Home Intel’s Core i7-875K and i5-655K processors

Intel’s Core i7-875K and i5-655K processors

Scott Wasson
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In CPUs, it’s good to be king, because the king gets to decide things. If you’re not king, you may be able to get away with all sorts of shenanigans, but you ultimately serve at the king’s pleasure.

Take, for instance, AMD’s recent resurgence in desktop processors. Although Intel has held the overall performance crown in an unbroken run since the introduction of the first Core 2 Duo, AMD has been able to stay on the radar of PC enthusiasts through cunning and guile. When it had no hope of catching up to the fastest Intel chip in a given price range, AMD cooked up its Black Edition processors that removed clock speed caps and made overclocking dead simple—without the huge price premium traditionally commanded by Extreme Edition and FX processors. Even though Intel’s CPUs were more attractive by most conventional standards, folks wanting value and performance suddenly had to weigh another variable. When it couldn’t keep pace with Intel’s quad-core processors using four cores of its own, AMD uncorked the Phenom II X6 and priced it directly opposite Intel’s Lynnfield quads. You were quite literally getting more chip for your money from AMD, and the X6’s strong value proposition was enough to earn it positive reviews.

Smart strategy will only take you so far, though, when you’re not the king. Intel’s chips extract more computational throughput from a smaller silicon area while consuming less power. From manufacturing to design and architecture, it has every advantage. Intel is king. As a result, Intel gets to decide how much performance it will deliver to customers and at what price. And now it appears, the king is a little miffed—cheesed, peeved, horked off, if you will—about the Phenom II X6’s critical success.

Thus, the king has lifted his hand from the armrest of the throne and made a quick flourish—perhaps a slicing motion across the throat—and tilted his head in the general direction of the Phenom II X6. A little fiddling in the royal factories and a few marketing slides later, and the king’s official response rides forth across the drawbridge, the Core i7-875K and the Core i5-655K. Both of the K-series CPUs are priced attractively and have unlocked core and memory multipliers for easy overclocking. And they offer precious little room for those pesky Phenom IIs to breathe.

Make way for the K series
Neither of the K-series processors is new speed grade from Intel. The Core i5-655K is just an unlocked version of the Core i5-650, and the Core i5-875K is an unlocked Core i7-870. The K-series parts have the same core clock, Turbo frequencies, and thermal envelopes as their unlocked brethren. What makes them different is the ability to crank up and down the core and memory multipliers at will. Well, there’s one more major bit of news for the i7-875K, best illustrated by a look at how it fits into Intel’s lineup.

Model Cores Threads Base core
clock speed
Peak Turbo
clock speed
L3 cache
TDP Price
Core i5-650 2 4 3.2 GHz 3.46 GHz 4 MB 2 73W $176
Core i5-655K 2 4 3.2 GHz 3.46 GHz 4 MB 2 73W $216
Core i5-661 2 4 3.33 GHz 3.6 GHz 4 MB 2 87W $196
Core i5-670 2 4 3.46 GHz 3.73 GHz 4 MB 2 73W $284
Core i5-680 2 4 3.6 GHz 3.86 GHz 4 MB 2 73W $294
Core i5-750 4 4 2.66 GHz 3.20 GHz 8 MB 2 95W $196
Core i7-860 4 8 2.80 GHz 3.46 GHz 8 MB 2 95W $284
Core i7-870 4 8 2.93 GHz 3.60 GHz 8 MB 2 95W $562
Core i7-875K 4 8 2.93 GHz 3.60 GHz 8 MB 2 95W $342

Intel charges a bit of a premium for the 655K versus the 650, but the 875K is $220 cheaper than the Core i7-870—at least right now. One would expect the Core i7-870’s price to snap into line or the product to be canceled, but Intel says it has no plans to change the i7-870’s price “in the near term.” So it may just hang around as a singularly poor value. Whatever happens, the 875K gives you a more flexible CPU for a whole lot less cash.

In fact, the i7-875K costs roughly 50 bucks more than the Phenom II X6 1090T Black Edition, AMD’s fastest desktop processor, which is also unlocked. No doubt Intel charges a little more because, at stock speeds, the Core i7-875K is faster than the X6 1090T. We know this because the Core i7-870, with the same stock clocks, was faster overall in our review of the 1090T. Still, the price is close enough that 875K’s intended target is pretty clear.

