For those interested in achieving high overclocks with Sandy Bridge 2500K or 2600K CPU's using Offset Mode to control your voltages, I thought I'd share some information and invite others to post their own experiences and results. Though the typical user of these forums is probably quite technically savvy and sophisticated (in other words - Computer Geeks), I'll start with a brief explanation of Offset Mode and why you might want to use it.
OFFSET MODE AND IT'S ADVANTAGE
Traditionally, when setting Vcore voltages, an overclocker set volts to a fixed value. No matter what load the computer was experiencing, the CPU was fed that voltage. Even if SpeedStep and Turbo modes down-clocked the CPU, the voltage stayed at that manual setting. This is not ideal obviously, since feeding higher voltages all the time - even when not needed - is unnecessarily wearing on your CPU, generates higher temperatures, and wastes energy. The alternative is using Offset Mode.
This allows your Vcore to scale on demand, so that when your computer is idle or lightly loaded such as web browsing, those nasty volts drop when the CPU steps down. For someone like me who runs the computer 24/7, this is like having your cake and eating it too. Maximum performance when actually needed and uber safe voltages and efficiency the rest of the time, all happening automatically.
OFFSET MODE DOWNSIDES
You often read that Offset Mode is fine for mild overclocks, but above 4.4ghz-4.5ghz it does not work. That is something that just is not true as you'll see. I have gotten to a 5.0ghz overclock using this method, at sane voltages. Just for fun I may see what it takes for 5.1ghz if the Vcore demands don't go above what I am willing to go to. There are three issues with Offset Mode. First, it's a bit more complex initially until you get the the hang of it, but nothing that bad. In fact it sort of puts the fun back in overclocking, as you go through what works for you. Second, people that attempt to use "Auto" for an Offset voltage setting tend to get unsatisfactory results. Either the voltage is too high on load, or too low for stability on idle. Last, to get a high overclock stable, you must change more than just the offset. I'll share what has actually worked for me.
SUGGESTED GENERAL APPROACH
Go slow and and start with mild overclocks to get the hang of things before you reach for more demanding overclocks. Take it in small steps, one multiplier increase at a time. A 4.2ghz or 4.4ghz overclock may be a good place to start if you are not experienced at this. Verify everything thoroughly with each change in voltage. I worked myself up to a stable overclock for a multiplier by decreasing or increasing .005 volts at a time. Then boot into Windows, open a good temperature and voltage checker, and see where you are. If the voltage is higher than expected, don't bother stability testing. Go back into bios and step it down. If the initial voltage readings look OK, and the temps are decent, I then use prime95 on blend to test for stability.
Personally, I find with my cooling, a dozen passes successfully run without temperatures climbing is sufficient, but that is up to you. I monitor voltages and temperatures throughout and stop Prime95 if anything is not looking good. More often, because I start out very conservatively on offset voltage, Prime95 or Windows itself crashes. Each time this happens, I bump up the volts by a single increment, and try again. This can take awhile, with a number of crashes, but it really will result in the best eventual outcome. Once Prime95 runs successfully and no other toward events occur, you know you have hit the lowest stable voltage for that multiplier.
I save those settings in an overclock profile in bios at that point. Then whenever I want to change my overclock level, I just load that profile.
Don't rely on software that reports a single CPU temperature. For instance, AI Suite II might tell you that your CPU temperature is 22.0C. OC3D Hardware Monitor or Real Temp will give you a far more accurate reading on each core, and the CPU package. This is usually 10.0C higher than my Asus software shows. Make sure you have sufficient cooling capacity. Don't let those temps soar.
Let your own CPU, motherboard, and cooling tell you what it is capable of – rather than assuming you can overclock to any pre-concieved level.
GETTING TO THE GOOD STUFF
OK, here we go with actually overclocking. I went through much trial and error, and you may have to as well, but I hope I can at least give you some direction. I'd suggest changing all the other settings BEFORE changing your multiplier and Vcore Offset. After that try your first overclock level. Here are what works for me as sort of a "base" Bios configuration, your mileage may of course vary. I begin by loading the optimized defaults with the F5 key, save and reboot. If this works with no problems I re-enter bios and configure as follows, and then reboot again to make sure everything is fine.
(The Bios settings and terminology are for an Asus Maximus IV Extreme-Z, other motherboards will have different features and terms.)
