Turbos at Altitude

[ptimmerm@mashtun.JPL.NASA.GOV]

The turbo book I have from HP press talks about turbo controls
and operation at altitude.  Two examples are Pikes peak cars
and airplanes.  Two points that I remember are, (1.) that you can
achieve AS MUCH boost at high altitude as low.  But, (2.) to achieve
boost at higher altitudes, the turbo has to spin faster.  So
tyically a high altitude app requires some overspeed protection.
With the fast spinning K-24, that may be what audi is doing.
Simple enough to do with hot wire air mass sensors, and not 
much to the electronics. In any late model freq valve controlled
wastegate system you just modify the map for the WGFV.

Getting into why as much boost at high altitude, the reason seems
to be because pressure differential effects both intake and exhaust
sides are reduced.  Drop in P in the exhaust is larger and drives 
the impeller with more force, and the intake has less to chew on, so
it spools up til it reaches a new equillibrium, with is the same
level of boost, but at a higher speed.

Living in CA I regularly drive my car to 7-8K feet and have little
trouble finding the wastegate, even at altitude.  Sorry about the 
length of this post. Hope it reduces confusion at least a bit.

paul timmerman