in theory, a rad with more widely spaced fins presents less backpressure to the fan, so airflow is higher. however there is reduced surface area, so one has to ask if the increase in airflow can compensate for the decrease in surface area; according to chylld's experience, clearly not.
however the papst fans are known for having good pressure characterisics, so one should maybe not generalise that all fans will handle dense rads so well.
very true... apparently the smallest difference in fan blade geometry can make a big change.
one thing that i forgot to mention in my last post, was that i am running my fans in a 'pull' configuration; which for me at this airflow with this heatercore, works 3-5 degrees C better than a 'push' configuration. i also have a good 2-3" between the fans and the radiator which helps to reduce the effect of any dead zones (e.g. the areas immediately behind the fan's hub)
(someone correct me if i am wrong) that airflow is more turbulent leaving a fan than it is entering a fan, i.e. in the case of rotary case fans, the airflow is 'smoother' as it speeds up, but 'rougher' as it slows down. this might explain why you can get such good performance using low speed fans in a pull configuration.