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Intel’s Core i5-655K & i7-875K Unlocked Processors
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by Rob Williams on May 28, 2010 in Intel Processors

Most of today’s desktop CPUs, including budget models, tend to be good for overclocking. But for those who are looking for the ultimate in tweaking ability, Intel’s mainstream models have left a bit to be desired. With the K series, though, it aims to remedy that situation by offering unlocked models at affordable prices.

Overclocking Intel’s Core i5-655K & Core i7-875K

Before discussing results, let’s take a minute to briefly discuss what I consider to be a worthwhile overclock. As I’ve mentioned in past content, I’m not as interested in finding the highest overclock possible as much as I am interested in finding the highest stable overclock. To me, if an overclock crashes the computer after a few minutes of running a stress-test, it has little value except for competition.

How we declare an overclock stable is simple… we stress it as hard as possible for a certain period of time, both with CPU-related tests and also GPU-related, to conclude on what we’ll be confident is 100% stability throughout all possible computing scenarios.

For the sake of CPU stress-testing, we use LinX. Compared to other popular CPU stress-testers, LinX’s tests are far more grueling, and proof of that is seen by the fact that it manages to heat the CPU up to 20°C hotter than competing applications, like SP2004. Also, LinX is just as effective on AMD processors. Generally, if the CPU survives the first half-hour of this stress, there’s a good chance that it’s mostly stable, but I strive for a 12 hour stress as long as time permits.

If the CPU stress passes without error, then GPU stress-testing begins, in order to assure a system-wide stable overclock. To test for this, 3DMark Vantage’s Extreme test is used, with the increased resolution of 2560×1600, looped nine times. If this passes, some time is dedicated to real-world game testing, to make sure that gaming is just as stable as it would be if the CPU were at stock. If both these CPU and GPU tests pass without issue, we can confidently declare a stable overclock.

Overclocking Intel’s Core i5-655K & Core i7-875K

To preface our overclocking report, I’d like to mention that due to the overall tight timing of this embargo, I wasn’t able to devote as much time to overclocking these two processors as I would have liked, due to other performance testing for other content needing to be done at around the same time.

For stable overclocks, I like to stress test for up to eight hours if possible, but here, I just went for 40 minutes for each run of LinX for the sake of time. If I hit that, then I decided to benchmark with it. I can’t consider these clocks truly stable, however, due to the fact that I couldn’t stress them for far longer.

To kick things off, the i5-655K overclocked like a dream, as was to be expected given that it’s a dual-core model, and that our previous Clarkdale overclocking adventures delivered similar results. The difference here, though, is that our RAM speeds didn’t leave DDR3-1333.

To reach 4.56GHz, I had to use a rather high voltage of about 1.475V, and 1.500V for the QPI bus. Even though the BCLK wasn’t drastically changed, the extra voltage was needed for any sort of stability – even when we kept it at 133MHz.

Intel Core i5-655K Overclock

Intel Core i5-655K 3.20GHz (Overclock: 4.56GHz)
Benchmark
Stock
Overclock
Increase
Autodesk 3ds Max 2009
Dog Render
Bathroom Render
307 s
731 s
194 s
490 s
36.81%
32.97%
Cinebench R10
Single-Thread
Multi-Thread
4866
11020
6267

13921
28.79%
26.32%
POV-Ray 3.7
Single-Thread
Multi-Thread
881.61
2257.24

1169.88
3015.26

31.70%
33.58%

Adobe Lightroom 2.0
Convert 100 RAW to JPEG
129.46 s

122.10 s

5.69%

TMPGEnc Xpress
HD Video Encode
Mobile Video Encode

265 s
110 s


213 s
89 s


19.62%
21.24%

ProShow Gold
HD Video Encode
DVD Video Encode

451 s
113 s


349 s
88
s


22.62%

22.12%

Sandra Arithmetic
Dhrystone SSE4.2
Whetstone SSE3

50280
MIPS
36390 MFLOPS


56590 MIPS
40964 MFLOPS


12.55%
12.57%

Sandra Multi-Media
Int x16
Float x8
Double x4

91.41 MPixel/s
68.92 MPixel/s
37.5 MPixel/s


121.94 MPixel/s
91.94 MPixel/s
50.35 MPixel/s


33.41%
33.40%
34.27%

Sandra Cryptography
AES256
SHA256

3623
446


2960
594


-18.3%
33.18%

Microsoft Excel
Monte Carlo
Big Number Crunch

27.268 s
8.018 s


21.403 s
6.381 s


21.51%
20.42%

3DMark Vantage
Performance CPU

10053


12739


26.72%

As expected, the performance increases with our overclock scaled rather well, with the exception of some benchmarks, such as Lightroom. Oddly, the AES256 saw the same kind of decrease from overclocking that we saw with our Core i7-980X Extreme Edition review. I’m unable to explain this, as every-single other benchmark saw an improvement, but it was consistent.

Although it’s more difficult to overclock a quad-core compared to a dual-core, I had an even more difficult time overclocking the i7-875K than I expected. To hit 4.12GHz was a challenge. I didn’t have to use voltages quite as high as I did for the i5-655K, but that’s because it didn’t seem to help at all. But again, more time might need to be spent in order to get a better overall clock.

Intel Core i5-655K Overclock

Intel Core i5-875K 2.93GHz (Overclock: 4.12GHz)
Benchmark
Stock
Overclock
Increase
Autodesk 3ds Max 2009
Dog Render
Bathroom Render
189 s
404 s
126 s
276 s
33.33%
31.68%
Cinebench R10
Single-Thread
Multi-Thread
4431
18371
5568
22961
25.66%
24.99%
POV-Ray 3.7
Single-Thread
Multi-Thread
867.44
3937.3

1155.08
5041.33

33.16%
28.03%

Adobe Lightroom 2.0
Convert 100 RAW to JPEG
96.17 s

89.89 s

6.53%

TMPGEnc Xpress
HD Video Encode
Mobile Video Encode

197 s
105 s


167 s
88 s


15.23%
16.19%

ProShow Gold
HD Video Encode
DVD Video Encode

237 s
63 s


196 s
52
s

17.3%
17.47%

Sandra Arithmetic
Dhrystone SSE4.2
Whetstone SSE3

77718 MIPS
61068 MFLOPS


89640 MIPS
70631 MFLOPS


15.34%
15.66%

Sandra Multi-Media
Int x16
Float x8
Double x4

163.01 MPixel/s
125.22 MPixel/s
69.11 MPixel/s


214.43 MPixel/s
164.05 MPixel/s
90.69 MPixel/s


31.54%
31.01%
31.23%

Sandra Cryptography
AES256
SHA256

581
532


789
723


35.80%
35.90%

Microsoft Excel
Monte Carlo
Big Number Crunch

13.571 s
4.088 s


11.351 s
3.553 s


16.36%
13.09%

3DMark Vantage
Performance CPU

19017


23600


24.10%

Once again, the scaling is quite good. We don’t have the oddball AES decrease here, as we saw with the i5-655K, so it seems likely that the AES instruction set has something to do with that oddity. It does seem strange that overclocking can worsen the performance there, and it’s something I plan to investigate in the near-future.

Overall, these processors deliver just the kind of overclocking I was expecting. As far as I can tell, and based on all of the information Intel has given us, K processors are not cherry-picked. That means that aside from their unlocked likeness, their top-end overclocks shouldn’t vary too much from current Clarkdale and Lynnfield models.


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