this one falls squarely under 'beats the hell outta me' at the moment cause I haven't had enough time to try to figure it out but I first noticed it when I was screwing around with a core similar to the one pictured above. I tried it one way and then the other and it definitely seemed to be slightly better with the inlet tube hooked to the side originally intended as the inlet on the 6/7 core [odd core maybe??]
Anyway, I've pulled enough of them apart to make a few observations - the bottom tank is basically symmetrical and as far as I can tell, the waterchannels are pretty much the same across the core - the only difference is the number per side and the direction of flow - provided the core is mounted in an upright position, the 6 channels handle water flowing downward where the seven channels handle water flowing upward. The top tanks are obviously asymmetrical - the one on the original outlet side is a decent bit larger than the one on the original inlet side.
the entrance to the water channels all look about like this on either the top or the bottom:
of course, those are just the entrances (or exits) from the channels - I suppose there could be a difference in the length of the waterchannel - that will require pulling one a part to check
on the bright side, pulling cores apart is always kinda fun:
for comparison - the '77 b'ville with ac (2-302) is a dual pass core that is setup with the same number of channels on either side - it has 6 on both sides and the top tanks are the same size. on this one, there is no noticeble difference regardless of which side is used as the inlet and which is used as the outlet.
hmmm...now that I think about it, all of the cores I have seen with an odd number of channels are set up the same - the extra channel and larger top tank is always on the outlet side...maybe a core manufacturer would be willing to give me a good reason for why they are set up like this. if it makes no difference, you'd think at least some of them would have the extra channel with the larger tank on the inlet side - especially on cores that use the same size inlet and outlet tubes. too tired to think about it at the moment but I will crunch this one a little more when I get a chance to swap some more cores around and maybe pull one of the 6/7 cores apart to more closely inspect their internals.