Second, I get emails from some readers who buy heatsinks and find performance does not match what they read in our and other reviews. The first question I ask is "What is the temp inside your case??" Invariably the answer is something like "30/35/40 C" - sometimes higher!
The air temp going INTO the heatsink or radiator fan is the floor - all other temps you see are added onto this temp. If the air temp is 35 or 40C going into the fan, then there is NO WAY this heatsink will give you 40C at the CPU. The following table lays out what to expect for a 100 watt CPU:
As the table shows, the higher your case temp (and fan inlet temp), the hotter the CPU temp. It's no wonder that many are finding fresh air ducts to be a very effective solution at lowering CPU temps. Effective CPU cooling requires BOTH a good heatsink and airflow through the case.
To test out how much case temp impacts your system, open the side of the case, aim a house fan at the motherboard and record temps under stress. This is "best case" for airflow through the case.
To calculate what to expect for other CPUs, for every watt the CPU radiates, the heatsink will cool the core by the (C/W x watts) plus ambient temp. For example, at a fan inlet temp of 25 C, a C/W of 0.25 with a CPU radiating 50 watts means that the CPU temp will be 50 x 0.25 = 12.5 C over ambient temp, or 37.5 C.
Last, remember that in-socket thermistors are not accurate measures of CPU temps. Every heatsink affects an in-socket thermistor differently, as does airflow over the motherboard. There is no consistency between motherboards as well. At a minimum, you can factor +/- 5 C to these temps.
