I'm sure most P4C800 users have already figured this out, but I didn't notice it until now. I've thought it odd that changing vcore from 1.60V to 1.625V actually hurt my overclock. I usually get an idea of my max overclock when running memtest86 and bumping vcore up in .025V increments. I've normally been cpu limited, rather than memory limited. My overclock would improve up to 1.60V, but changing the voltage to 1.625V or 1.650V would make things worse, and I never tried higher. Today I decided to check the voltage with mbm with the processor idling. Here's what I got (first number is bios, second is mbm):
1.575V -> 1.63V
1.600V -> 1.66V
1.625V -> 1.62V
1.650V -> 1.65V
1.675V -> 1.68V
1.700V -> 1.71V
This actually makes sense because the vcore ADP3180 controller normally supplies .8375V to 1.600V. In order to get above 1.60V the motherboard is adding an offset voltage to the controller to get the voltage up to 1.90V. This offset value is obviously not very accurate, hence the vcore voltage drop when changing from 1.60V to 1.625V or 1.65V. Changing the bios setting enough eventually starts giving voltages above the value that a 1.60V setting gives.
Rather than being stuck at a max stable frequency of 3905mhz, it's now looking like I should be able to get above 4ghz with complete stability. As I said, I'm sure others have already figured this out, but I thought I'd post about this in case other P4C800 users have wondered why increasing voltage above 1.60V to 1.625V or 1.65V degrades performance. Until a short while ago I just assumed the limitation was my processor.