The Core i5-655K isn’t such an obviously good value. At $216, it slots into the existing price structure for Clarkdale-based dual-cores, and we’ve long thought Intel charges too steep a premium for the higher clock speeds of those CPUs. Heck, the 655K costs more than the Core i5-661, which has higher stock clocks for both the CPU cores and the integrated graphics, so there’s a premium for the unlocking, too. If you leave overclocking out of the equation, we’d prefer Intel’s $199 quad, the Core i5-750. The closest competition from AMD has more cores, as well: the Phenom II X4 965 Black Edition is $185, and the Phenom II X6 1090T is $199.

Then again, you can’t really leave overclocking out of the conversation when you’re talking about a 32-nm Clarkdale processor, for reasons we’ll make clear shortly.

The new value mix
Since the 875K and 655K don’t break any new ground in terms of stock clock frequencies, we’ll spare you the massive CPU benchmark comparison. You can look at our Phenom II X6 review if that’s what you want. We’re instead going to focus on overclocking the K series.

Before we go there, though, we should take a quick look at how the effective price cut heralded by the 875K’s release changes the value picture. To get a sense of that, we’ve taken the performance and value data from our Phenom II X6 review and modified the prices slightly. Since we’re only considering stock clock speeds here, we’ve used performance data from the Core i7-870 to represent the 875K—the two should perform identically. Only a handful of the CPU prices have changed since that comparison, mostly for lower-end Athlon II processors.

A simple look at the price-performance ratio gets us this:

The cheaper processor almost always comes out on top if you run the numbers like this. As you can see, the 875K doesn’t fare so well on this basis. We don’t think that’s entirely fair, though, which is why we came up with our scatter plots that show both price and performance at the same time.

That looks much more promising. In fact, the 875K is in a very nice spot. To get higher performance than it, you’ll pay substantially more money.

Then again, CPU purchase decisions are usually made as part of a complete system build, and the math changes quite a bit when we consider price and performance in that context. In our X6 review, we priced out a comparable set of system components for each platform we tested, and it wound up looking like so:

Platform Total price Motherboard Memory Common components
AMD 890GX $656.94 Asus M4A89GTD Pro
4GB Kingston DDR3-1333
XFX Radeon HD 5770 1GB graphics card ($159.99), Western Digital Caviar Black 1TB hard drive ($109.99), Samsung SH-S223L DVD burner ($26.99), Antec Sonata III case with 500W PSU ($114.99)
Intel P45 $656.94 Gigabyte GA-EP45T-USB3P
Intel P55 $649.94 Gigabyte GA-P55-UD3
Intel X58 $789.94 Gigabyte GA-X58A-UD3R
6GB OCZ DDR3-1600

If you consider performance per dollar in the context of a mid-range system build like this one, the results change dramatically.

The 875K is our new value leader, just ahead of the Phenom II X6 1090T Black Edition. There’s very little daylight between the 875K and the 1090T, but the 875K comes out ahead.

The scatter plot illustrates the dynamic: the 875K’s additional performance offsets its higher price. Of course, none of this analysis takes overclocking into account, so it is just a starting point. Not a bad starting point, though!

We have a new leader in power efficiency per dollar, as well—by a very tiny margin over the old champ, the Core i5-750. The X6 1090T just isn’t as power-efficienct as Intel’s best chips.

Truth be told, overclocking most processors isn’t exactly difficult to do these days. An unlocked K-series chip does confer certain advantages, though. You can raise the multiplier to increase the core frequency while keeping the base clock steady, which is handy since an overclocked base clock can cause other system components to run at funky speeds. The K series gives the user total control over Intel’s Turbo Boost feature, too, so you can balance the CPU’s peak clock speeds against power consumption and different workload types. Also, you get unlocked memory clock multipliers with the K series, so higher memory frequencies are possible without altering the base clock.

A good motherboard will shepherd you around many of these difficulties, by locking down the PCIe clock to the correct speed, for instance. An unlocked CPU just makes everything a little easier and, with the right BIOS, gives you near-total freedom.

For our K-series overclocking attempts, we chose the Asus P7H57D-V EVO motherboard and a Thermaltake Frio CPU cooler; the combo is pictured above. We chose the Frio because its five heatpipes, large surface area, and dual fans will dissipate a tremendous amount of heat. We chose the EVO board because Asus was able to provide us with a special BIOS that supports fine-grained tweaking on K-series processors. Have a look at the menu below for a sense of the control it offers.

The base multiplier, or “CPU ratio setting” goes as high as 63, ridiculously enough, so there’s plenty of leeway for setting clock speeds there. The 875K’s default ratio of 22 multiplied by the 133.33MHz base clock yields a 2.93GHz core speed. The BIOS also allows us to set the additional multiplier increments added by Turbo Boost, depending on how many CPU cores are busy—those go as high as 15 beyond the base multiplier.

