My CPU is i7-4790K with FSB 100Mhz i think cause it max x40 when I set in Bios. And I don't know how to determine the correct CPU voltage, I just know the voltage runs between +- 1V. For example, if I underclock the multiplier to x30 (3000mhz) or I overclock to x50 (5000mhz), what is the correct manual voltage? Btw, so thanks for the answer.
4GHz base and 4,4GHz boost with 1.25 to 1.32V respectively and at load should be some normal voltages at auto settings so such should be applied when setting frequency manually. When overclocking, max 1.4v should still be safe providing you have good cooling. When underclocking (less than 4GHz), similar spread of voltages under 1.25v can be applied. Because not all CPUs are made identical (silicone lottery), some voltage variations , typically +/- 0.01 - 0.02v could be normal.
That's how it works when over/underclocking manually any CPUs, just those values can be different. Basically, you determind maximum safe voltage for max overclock and minimum stable voltage for underclock so voltage +/- raise/fall is fairly linear for those frequencies. For more precise tweaking a lot more of testing is needed because of above mentioned "Silicon lottery". Temperatures are a good indicator for particular settings, specially max temperatures when overclocking because if it starts overheating CPU will throttle down and you'd have negative effects of overclocking,
Conversely, if underclocking or undervolting for lower temperatures and/or power usage, stability may be compromised,
If BIOS permits, some small corrections to FSB frequency may also be possible but have to be extra careful because it also affects memory and PCI frequencies which can induce stability problems unless is also possible to set them manually to their "normal" values. If that is possible, FSB frequency will influence only CPU frequency.
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