Generally negative pressure increases cooling performance. endorphines and GeOMan are talking about VERY negative pressure. RetiredChief is the closest, you want close to balanced, though slightly negative is the best, very simple example: (all same fans) 5 exhaust and 4 intake. Positive pressure does not cool better than negative, but too much of both is the worst of all. I find that the 900 with the extra side fan is still slightly negative so it cools very well.