Using the Advanced Mode in Bios (Bios Version 0902) - Extreme Tweaker Tab
Ai Overclock Tuner: Auto
BLCK/PCIE Frequency: 100.0
Turbo Ratio: All Cores Mode
Maximum Turbo Ratio: This will be set to your target Multiplier
Internal PLL Voltage: Auto
Xtreme Tweaking: Disabled
EPU Power Saving Mode: Disabled
Extreme OV: Disabled
CPU Voltage: Offset Mode
Offset Mode Sign: This will be positive or negative depending on your target.
CPU Manual Voltage: Value will depend on your target.
CPU PLL Voltage: 1.850
CPU Performance Settings (Sub-Menu)
CPU Ratio: Auto
Enhanced Intel SpeedStep Technology: Enabled
Turbo Mode: Enabled
Maximum Power: Disabled
(All other sub-menu items on Auto)
Digi+ VRM (Sub-Menu)
VCore PWM mode: Extreme
Vcore MOS volt Control: Auto
Vcore Load-Line Calibration: 75%
Vcore Switching Freq: Manual
VRM Fixed Frequency Mode: 350
Vcore Phase Control: Manual Adjustment
Manual Adjustment: Fast
Vcore Over-Current Protection: 140%
THE FIRST OVERCLOCK
Once you have a stable "base configuration" (I save this as a profile), it's time to bump things up. From now on you can probably do all your overclocking by changing just three settings. The Maximum Turbo ratio is where you enter the CPU multiplier, say 43 for a 4.3ghz overclock. For the first overclock I'd suggest working "backwards" from how you'll subsequently do it. By this I mean try a mild overclock multiplier. Leave the CPU Manual Voltage on "Auto". Save and boot into Windows. Check your voltages and temps. Try a prime95 run and assuming it goes alright, it's time to tune a bit.
Your Vcore voltage is probably higher than it needs to be to sustain a stable overclock. Go back into bios and we'll try the first manual setting on volts. Set the Offset Mode Sign to a negative (-). This means the voltage offset you will enter will be subtracted from what the motherboard set automatically. Enter a CPU Manual Voltage value of .005, save and reboot. Check your voltage and it should be lower than the initial value using an Auto setting. Before proceeding wait for your processor to "step down" to 1600MHz. Then re-check the Vcore.
This time you are looking at the Vcore not to make sure it's not too high, but rather that it has not dropped too low. For me, it needs to stay around 0.975 or better. If all is well, try a prime95 run. If it's successful, re-enter bios and lower the voltage another step, to a -0.010, save and retest things. I keep going until either the down clocked 1600MHz Vcore is too low for stability, or on the Prime95, I get instability. Once I find that that level, I bump the volts back up to the last stable value, and we are there. I save that configuration with a name of the multiplier, and then whenever I want that level of overclock, I just need to load it.
GOING ONWARD AND UPWARD
From here it's a matter of increasing the multiplier by one, save and reboot, verify things and test. You may be lucky and not have to increase your voltage offset on mild multipliers, but if the first try is unstable, go back into bios. This time you will need to change the "Offset Mode Sign" to a positive (+), and bump up the Manual Voltage by 0.005, save and reboot, testing things again. Keep bumping it up ward one step at a time until you reach stability, or the temperatures or voltage levels tell you that for your system, that multiplier is too high. Again, I save every successful multiplier as a profile. How high can you go? That is entirely a matter of your particular system, and your own risk tolerance.
I'll share the data for my system at a optimized 3.8ghz, 4.8ghz, 4.9ghz, and 5ghz in an attachment. I would not just plug in these values. Every system is different, these are what I worked up to. I'm also attaching the 5ghz results from OC3D Hardware Monitor (apparently all the attachment space I have).
Overclock Data Summary.JPG
I hope some of you who have not tried Offset Mode overclocking found this helpful. One thing I should note is that the results I show were obtained with full fans and pump speeds. In practice I have my pump and fans controlled by a user profile in Fan Xpert. This gives me very quiet general computing even at a 5Gghz overclock, and automatically bumps up the cooling to whatever level is needed by more demanding use. I'd love to see how others are doing with Offset Mode.
Case: Corsair 650D
Motherboard: Asus Maximus IV Extreme-Z
Memory: 16 GB DDR3 @ 1600
Power Supply: Seasonic X650
Graphics Card: Still my old BFG GTX260
SSD: OCZ Vertex2 120GB
Hard Drive: F3 1TB
Sound Card: Asus Essence STX
Water Cooling – CPU only Single Custom Loop
Pump: MCP35X on PWM control
CPU Block: XSPC Raystorm
Radiator #1: XSPC EX240 with 2 GT AP15's
Radiator #2: Phobya 200mm with 1 Silverstone AP181