ptlas, yes it is a bit strange how the diameter vs volume sums...

ptlas,
yes it is a bit strange how the diameter vs volume sums work. We became aware of this when we first looked at replacing some underground irrigation mains. We upgraded some of the round poly pipe from 2" to 3". I found it easiest to understand by thinking of the pipe as square (much the same volume) Thus a 2" square pipe was 2" X 2 " equals 4 square inches and the 3" pipe was 3" X 3" equals 9 square inches, more than double the flow.
Even more noticeable with some of the surface lines upgraded from 1" to 2" as 2" gives 4 times more flow.

From those great photos Relax posted some of the spirals certainly are not just a metre in diameter, they definitely are larger but certainly not 2 metres. So I'll definitely go with your 1.4 metres.

regards, EB