What to look for?
For the most part the core changes are the same across the different motherboard manufacturers, these include:
Base Clock: Just like the LGA1366 Core i7 range, and as a replacement for the old Front Side Bus clock on Core 2 and Pentium products, the base clock is the underlying frequency that changes all of the CPUs clock domains, not just the core clocks.
The standard frequency for the Core i5-750 is 133MHz. Increasing this affects the core frequency, CPU-northbridge frequency and QPI frequency - your CPU may be limited by the latter factors that do not have the availability of voltage increments to compensate, although increasing the frequency of the CPU-NB clock does directly increase memory performance (especially memory write speeds).
Intel TurboBoost: This is Intel's auto-overclocking feature that increases the CPU multiplier beyond its stock value when fewer than all four cores are being used. TurboBoost has two levels of multiplier increment, which for the i5-750 are 21x and 22x. On some boards the 21x multiplier can be forced on permanently, however whether this actually "holds" is dependent on the BIOS and board. Gigabyte claim it's impossible, however we've certainly seen it work with the MSI P55 GD65. When overclocking it's typical to turn this function off.
CPU Multiplier: Like we said above, some boards allow the 21x multiplier which makes hitting above 4GHz much easier, however since all the boards with an i5-750 will have the 20x multiplier, a nice, round 4GHz with a 200MHz baseclock should easily be achieved. A word of advice from the OC community though: Uneven multipliers typically overclock the CPU better.
Intel SpeedStep/C1E/Power Saving Technology: This underclocks the multiplier when the CPU is idle to save power, however it can also affect performance sometimes, especially SATA throughput as we've seen in a few cases. Considering we're going to be aiming to heavily overvolt to achieve high clock frequencies, saving power is not the greatest concern, so this can be disabled.
DRAM Frequency: Core i5-750 CPUs have one less memory multiplier available than the Core i7 LGA1156 products. The memory clock is reliant on the base clock, and there's still 6x, 8x and 10x multipliers available allowing a 2GHz DDR3 clock at 200MHz base clock, or, a more usual 1,600MHz with the 8x multiplier. Better motherboards will show the total frequency as the base clock changes in the BIOS so be wary of what memory is in your motherboard, and try not to run it much faster than it can handle. If you need to drop a memory multiplier, offset the loss of bandwidth by dropping the memory timings as much as possible.
DRAM Channel Timings: Intel allows mixed modules in each memory channel, so the timings can be set separately in most BIOS'. For our needs though, it's best to have matched modules. Some motherboards don't include the option to change both memory channels at once so make sure you change the timings which relate to the channel your memory is plugged into, or, if you're using all four slots (both channels) that the settings are uniform across them both, which can sometimes mean setting everything twice.
QPI Frequency: Anything over 150-160MHz base clock, select the lower multiplier. It does not affect performance, but it is also connected to the base clock and will potentially limit the total overclock.
PCI-Express Frequency: Unlike other chipsets, no additional voltage can be applied to the PCI-Express core specifically, however there is sometimes a PCI-Express clock signal amplitude which will substitutes for this. Increasing the clock heavily can certainly give some additional performance in multi-GPU scenarios to make up for the fact it only offers two lanes of x8 bandwidth PCI-Express 2.0. It also depends heavily on the quality of graphics card.
We've read this option is also directly linked to the SATA frequency - whether by the same clock generator, or perhaps the DMI link to P55 is overclocked? We don't specifically know yet, but increasing the SATA frequency can potentially cause data corruption, so be mindful of this.
Why would you ever overclock the PCI-Express then? Well MSI claims it has a positive effect on base clock overclocking, however since all boards should do 200 without fussing over it, we don't see the need to change it unless you're overclocking very heavily.
CPU Clock Amplitude: We typically increase this slightly, however it can't just be maxed all the way because that can lead to just as many potential problems of clock signal overshoot, depending on the quality of motherboard, its physical traces, the size of the clock data eye and possibly even electromagnetic interference.
Load Line Calibration: This factor is a compensator for voltage drop/droop as the CPU goes from idle to load. We recommend enabling this where possible, although some boards offer different levels offer a percentage change, which can require more research to know the specific factors.
CPU Voltage: This is often given as either an offset or absolute voltages, depending on the motherboard. Personally we prefer absolute voltages for a heavy overclock, but for a "light" mid-3GHz overclock an offset of up to +300mV will do. Even stock voltages will go quite far with a good CPU. For a heavy 4GHz overclock ~1.38 to 1.425V is normal, and above 4GHz usually the recommendation from motherboard vendors is anything from 1.45V-1.5V. That is certainly getting dangerous for long term use rather than just benchmarking, and it's very difficult to keep cool. Of course, your mileage will vary mostly on this factor so as a rule of thumb start aim for the least voltage you can get away with.
