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How to adjust Vcore
To test for max overclocking, CPU clock is increased (bigger step at beginning and few MHz step at the end). And when the system becomes unstable, you can increase Vcore by 25 mV stepwise to see whether it can make the CPU to run faster so that the system can become stable again.
Repeat the above until high Vcore is reached to a point that CPU clock frequency cannot go higher practically, (say less than 10 MHz per C temperature increase, by raising Vcore (25 mV stepwise) for Tbred B/Barton), reaching zone of diminishing return on frequency. That is the heat/temperature increase will slow down more than 50% of the intrinsic CPU frequency increase (beyond break-even). From experience, 1.9-1.95 V should be OK for typical Tbred B overclocking (at least for testing), system stability (e.g. running Prime95 for CPU overclocking) is key, while keeping die temperature under control (take 50-55-65 C loaded as reference rather than the perceived absolute limit). Good HSF is needed to keep temperature low.
There are different level of stability. Minimum is that it can boot in the operating system and run Sandra CPU. Prime95 is considered to be a reasonable way for CPU stability and stress test.
Overclocking is a process, not by sudden boosting the Vcore and hope it will go to 2.5 GHz. Then it would not blow up the CPU. Don't sudden jump Vcore by multiple steps since you will lose sight of the CPU stability and die temperature increases.
During this process, watch for
- system stability,
- rate of increase of clock frequency per step of Vcore increase
I consider 5 MHz per 25 mV Vcore increase as near the overclocking limit. The frequency increase has decreased to less than 1/4 of its original overclocking rate on Vcore increase.
- die temperature, keep an eye on the rate of temperature increase per step of Vcore increase:
1. When overclocking reaches around 10 MHz increase per C increase for loaded CPU, it reaches the break-even point, the CPU overclocking frequency begins to slow down faster due to heat than helped by voltage (diminishing return).
2. I consider 2-3 MHz increase per C increase for loaded CPU at 25 mV step of Vcore increase as reaching the overclocking limit, take 50-55-65 C as relative rather than perceived absolute limit
Example for Vcore increase
Here use a Tbred B 1700+ DLT3C as an exmaple. If you have a CPU that can do 2.2 GHz @ 1.5V, it is a good CPU for oc.
(If you chip can only do 2.1 GHz @ 1.5V, then substract the CPU clock by 100 MHz and 0.5 in the multiplier in the following example, etc, etc.)
Keep FSB = 200 MHz
Raise the multiplier to 11.5, i.e. freq = 2300 MHz
Increase Vcore to 1.6 V+- should get it stable. Run prime95 to test stability.
Then repeat again with FSB = 200 MHz, multiplier 12. I estimate Vcore around 1.7 V +- to get it stable to 200 x 12 = 2400 MHz. Run prime95 to test stability.
etc, etc.
At the early stage, one would expect to get 100+ MHz for each 0.1 V or 100 mV Vcore increase. When close to the overclocking limit, that number will drop to around 50 MHz / 100 mV, and eventually down to 5 MHz / 25 mV eventually, and the end is almost there. Also at the last % of overclocking, the temperature would increase much faster per step of Vcore increase, I consider 2-3 MHz / C is about reaching the limit. It will be very costly in terms of voltage, power and temperature to get the CPU to run a % higher.
This is just an estimated scenario, try it out first. If you want to go further, you may need to try a better HSF such as SK7 or SLK-800U/900U with a variable speed high CFM fan such as TT SFII or even a Vantec Tornado (with VR mod).
When you can reach 2.4/2.5 GHz, depends on the HSF and PSU, your are probably 100 MHz away from one of the best oc of a 1700+ on air.
