Intel's K-series processors offer fully unlocked multipliers that place few restrictions on overclocking. With Sandy Bridge and its Ivy Bridge successor, Intel has also allowed limited overclocking on regular Core i5/i7 CPUs that aren't part of the K series.
On those regular-model chips, one may increase the maximum Turbo multipliers by four "bins" above stock, effectively delivering up to a 400MHz overclock. That boost won't get you into the near-5GHz territory attainable with fully unlocked versions, but it's a nice freebie that most CPUs and coolers can tolerate.
When we were discussing Haswell overclocking on this week's podcast, we weren't sure whether Intel had extended limited multiplier control to non-K Haswell products. We checked with the firm and are sad to report that this feature has been removed.
We've already reported in our 4770K review that, although Haswell offers more flexibility the form of additional base clock straps, access to those straps is disabled in non-K parts.
As a result, with Haswell, overclocking support is now almost entirely confined to K-series CPUs.
Intel says it's targeting non-K chips at "the business and consumer market where overclocking is generally not performed."
Based on what we've seen in our labs and heard from industry sources, K-series Haswell CPUs have less clock headroom than their Ivy and Sandy Bridge predecessors. The Core i7-4770K being tortured on my test rack peaks at 4.5-4.7GHz, depending on the motherboard, and it requires potent cooling at those speeds. A more modest 400MHz bump shouldn't require an exotic cooler, but getting it will require paying the premium for a K-series processor.
Buying a K-series Haswell processor will set you back an extra $20-$30 over the equivalent standard model. The Core i7-4770K is priced $30 higher than the Core i7-4770, while the Core i5-4670K is $20 more than the Core i5-4670.
Paying the extra for a K-series product also means giving up support for one of Haswell's key features, the TSX extensions that enable transactional memory. Intel has stripped out the VT-d device virtualization and vPro management features in the K series, as well.
In the end, enthusiasts face a rather unfortunate set of choices in Intel's Haswell-based product offerings. We can't help but think this situation wouldn't exist if AMD were putting more competitive pressure on Intel.