Aside from their markings, there isn't anything that physically differentiates the new Core i7-875K and Core i5-655K from other Lynnfield and Clarkdale based processors.
As you can see in the images above, the Core i7-875K and Core i5-655K use the same LGA1156 packaging, but the surface mounted components on their undersides are different from each other.
Above are a couple of screen captures from the latest version of CPU-Z showing the Core i7-875K's and Core i5-655K's pertinent details. Looking back at our previous coverage of the first Lynnfield and Clarkdale chips to arrive in the lab, it appears that the Core i7-875K uses the same stepping and revision as previous Lynnfield-based Core i7 processors, but the Core i5-655K is based on a new stepping and revision of the Clarkdale chip (5 vs. 2).
This next set of CPU-Z screenshots shows our max stable overclocks using the new Core i7-875K and Core i5-655K processors in an Asus P7H57D-V EVO motherboard, outfitted with a new BIOS that properly supports the chips. Other than their core speeds, what's important to note here is that all of the overclocking gains were achieved by simply increasing the processors' multipliers. By doing so, the CPU frequency is increased, but all of the other components that derive their clocks from the base clock frequency (incorrectly labeled Bus Speed by CPU-Z) are running perfectly within spec. With a good motherboard that has a BIOS specifically tailored for these new K-Series processors, users have complete control over CPU, Memory, and Turbo mode multipliers and can quickly and easily tweak any of their settings.
To overclock the Core i7-875K and Core i5-655K, we first increased each processor's voltage by .15v and then increased their multiplier until our test system was no longer stable. Turbo mode was disabled to prevent any unwanted frequency spikes, but we left HyperThreading enabled. In the end, we hit 3.9GHz with the 655K and 4GHz with the 875K.
At those speeds, using a stock Intel cooler, the 655K idled at around 37'C and peaked at about 67'C under load. The 875K idled at around 44'C and peaked at about 72'C.
Just in case you're wondering how much additional performance was gained by the overclocks, the Core i5-665K had an MT score of 10,459 in the Cinebench R10 benchmark in its stock config, and an overclocked score of 12,126--and increase of almost 16%. The 875K had a stock score 18,659 in the same test, with an overclocked score of 22,361--and increase of about 19.8%. Of course, with overclocking, your mileage will likely vary.