I measured the performance of my 280 rad, see a thread in overclocking section. Here are the least wrong numbers I produced so far:
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Let us say we have ambient temp 20°C, coolant temp 35°C, one rad blowing air in the case and one rad blowing the warm air out of the case.
I actually measured the temp of the warm air from the intake rad to be 32°C.
The intake rad works with delta 15°C and has cooling power 466W. The exhaust rad works with delta of only 3°C (using 32°C air for cooling) and has cooling power 100W (see in the measurement thread how I got this number from the thermal resistance I meaured!).
Total cooling power of these rads would then be 466 + 100 that is 566W.
Let us say we are really loading these rads with 566W. If we made them both use the ambient cold air and one rad would be dealing with 283W, the corresonding delta is 9°C. The coolant temperature would then be 20+9, that is 29°C.
Recapitulation:
With heat load of 566W, using one rad as an air intake in the case and the second as an air exhaust would lead to coolant temperature of 35°C. Using both rads as air intakes would lead to coolant temperature 6°C lower.
Adding a rad on the case that works with warm air produced by other rads in the case really has a limited effect, in this example improving cooling power only by 20%.
The difference is in the coolant temperature - with 566W load,
one rad would cause 18°C delta,
one rad as an air intake and one rad as an air exhaust 15°C delta
both rads as air intakes 9°C delta.
Now I actually swapped the top rad to be an air intake too, the temps really improved, and now my case is swamped with warm air and I have just one 140mm fan and holes in the case to get rid of it. I do not like that too much...