Overclocking limit and cooling cost for different types of overclocking:
When running at full load (e.g. Prime95) to test for stability by adding small step of Vcore (e.g. 25 mV), if the increase in CPU frequency per CPU temperature increase is more than 10 MHz / C, I think there is still have some more overclocking to go, for a given cooling setup. For extreme cooling, one could use 12 MHz / C instead of 10 MHz / C.
If it is less than 10 MHz / C, it is beyond the break-even point or point of diminishing return, it would be getting harder and harder to overclock.
At 2-3 MHz / C, it reaches the limit. Then a better cooling would be needed, such as a better HSF with a smaller thermal resistance C/W, or going to water, or extreme cooling.
The above rules can be applied to various type of coolings, including air, water, extreme cooling
But usually, going to a better cooling becomes less and less price-performance efficient, as shown below, except for absolute higher MHz and environmental reason, noise reduction in the case of water cooling.
The cost in terms of overclocking MHz for various cooling, using Tbred B 1700+ DLT3C as an example:
- stock air HSF: from 1500 to 2100-2200 MHz, 60 C
$10 for 47% overclocking => ~ 4.5% / $ or 60-70 MHz / $ (best price)
Recently, I tested a new AMD stock HSF, I got 2300 MHz at 65+ C, fan at 3100 rpm (very quiet)
For actual MHz per $, it is 210-220 MHz / $.
- high performance air (e.g. SK-7 / SLK-800U): from 1500 to 2400-2500-2600 MHz, 50 C
$30-40 for 60% overclocking => ~ 2% / $ or 30 MHz / $ (best price-performance tradeoff)
For actual MHz per $, it is 60 - 80 MHz / $.
- DIY high end water: from 1500 to 2500-2600-2700 MHz, 30-40 C
$150-200 for 80% overclocking => ~ 0.5% / $ or 7 MHz / $ (not price-efficient, but good for the ears)
For actual MHz per $, it is 14 - 18 MHz / $.
- extreme cooling: from 1500 to 2700-3000 MHz, -10 to 20 C
$300-500-1000+ for 100% overclocking => 0.1 - 0.3% / $ or 2 - 4 MHz / $
For actual MHz per $, it is 3 - 10 MHz / $.
Without going into each individual type of extreme cooling, and the still evolving setup such as thermoelectric, chilled water, phase change, liquid nitrogen, ... , the cost per MHz for each type of extreme cooling is very high.
Observation, the MHz/$ is roughly about 2-4 times more costly as moving up the cooling chain each step from stock air HSF, to high performance air, to water, to various exterme cooling. The improvement from stock HSF to high performance HSF (such as copper) is noticeable (~ 15-20%) and worth the added price. The gain in MHz from high performance air to water is minimal (100-200 MHz), except for environmental reasons such as noise reduction, also water cooling requires more setup time and more maintainence time.
The numbers used are for illustration purposes to show the trend and the cost involved of the various types of CPU cooling. The exact MHz and $ amount may vary by a slight amount and would not change the overall picture and general trend.