock with a final CPU clock speed of 2.84GHz. The system actually booted at higher speeds but couldn’t make it through our suite of benchmarks without crashing, so we backed off a bit and stabilized the system at 237MHz.

A CPU clock speed of 2.84GHz is about an 18.5% increase over the Athlon 64 X2 4600+’s default 2.4GHz frequency; enough of a boost to put it past the flagship Athlon 64 FX-62. Not bad for a $250 processor. With more exotic cooling and perhaps a bit more voltage, we suspect frequencies higher than 2.9GHz are a possibility with the Athlon 64 X2 4600+, but we’re pleased with these results. Our processor and motherboard combination played very well together and it definitely showed in our benchmarks.

by Marco Chiappetta