A whole lot of confusion here between Base vs Turbo vs MCE vs Overclock:-
- Base clocks on i5//i7 Turbo are applicable only if Turbo Boost is disabled in the BIOS or if the motherboard is set to have a power / time limit for Turbo for thermal / power envelope reasons (often done on laptops to ensure, eg, constant maxed out Turbo for hours on end doesn't fry a 55w power brick PSU even if the CPU cooler can handle it). For desktops, as long as the thermals are good, the Turbo's are:-
i7-8700K = 3.7GHz Base. 4.7 (1C) / 4.6 (2C) / 4.4 (4C) / 4.3 (6C)
i7-8700 = 3.2GHz Base. 4.6 (1C) / 4.5 (2C) / 4.3 (4C) / 4.3 (6C)
i5-8600K = 3.6GHz Base. 4.3 (1C) / 4.2 (2C) / 4.2 (4C) / 4.1 (6C)
i5-8400 = 2.8GHz Base. 4.0 (1C) / 3.9 (2C) / 3.9 (4C) / 3.8 (6C)
i3-8350K = 4.0GHz Base (No Turbo but Unlocked)
i3-8100 = 3.6GHz Fixed
- These Turbo's are guaranteed to work at those freqs under constant load, ie, an i5-8400 is pretty much a 3.8-4.0GHz chip and an i7-8700 a 4.3-4.6GHz chip, as long as temps aren't hitting throttle limit. That's what separates a Turbo from an OC - Turbo is officially guaranteed (even on non-K chips), an OC isn't (even on K chips).
- Z boards allows overclocking (and raising clocks on a K chip to above Turbo speeds will disable Turbo anyway), but all boards including cheaper H and B boards can hold Turbo at rated speeds just fine.
- MCE forces "all load cores" to the highest 1T core frequency but this isn't an official feature and whether this works or not may be motherboard vendor specific
- CPU speeds normally fluctuate under non maxed gaming loads due to SpeedStep and you'll often see 3-4GHz variations as cores are lightly loaded (down to 1GHz for lightweight / older games). This isn't the same as thermal / power / Turbo throttling. Disabling this whilst benchmarking (High Performance Power Option or at least advanced setting "Minimum Processor State" to 100%) can clear up confusion.
- There's nothing abnormal about low listed base speeds on the slowest i5 variant and is simply how Intel's marketing works on all previous chips too (eg, i5-7400 = 3.0GHz, i5-6400 = 2.7GHz, i5-4430 = 3.0GHz, i5-3330 = 3.0GHz, i5-2300 = 2.8GHz, etc). It has nothing to do with "binning limitations because there's a big gap". Remember prior to Haswell, both Sandy + Ivy Bridge chips had that +4-bin limited OC on non-K chips that allowed +400MHz on top of max Turbo which created similar large gaps, eg, i5-3450 = 3.1GHz Base / 3.9GHz max Turbo.
Pretty much any software monitor that polls then logs CPU freq several times per second (eg,
HWInfo64) under an all core load shows this up quite clearly. As for 8400's "real speeds", it's exactly the above Turbo speeds as multiple review sites have already checked : For 6C loads (Examples
1,
2), 2-4C loads (Example
1) and 1C loads (Examples
1,
2).