Legend has it that Intel prepared the Core i5-7660X three years ago, but never officially released the processor. A Chinese Twitter user (opens in new tab) recently obtained a working sample of the forgotten child from the Skylake-X family.
The Core i5-7660X clearly uses the Skylake microarchitecture. Therefore, the processor is manufactured on Intel's 14nm+ process node and slides into the LGA2066 socket just like any other Skylake-X chip. The Core i5-7660X inherited all of the attributes of Skylake-X, like support for quad-channel memory (128GB max) and AVX-512 instructions.
Had Intel released the Core i5-7660X, the hexa-core processor would have been an interesting offering at the time. The Core i5-7660X would not only be the chipmaker's first Core i5 HEDT (high-end desktop) processor, but also the first Core i5 SKU to support quad-channel memory.
|Model||Cores / Threads||Base / Boost Clock (GHz)||L3 Cache (MB)||TDP (W)|
|Core i9-7980XE||18 / 36||2.6 / 4.2||24.75||165|
|Core i9-7960X||16 / 32||2.8 / 4.2||22||165|
|Core i9-7940X||14 / 28||3.1 / 4.3||19.25||165|
|Core i9-7920X||12 / 24||2.9 / 4.3||16.5||140|
|Core i9-7900X||10 / 20||3.3 / 4.3||13.75||140|
|Core i7-7820X||8 / 16||3.6 / 4.3||11||140|
|Core i7-7800X||6 / 12||3.5 / 4.0||8.25||140|
|Core i5-7660X||6 / 6||3.4 / 5.0||8.25||140|
The CPU-Z screenshots show the Core i5-7660X with six cores, six threads and 8.25MB of L3 cache. These parameters alone are enough to assume that the Core i5-7660X is closely related to the Core i7-7800X.
The Core i5-7660X comes equipped with a 3.4 GHz base clock, which isn't a big deal. Its boost clock, however, is pretty impressive. Previous Skylake-X offerings maxed out at 4.3 GHz, and the Core i5-7660X exhibits a 5 GHz boost clock, the highest of all the Skylake-X SKUs.
At the end of the day, the Core i5-7660X is essentially a Core i7-7800X with higher clock speeds but without Hyper-Threading. It's rated with the same 140W TDP (thermal design power), but by disabling Hyper-Threading, Intel could jack the Core i5-7660X's boost clock through the roof.
As a matter of fact, the Core i5-7660X could have been a product of Intel recycling subpar silicon that didn't meet the standards for a Core i7-7800X. This would certainly explain the close similarity between the two HEDT processors.
Not that it matters anymore but, the Core i5-7660X provides 28 PCIe 3.0 lanes and supports DDR4-2400 memory modules just like the Core i7-7800X. As you might recall, only the Core i7-7820X and above natively supported DDR4-2666.
It remains a mystery why Intel never released the Core i5-7660X to the public. Probably Intel was concerned with either lack of interest, or it wanted to keep HEDT as a platform for Core i7 and above (never mind the short-lived i5-7640X). Word on the street is that some i5-7660X samples actually made it to the hands of a few selected overclockers.