Alright I was reading on the original thread, and there seems to be some mass confusion as to how to rate heat exchangers (i.e. rad).
http://forums.stangnet.com/showthread.php?t=703015
They are rated in heat (i.e BTU) rejection rate.
To compare two heat exchangers:
The primary method to compare two heat exchangers for a single certain application should be comparing the surface area/volume ratio. While many other factors affect the amount of heat rejected from a heat exchanger, for a single application we need to hold these constant (ambient air temp, air speed across fins, inlet coolant temp, etc.) unless something in either of the designs changes them.
Many of you were talking about comparing the overall size of the heat exchanger, and that is only one part of it, we need to know fin area, tube diameter, coolant volume, etc.
Anyone that designs heat exchangers (i.e. this company) will have this data (unless they are rebadging/ripping off designs). While I understand this data can sometimes be proprietary, I would love to have this published so we can inspect.
Here is some good reading if you want to learn more about the wide world of heat transfer and thermodynamics:
http://en.wikipedia.org/wiki/Heat_transfer