Connection of the Float Chamber Venting Lines to the Rotax Airbox

HKK

"The float chamber venting lines have to be routed into a ram-air and vacuum free zone or into the GENUINE ROTAX® airbox,..."

The term Ram Air is usually used to describe relative (???) high velocity & pressure air eg air as it enters the forward facing cowl duct, is often high velocity and relative to further back in the cowling, pressure.

Vacuum Free in this context simply means an area that is not experiencing negative pressure, relative to other areas within the cowling eg cowling exit air stream, may have lower pressure than further up/forward in the cowling (where the carburettor are located).

Note; my use of the word relative.

In both the Ram & the Vacuum instructions, the installer is being advised not to plumb the vent tube termination into either of these air pressures RELATIVE to the float bowl pressure.

Rotax want the installer to have the carburettor float chamber pressure and the carburettor air inlet/throat at the same pressure. It does not really matter what this pressure is (eg turbocharged/naturally aspirated) as long as its the same.

Your hunt for  the "pressure neutral zone" is commendable but not entirely necessary, as you could just have plumbed the float chamber vent onto/into the air filter and been fairly sure that the bowl & inlet are at the same pressure due to close proximity.

Mr H. Kiefel dissertation on the subject, Pressure Neutral Area, is creating complexity/uncertainty where non need exist. I have not attempted to read all of his "paper" however my impression is one of "making mountains out of molehills".

The nice thing about an air box is that you don't have to worry about relative pressures, as by plumbing the vent line into the air box, you are automatically equalising the float bowl pressure with the carb inlet pressure.😈

Yes...since I have an airbox, I'm not worried about this.  Thanks for the discussion 

Mike