Optimal clearance between radiator and case for fans?

[ixtapalapaquetl]

I am going to be pulling 3 Sanyo Denki 1011s through a PA120.3 radiator (thickness approximately 2.5 in). I am going to mount the radiator externally on top of my case, and realize I obviously need to allow for a gap to allow the fans to draw air through the radiator. My question is this: how many inches should I allow between the case and the radiator to allow the fans to perform optimally?

If, for example, the radiator was placed directly on the solid plane of the case, I would strongly suspect performance loss because the fans would be unable to easily draw in air through the radiator. Similarly, if the radiator is a foot above the case, the fans would feel no effect from the case and draw air freely through the radiator. So I guess my question is how close can the radiator be placed to the case before the fans begin to get choked off?

Here is a silly diagram. I am looking for the distance represented by ? . The arrows indicate the direction of air flow.

|case|---?--->|PA120.3|Fans|----->

I realize this is one of those .02C difference type of questions, but I was curious if anyone had any science one way or the other.

[Nate P.]

Why don't you just cut holes in the top for the fans?

[ixtapalapaquetl]

Why don't you just cut holes in the top for the fans?

While that would be an excellent solution, I would prefer not to hack up this case too much as I will eventually move my WC setup to a different case and use this one for an air system.

And just to be clear, the radiator is going to be mounted outside and above the case using some struts of some kind. So your suggestion aside, I still wonder if there is a theoretical limit to how close the radiator can be placed.