Gigabyte’s recent BIOSes do support the K-series chips’ unlocked multipliers, but they don’t give you control over individual Turbo multipliers.

The version of the Asus BIOS we used for the bulk of our testing exposed memory clocks up to 1600MHz for the 875K and up to 1333MHz for the 655K. A later revision added 1600, 1866, and 2133MHz memory speeds with the 655K, though we weren’t able to get the system stable at memory frequencies beyond 1600MHz in our few attempts with a couple of different fancy DIMM sets, even with loose memory timings and memory voltages over 1.65V.

We decided to overclock the two K-series processors by turning up the Turbo Boost multipliers in the BIOS, mainly because we could. We initially set all four Turbo multiplier offsets the same, however, since we had good cooling and didn’t see the need to limit performance. Although the CPU’s use of Turbo Boost frequencies is typically capped by certain thermal limits, the EVO’s BIOS appears to have removed them. During our stability tests with Prime95, our fully loaded CPUs didn’t appear to waver from their Turbo Core peak multipliers, even at high voltages and frequencies, when temperatures crept up above 70° C.

In fairly short order, as these things go, we got the 875K up to 4.13GHz by raising the Turbo offset for all cores to +9 and the core voltage to 1.275V. CPU temperatures hovered around 64° C at those settings, well within hand.

Bumping the multiplier up another notch just wasn’t entirely stable, even at 1.3V and beyond. The system would POST and boot into Windows, but it crashed during our stability tests. To compound the problem, at those settings, CPU temperatures climbed into the mid 70s and were on the rise. We were at the limits of both the chip and our cooling. Had we only been at the limits of our cooling, we could have used individual Turbo multipliers to get a little more frequency out of one or two cores. However, even just one core set to a +10 offset produced a system crash.

4.13GHz ain’t too shabby, though, all things considered. Your mileage may vary, but I’d say these results are fairly typical for casual Lynnfield overclocking with air cooling.

The 655K upstaged its bigger brother by reaching 4.4GHz without a fuss. That’s at a Turbo offset of +9 for both cores, adding to a base multiplier of 24. We had to raise the voltage to 1.38V to get there, but temperatures remained relatively low at around 61°C in our stress tests. An offset of +10 wasn’t stable, even at over 1.4V.

We’ve hit similar speeds with non-unlocked Clarkdales in the past, so this result isn’t unexpected. Seeing a CPU’s ticker going that fast doesn’t fail to impress, though.

I had an awfully good experience with overclocking the Phenom II X6 1090T Black Edition using AMD’s Windows-based Overdrive utility. I’ve not been favorably disposed to Windows-based overclocking tools in the past, but Overdrive exposes all of the X6’s knobs and dials exquisitely and is very easy to use. Given that, I figured I should try out Intel’s Control Center app, as well.

Unfortunately, this app only works with Intel’s own motherboards, and only certain ones. I installed the 875K into the supported DP55KG mobo and attempted to replicate our overclocking success from the Asus board.

Control Center will let you set all of the multipliers and raise the CPU voltage, and it even gives the user the ability to adjust Turbo Boost’s wattage and amperage limits. However, you’ll notice in the picture above that all of the items marked with a blue asterisk require a reboot in order to take effect. Many of the good ones, including CPU voltage, are subject to this limitation. In my view, that makes this application nearly useless. The whole point of Windows-based overclocking is to avoid the reboots required with BIOS-based tools

Worse yet, I found that voltage settings in the app didn’t always persist. If the system crashed and rebooted, it would often come up at a lower voltage than the one I’d just been using. I’d have to select the voltage I wanted and, yes, reboot once again. Ugh.

We had some trouble replicating the results we got on the Asus for another reason: the CPU voltage on the Intel board tended to droop under load, and that often led to a lockup or crash. This happened even after we raised the Turbo wattage and amperage maximums well beyond their defaults. The solution was to crank up the CPU voltage to 1.3875V in Control Center; it would then droop to 1.29V under load, as monitored in Control Center, and the system would remain stable at 4.13GHz.

Obviously, the total package here isn’t great. AMD’s Overdrive is miles ahead of Intel’s utility, and Asus’ BIOS is faster and easier to use. If you are using an Intel motherboard and want to overclock a K-series processor, the BIOSes on Intel’s enthusiast-class boards have allowed control over Turbo multipliers on unlocked chips for some time now. That capability has been limited to expensive Extreme Edition chips in the past, but no longer. You’re probably better off just using the BIOS, provided your board has the option.

Overclocked performance and power draw
Here’s a quick look at the performance of the overclocked 655K and 875K processors compared to some other chips we’ve abused recently. Note that we had the Phenom II X6 at a base clock of 3.9GHz and a three-core Turbo peak of 4.3GHz.

