>Bypassing the filter at altitude may be a
> reasonable thing to do,

Or maybe not.  "Van" claims they ran a RV6 with and without a K&N Filter to
determine the difference.  They use a straight in plenum (aprox. 3" opening
gradually expanding to about 7" wide) which required the induction air to
turn 90 degrees about 14" behind the inlet to enter the updraft carburetor.
The result was a significant increase in MP at all cruise and climb
airspeeds, confirming the K&N claims.

A friend put a new Bracket brand, oil soaked foam filter on, flew from LA to
Arkansas, no visible dust and the filter was clogged enough for the engine
to run better with carb heat pulled by the time he got to Arkansas, just
another reason to have a filter all the time.

Steve Colwell   Legacy 2K  Placerville,CA   (530) 621-3408