Overclocking the Core i7-3820 is a little different than the higher-end Core i7-3930K because the i7-3820 is not fully unlocked. The max multiplier is only 43x. So if we simply move the CPU Clock Ratio all the way up to 43 it will give us an overclock of 4.30GHz.
We could try and mess around with the base clock in order to get a higher clock, but doing this causes many systems to become unstable and if you go past 105MHz good luck getting the system to boot at all. So are we stuck at 4.30GHz on the i7-3820? No we are not! This is where the Gear Ratio (may be called something different on your motherboard) on the Sandy Bridge-E processors come in. It is used as a master multiplier for both the processor and memory. It can be used as 1.00x, 1.25x and 1.66x. To give you an example if you are using the base Clock Ratio of 36 and have your memory set at DDR3-1600 when you change the Gear Ratio from 1.00x to 1.25x your CPU Frequency will jump to 4.5GHz from 3.6GHz and memory to DDR3-2000.
So using this system we were able to bring our system all the way up to 4.87GHz with the CPU Voltage set to 1.5V. I really wanted to get to 5.0GHz, but Windows would not load even with the voltage set to 1.52V. 4.87GHz did not prove to be that stable so we went down a notch to a 38x clock ratio giving is a max stable overclock of 4.75GHz. A 1.15GHz overclock, not to bad at all, and extremely simple to to.
I ran a few of the benchmarks at this speed to show you the difference between the two speeds.