Intel Core i7-875K And Core i5-655K Battle Beyond 4 GHz

Intel: 4.8 GHz On Air? Sure!

But that’s not the only curveball we’re throwing. It’d really be pointless to benchmark either of these new CPUs at their stock frequencies. Nobody pays a premium for an unlocked processor only to run it at the same speed as a couple of less expensive models. Instead, we’re pushing both chips as far as they’ll go on air (cooled by Noctua’s NH-D14, that is). The point here is to overclock via the unlocked multipliers, testing to see just how much headroom is available from Intel’s 45 nm and 32 nm nodes.

Additionally, we’re adding a pair of Black Edition parts—the Phenom II X4 965 and Phenom II X6 1090T. The two AMD processors are priced at $185 and $310, respectively. Though Intel’s offerings are more expensive ($216 and $342), the quad-core Core i7-875K goes up against the six-core Phenom II X6 1090T fairly well, while the dual-core Core i5-655K takes on the quad-core Phenom II X4.

Right out of the gate, AMD has the advantage on cost, so Intel has to prove itself in performance beyond 4 GHz.

Core i7-875K

Lynnfield’s “limitations” are fairly well-known, so it was hardly surprising to see the Core i7-875K reach a stable 4.13 GHz before exhibiting a bit of instability.

We were actually able to run most tests at 4.26 GHz before determining that this CPU just couldn’t take the heat over the long term. Using Patrick’s Core i5 clock rate guide as a starting point, we got up to 1.5V before backing down and deciding 4.26 GHz just wasn’t going to happen. Naturally, heat was the enemy here, and CoreTemp saw us consistently butting up against the processor’s 99 degree Tj limit. Of course, it didn’t help that the fourth core ran particularly hot, cresting the limit as the other cores were just hitting 90 degrees.

For all testing, Turbo Boost was disabled, giving us a static overclock. Hyper-Threading is left enabled, and Enhanced SpeedStep is on as well.

Core i5-655K

I was frankly most excited to work with the Core i5, a $216 part manufactured at 32 nm and set to run at 3.2 GHz by default. We’ve already had a couple of Clarkdale-based voltage-related fatalities here in the lab, so I wanted to exercise at least some caution. But ambition got the best of me and I ended up testing up to 1.45V—right about where Don lost his Pentium G6950.

Nevertheless, I was able to boot at 4.93 GHz. On air. Without killing the chip. I managed to run a number of our benchmarks at 4.8 GHz, but crashing in 3ds Max 2010, for instance, compelled me to drop another notch to 4.66 GHz. Still, an extra 1.46 GHz isn’t bad (at a reduced voltage of 1.4165V).

Again, Turbo Boost was turned off for this one, with Hyper-Threading and SpeedStep turned on. As with the Core i7-875K, 8 GB of DDR3 memory ran at 1,066 MT/s with 7-7-7 timings. Really, the beauty of these unlocked parts is that you don’t have to worry about modules able to accommodate tons of headroom. Of course, if you have enthusiast-class DDR3, unlocked memory ratios let you scale data rates up and down (though there’s really little reason to go beyond the official 1,333 MT/s these chips support).