More fin area to compensate for thicker rad?
 

The optimal thickness of a radiator is around 1"... at least with weak fans.

Looking at the thermochill rads, or blackice ones too i guess, they're thin, however the fin area isn't all that great. IE 120x120mm fin area thats 1 inch thick vs a heatercore thats 170x170mm fin area and 2 inches thick... do the 2 cancel eachother out? The 14400mm^2 vs 28900mm^2... twice the fin is about 1/2 the resistance and twice the thickness is twice the resistance... does it work out that nicely?

Also does it matter how much airflow is tring to be pushed though? I'm looking at a fan that'd be pushing 10-15CFM in freeair... does the less air you try to move make the resistance affect it less?

Also holding a heatercore horizontal... does heat rising add up to anything? The deltaT is probably 10C or so between the water and air. I know exact numbers arn't really possible but can they ever add up to anything worthwhile? I mean even if .5CFM moved though the rad on its own thats 5% of what the fans would be doing.

How do you know this? 2" is clearly too thick, but I'm unconvinced that the BI pro (at about a half inch) is too thin. I suspect there's an interplay between volume (so air velocity) passage size, total opening size, surface smoothness (turbulence) and thickness.

Frontal area seems to be the ruling factor. Changing frontal area changes radiator effectiveness a lot. Changing thickness seems to make a much smaller impact on thermal performance - but presents less resistance (which in turn may allow greater air volume at the same noise level).
If you are asking does a 120x120x12 rad have the same air resistance as a 170x170x50 one, given that the 170x170 is about two and a half times the frontal area (so lower air velocity for the same air volume) - I don't know. I can answer for a 150x150x50 (common chevette/montego core) though - which is that it does not. A Panaflo L1A at 5V has no problem moving air through a BIP, but is essentially in stall if shrouded to a 6x6x2 heater core.

[quote]Also does it matter how much airflow is tring to be pushed though? I'm looking at a fan that'd be pushing 10-15CFM in freeair... does the less air you try to move make the resistance affect it less?[quote]Yes. Friction goes up with velocity. Cross section stays the same, so velocity goes up with volume, so friction (AKA resistance) goes up with volume.

Beats me. It's certainly do something - particularly if your delta was 10C. Hopefully it's a third of that, or less, though...

There's also fins per inch and the surface treatment of the fins (rough or smooth). Heater cores were not designed for what we're doing - it's just that they have so much heat shedding capability compared to what we need that they do a good job.
There is some discussion of it here and (in another thread I can't find right now but ought'a be stickied) Cathar did an outstanding analysis of why radiators in air-flow-series don't work much better than a single rad - and a thick rad isn't much more than a couple of thinner ones in air-series.