thanks for the explanation karl
like you, i am also a trained (although not chartered) mechanical engineer in the automotive industry and (more or less) fully understood these things before you made your point.
not meaning to crtiticise you for replying in an intelligent and constructive manner (which is appreciated by me more than most on a lot of topics), in this case i made my post in a deliberately simple way in the hope that it would bring further understanding for others.
you answered in exactly the way that i had hoped, and saved me basically typing out the answer to my own question.
and it's a fact that your word's of wisdom are taken a lot more seriously on here than mine.
so my cunning tactic worked
