IMPORTANT: TDP, Maximum Power and °C/W
°C/W °C rise over ambient per watt of CPU heat is calculated by dividing the temperature rise over ambient by the heat (in watts) of the CPU. The lower this number, the greater the cooling power of the heatsink. It can be used to predict the maximum temperature with CPUs other than the one used to test the heatsink. It's also allows a fair comparison between heatsinks, regardless of ambient temperature or CPU, as long as the same fan/speed is used.
The number used for W, the power dissipated by the CPU, has a major effect on °C/W. For P4 heatsinks in SPCR reviews to date, we have used Intel's Thermal Design Power (TDP) specification. Questions about the P4's actual maximum power dissipation have been floating around ever since Intel first devised their TDP specification.
Processor Electrical Specifications, our favorite CPU power reference, quotes Intel on TDP as being the
"worst case power dissipated by the processor while executing publically available software under normal operating conditions at nominal voltages that meet the load line specifications... The Intel TDP specification is a recommanded design point and is not representative of the absolute maximum power the processor may dissipate under worst case conditions... Processor power dissipation simulations indicate a maximum application power in the range of 75% of the maximum power for a given frequency."
We recently found another website, CPUHeat & CPUMSR Projects, which has calculated the Maximum Power Dissipation of Intel processors:
"Intel hides real power consumption behind Thermal Design Power. TDP is a power consumption of a processor while executing normal software. That is not while executing a stress test software like BurnK7.
"While TDP may be a useful number for CPU cooler manufacturers, it's not useful for end-users. This is because in real world use, there can be application that forces Intel processor to drain more power than TDP."
They have compiled is a list of computed maximum thermal power of Intel processors based on their TDP number.
We believe CPUHeat & CPUMSR Projects' Maximum Power Dissipation (MPD) is a more accurate estimation of P4 CPU power dissipated during SPCR heatsink testing. We will be transitioning to use MPD figures to calculate °C/W numbers for P4 heatsinks. To ensure the data from previous HS reviews can be compared to new ones, two sets of °C/W figures will be presented, one based on the TDP and one on the more realistic MPD.