Quote Originally Posted by Waterlogged View Post
Actually, only 1 rad has used copper tubes and fins, that was the X-Changer. Everything else used brass tubes, copper fins and aluminum does have a higher thermal transfer than brass.
Indeed. The conductivity of brass is ~110 W/mK, aluminum 250, copper 400. There could certainly be situations where there is enough brass and not enough copper that the average conductivity of the two metals is worse than aluminum.

On a one-dimensional basis, you can see by inspection that the ratio is about 50%. Most radiators have brass tubes that have much thinner walls than the length of the copper fins, so unless the brazing is particularly restrictive this should be more efficient than solid aluminum by a fair bit.