Beek and Timo,
You are both right.
About carbs and flow I really don't know.
Hey that rhymes.
I go back in time to the flow bench and Sonny Branch making Harley's go fast on the flat track.
Out of all that I got it that air is an invisible, compressible fluid.
So birds fly and so do airplanes and I always loved the look of a velocity stack.
Air flows like water. It's a fluid. Birds swim in the air.
Being a fly fisherman I spent many years of time watching water.
It flows and is predictable in my experience.
That said, we understand that KTM used a BST on the 640 travel bike because the Adventure was not designed as a racer. The BST is a carb that self adjusts to altitude changes and is a frugal user of fuel.
If I was running a FCR carb on a 640 Adventure I'd have to see the layout of the 660 racer's airfilter and airbox, I'd build that as best I could....
The factory always cheats.
They use a dyno.
As for dirt, no worries, chrome bore and chrome rings.
The foam filter in my experience doesn't pass dirt, it will plug eventually but it is easily cleaned with tank gas and oiled using a bread bag, engine oil and some EMT gloves.
'02 KTM 640 Adventure-lowered
"On the road there are no special cases."
Cormack McCarthy-The Crossing
The faster it goes the faster it breaks.
And high performance=high maintenance.