BIOS settings Continued:
VTT voltage (or IMC voltage): While there's no specific "uncore" voltage for Lynnfield like with LGA1366 Core i7 CPUs, the VTT (or IMC as Asus call it) voltage helps with baseclock overclocking. Reports vary on exactly how much voltage needs to be applied, but usually we've seen that a 200MHz base clock will require 1.35-1.4V depending on the motherboard and CPU. Above 200MHz anything up to 1.45-1.5V can be needed on air, but again the story is the same as the CPU voltage on danger and cooling. "Light" overclocking will often require no change. Once you've found your happy frequency, drop this voltage until you lose stability then sit it just above it.
CPU PLL Voltage: At first we assumed this required heavy overvolting like Core 2 products, applying over 2V, however the unanimous reply from Taiwanese engineers was that very little extra voltage is needed - if any. At any base clock 200MHz and over we typically applied 1.9V.
PCH Voltage: The PCH is the P55 chipset, and as a glorified southbridge it requires very little, if any, additional voltage. Given the fact it's cooling is often limited and it sits under hot graphics cards, adding more voltage and heat will create just as much problems as it attempts to solve. Change this as a last option.
DRAM Voltage: As with the LGA1366 CPUs, a maximum 0.5V difference between CPU uncore voltage and memory is advised. Since there's no specific "uncore" voltage now (don't mistake CPU core voltage with the uncore that contains L3 cache, the memory controller and now PCI-Express controller), it does limit the extra voltage that can be applied to the memory and 1.65V is still the norm for most performance modules. We have run 1.8V for short periods of benchmarking to push our memory further, which the CPUs will handle fine, but we don't recommend long term use over the recommended settings.
DRAM Reference Voltage: This memory specific option can be left on Auto for 99 per cent of people, unless you're heavily tweaking and pushing the DDR3 memory bus (for which, you need high performance memory plugged into it). We've hit 2.35GHz without adjusting these values, however we know 3GHz is do-able with some Lynnfield boards. If you are interested in these values, our advice is to check with professional overclocking forums for tips.
CPU Clock Skew: Again, this can be left to auto, however if you are on the verge of stability at a high clock this factor can sometimes help where voltage cannot. The downside is that it's a completely blind art - unless you have an oscilloscope to SEE the clock generated, there's no way to easily tell how much skew to add to the clock to keep it all aligned. Short of spending a lot of time continually testing scenarios where you find the instability on each option, there's no easy way to narrow down the best setting. If one of the clocks is then changed, this process needs to be started again.
Hardware Prefetecher/Cache Prefetechers: Leave these ON! They will improve performance.
Virtualisation Technology: Unless you're using virtualised environments (an OS within an OS like XP Mode within Windows 7), this can be disabled.
ACPI 2.0 Support or HPET (High Performance Event Timer): Enable this/these.
BIOS Profile Saving: Each motherboard designer will have a different brand for this function, but ultimately all it does is save the current applied settings to a profile file which can later be re-enabled after a CMOS reset. Flashing the BIOS with an updated version will most likely lose these or invalidate these saved settings though. The better BIOS' will allow profile naming and multiple slots for ease of use, and even more ingenious ones will offer the end user the ability to load a profile after a CMOS reset has been done (MSI and Gigabyte). Our advice is to absolutely use this feature, just like you'd save any important document. Nothing is more annoying than having to start again, struggling to remember what every setting was.
Turn off anything you don't use! If you don't use Firewire, the floppy port, serial and parallel ports or extra SATA ports: turn them OFF! It saves system resources, reduces potential conflicts and power/clock dependencies on heavily loaded buses.
In BIOS Flash Utilities: USE THEM. They are generally more reliable than using Windows and far, far more convenient than digging out a floppy drive or making a bootable USB stick. It's incredibly important to reset the CPU back to its default settings because potential instability from elevated frequencies only increase the risk of flash corruption and failure!
Click to enlarge
The P55-GD65 from MSI we've already reviewed and for the most part the BIOS is identical the the CD53 above, however in the first picture we can see that the CPU VTT voltage option is available here. MSI sends it red at 1.35V, and for light overclocks this is more than sufficient, but pushing over 200MHz it needs to be upwards of this.
Here we also have the 21x multiplier enabled when the EIST state is set to auto or enabled: it signifies that it's additional by turning yellow.
Click to enlarge
Pushing the board as far as we can we shot for 4.37GHz at a 230MHz base clock and a 19x multiplier: remember the Lynnfield CPUs are reported to overclock better with odd multipliers. This seemed to be an absolute limit of our CPU, where the CPU-northbridge runs well over 4GHz. The VTT is pushed to 1.44V and the CPU 1.45V to keep it there, which can potentially overheat some less capable cooling solutions, and we also had to drop the memory multiplier down a notch too.
Click to enlarge
With the OC Genie button enabled we saw a similar change to the CD53, however the base clock here has been pushed even further to 210MHz, on the same 17x multiplier, and 1.42V is being pushed through the CPU. Again, we feel that 1.72V through the memory is not a wise choice for just 1,260MHz DDR3 (we were using 1,600MHz Corsair Dominator memory), so be mindful when using this function. ***
Views: 9233 
