The ICFX has the ability to increase and decrease GTL voltages for both dual and quad core CPU's straight from bios. The effect tweaking these voltages has can be quite profound as seen by Kris's article showing a quad core CPU
at 460fsb. Increasing values within bios does actually increase voltages, here I will try and give some guidance as to what does what so you know how and what to tweak and when certain voltages need to be tweaked in preference to others.
Kris has already shown how GTL reference tweaks have a profound effect on overclocking most motherboards do actually have to be hard modified at this time to adjust the GTL voltages but DFI have given us the ability in the ICFX to do this straight from bios.
Now the burning question is what do the values 0 to XXX actually do? There is no way to see the voltages adjusting thru bios readout, so we have to rely on DFI to give us some answers.
Looking here for the original information we see the actual settings give minute changes in voltages. For the sake of clarity I have reproduced DFI Wiki page here to integrate into the guide.
To start with you have to
Enable GTLRef to have the setting take some effect, Oskar has preprogrammed good auto settings but they can be improved on as each processor does respond in different ways to voltage tweaks.
In my original guide Oskar had informed me to tweak up from 0 to 48 for the CPU GTL voltages, we have now found this is not the best option. Thru the work of many end users using dual and quad core CPU's we have found increasing over 48 is best to increase the FSB the cpu will run on. Also please rememer DFI have locked core 0+1 and 2+3 together for adjustment, they did this as this is how they are latched onto the FSB. Here are some points to remember.
1 If using a quad core you should only need to increase GTL for core 2+3, signalling to 0+1 is usually OK at auto unless you are seeing a huge FSB hole.
2 Increase GTL values
BEFORE CPU VTT, this is VERY important, only increase VTT if GTL has zero effect or makes things worse.
3 Increase GTL voltages in preference to NB Vcore voltage. While an increase in NB core voltage does have a huge effect on FSB overclocking you do need to keep the NB core voltage as low as you can to ensure the NB has a long life.
Now the final question....What is the optimal settings for the highest FSB?
The answer is every CPU is different, some have found setting between 70 and 80 best on the CPU, some have even run 120+ so it all depends on what the CPU cores need to open up the FSB. be prepared for a lot of tweaking and testing...but as Kris has shown it is possible to gain quite a few MHz if you are willing to put in some time testing.