At stock speeds, the Phenom II X6 1090T is only a smidgen slower than the Core i7-870. When the 875K and 1090T are both overclocked to their limits, the margin grows, and the 875K’s lead is more pronounced. Remember, also, that we’re seeing four Intel cores outperform six AMD cores at similar clock speeds.

One might expect the Core i5-655K’s dual cores to have little chance against the quad-core competition, but its dizzying clock frequencies and strong per-clock performance allow it to close within striking distance. At 4.4GHz, the 655K trails the stock-clocked Phenom II X4 965 by only about five percent—and the 655K is over 50% faster than the X4 965 in the single-threaded test. The only other processors to break the 6000 mark in the single-threaded portion of Cinebench are other overclocked 32-nm Intel CPUs.

Even at 4.4GHz, the 655K’s power consumption is pretty reasonable; it draws about as much power under load as an Athlon II X4 635 or a Core i7-870—and substantially less than a Phenom II X4 965. The 875K’s power draw is much more considerable at 4.13GHz, but it’s still lower than our overclocked Phenom II X6’s. You’ll need good cooling to get the most out of the X6 or the 875K.

Cranking up the memory speeds
After we’d taken the core clocks on our K-series CPUs up to their peaks, it occurred to me that we should see how those overclocked CPUs would perform with faster memory, as well. We have incredibly capable Corsair Dominator DIMMs in all of our CPU test rigs, but we rarely push them to their limits when we’re not overclocking. Since the K-series processors have unlocked memory multipliers, we quickly turned up the DIMM clocks to 1600MHz at 8-8-8-24 timings with a 1T command rate. We did the same with the Phenom II X6 1090T. That’s quite a bump from our usual settings of 1333MHz at 8-8-8-20 and 2T. Here’s how it affected memory bandwidth.

The 875K posts some decent gains over the Core i7-870 at stock speeds, but none of these gains are all that exceptional. Still, faster memory should give our overclocked processors a little more headroom in some tests.

Putting it all together
Now that we’ve overclocked both the CPUs and the memory, let’s take another quick look at performance in some common applications. For these tests, I took the liberty of raising the 1090T’s Turbo Core limit from three occupied cores to four to give it a little extra speed.

The 875K’s four cores juuust manage to outperform the 1090T’s six in this nicely multithreaded application. The 655K’s two screaming cores, meanwhile, are scary close to the Phenom II X4 965, though they fall short of victory.

Blah blah four cores blah blah six blah blah fall short of victory.

A fast CPU simply isn’t a requirement for most of today’s games. With averages in the 50 FPS range, even the cheapest contemporary processors we tested will run this game (and many like it) quite smoothly. I wouldn’t worry too much about those frame rate minimums, either; they seem to be a little flaky in this test.

For what it’s worth in terms of future-proofing, though, both overclocked K-series processors are measurably faster than the overclocked Phenom II X6 1090T.

Like I said, it’s good to be king. Intel has calibrated its response to the Phenom II X6 quite carefully, and the result is a clean sweep. At stock clock speeds, the Core i7-875K is a better performer than the Phenom II X6 1090T—just a little bit in multithreaded applications, but quite a lot in single-threaded ones. The 875K is also a (very slightly) better value. When both CPUs are overclocked, the 875K retains its performance lead. Either way, the 875K is more power efficient than the 1090T, too.

The Phenom II X6 1090T is still an attractive product, but it’s not quite as good as the Core i7-875K. One gets the impression that’s exactly how Intel wants it. Meanwhile, Intel is no doubt giving AMD a look that says, “Don’t make me bring my 32-nm six-core over there. Don’t you make me do it.”

The Core i5-655K is another matter entirely. I’m not convinced it’s a great value compared to the quad-core alternatives, but it is the best way to get blistering performance with applications that use only light multithreading.

The 655K is also an official blessing of sorts for Clarkdale overclocking, and that’s like having a special pass from your doctor for eating all the bacon you want. When its two cores at 4.4GHz nearly match a stock-clocked Phenom II X4 965 in a video encoding or rendering test, it’s hard not to be a little taken. If you then notice that a system based on a 4.4GHz 655K draws 30W less under load than a system based on a Phenom II X4 965, you may start asking serious questions about how many cores your next computer really needs to have. I can envision an incredibly fast, exceptionally quiet system built around a 655K. The 655K may be a little pricey for what it is, but it’s very much an enthusiast’s play toy. You may be able to get similar overclocking satisfaction out of the Core i3-530 for nearly half the price, though, if you don’t mind doing a little more tweaking to reach stable settings.

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