Originally Posted by
George
I also forgot to mention that on "longer" runs I run 6mm as far as I can then reduce to 4mm just before the engine. I'm not sure if it makes a huge difference that way, but it gives me 'piece of mind' nonetheless.
While Oli's description covers one aspect of longer lines I'll give you another that comes from being a retired FireFighter; friction loss. Upgrading the line from a 4mm to a 6mm will make a HUGE difference in the friction loss over your stated distances. Think about what happens when you add another length of hose to your existing garden hose, that's friction loss.