Re: Turbos in high altitudes

[PDQSHIP@aol.com]

In a message dated 96-11-13 12:48:28 EST, you write:

<< >> Does a turbo make up for the loss of external air pressure - ie
 >> charge
 >> pressurization is the same regardless of altitude? Would the knock
 >Depends on how they program the wastegate in the computer.  Most likely
they
 >allow a maximum Absolute pressure which would mean that there would be
 >no loss in power.
 
 I'd make that a maybe.  There will be loss if the turbo as sized can't
manage
 the additional volume (not mass, which is volume x density) of airflow
needed
 to fully pressurize the engine at high rpms.  That's always a trade-off;
small turbos
 result in reduced lag but can only move "X" ft^2 of air.  Larger ones can
move
 more air, but lag more.
  Grant
 
  >>
Grant, make that a yes, audis are altitude compensating turbos, or absolute
pressure type in design.....  On a stock audi turbo, changes in altitude are
covered up to more than 15,000 ft in the mapping of all turbo with stock
computers, the newer 20v motors have a 15,000 ft limit for overspin
purposes....  This changes with the addition of higher boost pressure
computers....  The small turbo and large turbo theory is not necessarily true
anymore...   The size of the hot side and the cold side make more of a
difference than you'd think....  A small hot side with a large cold side
wheel and housing don't lag that much...  RS2's for instance... A modified
RS2 unit can rival a k24 at some pretty low rpm's....  And give all the gains
of a larger k26 based turbo at hi rpm's, passing the k24 at about 2200rpm....
 

Scott