Re: High Altitude???

[Robert Myers <rmyers@inetone.net>]

Pablo,

Any turbocharger can deliver more air than needed at higher barometric
pressures.  (Well, perhaps not *any* turbo, but the ones used by Audi
certainly can.)  That's why they have wastegate valves - to prevent
overboost.  At lower ambient pressures it just takes a bit longer for the
wastegate valve to open but open it will.  The ultimate pressure is arrived
at (in earlier turbo/ECU combos) by supplying pressure at so much above
ambient pressure.  This results in a lower total pressure at altitude but
still well above ambient.  The boost pressure is controlled by a pressure
differential across the wastegate diaphragm.  It is limited by the
wastegate spring.  A weaker spring --> lower maximum boost.  A stiffer
spring --> higher maximum boost.

More modern ECUs control more precisely and can compensate for altitude to
allow operation at a fixed and more or less constant maximum boost
regardless of altitude (within reasonable limits).  The ultimate boost
pressure is also controlled by the same pressure differential across the
diaphragm.  However, the more modern ECUs can manipulate the differential
by means of the wastegate frequency valve to raise/decrease the pressure
differential by adding a controlled amount of boost pressure to one side or
the other of the diaphragm.  This augments/decreases the effective strength
of the spring to increase/decrease maximum boost as the ECU determines
appropriate.

Since the turbo is capable of producing a rather large amount of compressed
air, unless ambient pressure is *extremely* low, there will be more than
enough to provide significant boost under any practical conditions
regardless of altitude.  At Pike's Peak (13,000 feet ASL), the power output
of my S6 was noticeably lower than at lower altitudes but the car was
running quite well and had significantly more power than non-turbo cars.

>> Pablo,
>> Any supercharger regardless of the method driven (exhaust or mechanical)can
>> compensate for altitude (that is why they are used in aircraft's)...
>> Avi
---snip---
>Avi:
>Thanks for input. How can an exhaust driven supercharger (Turbo ?)
>compensate ?
>Where am I wrong ? Please help me !
>I thought less volume of air at intake resulted in less exhaust volume.
>Therefore less CFM's to drive superchargers.
>Is it then that a Turbo at sea level and at 12000 ft. provide the same
>boost ? by compensating ?
>Mechanically driven turbochargers are of course self explanatory.
>Your help will be appreciated
>Thanks in advance
>Pablo
>P.s. I measured (long time ago ) internal combustion engine vacuum at
>12000 ft.  and it was about 2 inches of mercury less than at sea level.
>Engines seem to breathe less. 
>Compression drops substantially, Power  and Torque follow.
>I am getting old
>Pablo
>
>
>
___
   Bob
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*  '95 S6  Cashmere Grey                                          *
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