In our Core i7 review, we pointed out that the cheaper two i7 models—the 920 and 940—both had lower QuickPath Interface and L3 cache clock speeds than the i7-965. We also said the cheaper CPUs only supported DDR3 memory speeds up to 1066MHz. However, Intel apparently neglected to tell us about a small detail: retail-boxed Core i7-920 and i7-940 processors have no such restrictions on QPI and RAM speeds.
As Maximum PC reports, the two CPUs have the right multipliers for 4.8GT/s QuickPath and 1066MHz DDR3 memory speeds out of the box. However, users are free to pop into the BIOS and raise those multipliers, pushing the QPI speed to 6.4GT/s and the memory to 1600MHz—just like on a Core i7-965.
The interesting thing here is that Core i7 engineering samples did have limitations in place, so testers couldn't bump RAM or QPI speeds without raising the 133MHz base clock and potentially impacting stability. However, Intel quietly lifted the restriction from retail CPUs because of "requests from some . . . customers."
We received direct confirmation from Intel about the unlocking business, so that means one thing is certain: higher QPI and RAM speeds are possible no matter which Core i7 you get. And, as we've seen ourselves, going from DDR3-1066 to DDR3-1600 on a Core i7 can impact performance in some applications.
Additional performance factors may be at play, too. On the Core i7-965, we found that the speeds of the L3 cache and memory controller (and thus memory) were linked. That means running a low-end Core i7 with 1600MHz RAM might push the L3 cache to 3.2GHz, which could improve performance further. We'll obviously have to do some additional testing to get to the bottom of this, though. In the meantime, you can peruse our revised Core i7-940 review for the skinny on all that uncore, QuickPath, and memory controller mumbo-jumbo.
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