Now is that 6 inches wide as in the path the air takes across it, or does the air flow through the 12 inch section. Lots of intercoolers are designed for fit and not for effeciency. What I said is hard to describe, but is the intercooler long and narrow, or tall and skinny. Now that doesnt make and sence either, do you have a link?
So a 3.5 inch core is pretty thick for most normal fans to push air through. A front mount has an 8hp fan attached to it, so it can easily run air through. A top mount intercooler has a scoop to do it and and maybe an electric fan to help. I can tell you that I struggled to get any air at all through my 4 inch core with anything short of a 40 amp fan. I have a taurus fan I played with to test flow through the core. A normal cheapo 7 amp ebay fan couldnt drop my intake charge at idle 1 degree F in my driveway! Absolutely no air could be felt passing through the core at all. At highway speeds I was getting cooling, but effeciency was much lower (67%) than I thought the core could do. I think a good small spal fan would do the trick(40amps), but it doesnt get around the fact that the core is very very thick. Now stack a 3 inch fan on top of a 4 inch core and you get a serously tall set up for a top mount. A 3 inch would net greater passive effeceincy and would still have very little pressure drop at the cfms our engines put throught it and would fit much better.
The short of it is: if you want a more effecent intercooler, get one with lots of surface area that is shallow. Air flows through it easily with lots of internal volume for low pressure drop. Sadly, usually those kinds of intercoolers only like to fit in front of your radiator.
