From my understanding of discussions with Willy from HWLabs two years ago, the Black Ice radiator ratings were calculated with a 40C temperature differential between the coolant and the incoming air.
If I am misremembering that figure I will happily admit to being wrong as soon as some hard figures are shown by HWLabs directly as to just how, and under what coolant/air temperatures conditions such values were attained.
A quick persual of Bill Adams' tests of the ThermalChill 80.1, which is a radiator very similar to the BIM, shows that the 40C delta figure to be quite likely to be correct.
i.e. If your system is dumping 160W, and the radiator is rated for 275W, then the water temps will be 160/275*40 =~ 23C above room air temperature.
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