To fill in the details:
First off, there is discussion of some of these HC's
here.
[I hope I've got the following correct but with heatercores4u down, I'm not certain.]
Mine (Sean's) is a 2-274:
9.875" X 6.125" X 2", 13 flat tubes, two pass
two 5/8" tubes
Blackeagle's (Craig's) is a 2-342
9.5" X 6.125" X 2" 13 flat tubes, single pass
one 5/8" tube and one 3/4" tube
So, aside from a 3/8" difference in length, and the (relatively insignificant?) difference in inlet/outlet tubes, the difference between the two is the number of passes.
It looks like my HC has about 3.5 times the flow resistance of Blackeagle's.
I had expected mine to have more than 4 times the flow resistance of Blackeagle's due to the fact that my HC consisted of:
seven 9.875" 'paralleled' flat tubes in series with six 9.875" 'paralleled' flat tubes
whereas Blackeagle's consisted of:
thirteen 9.5" 'paralled' flat tubes
It appears that there is more 'overhead' flow resistance associated with the inlet/outlet tubes and the tanks than I had guessed. I'm assuming this accounts for the difference between the greater than 4X difference I'd expected, and the 3.5X difference measured.
Bill, if you want to send me the excel file, I'll take a shot at coming up with a breakdown of the relevant factors. (I haven't seen anything from gmat in a while. He'd said something about a vacation coming up.)
Edit: grammar