Re: Turbos at Altitude

[steveb@newkla.kla.com]

> >  As far as I can tell, our cars control it relative to atmospheric
> > pressure.  For instance, a stock '89-'90 200TQ allows 0.4 bar above
> > atmospheric (or 0.8 in the case of mine with modified wastegate spring,
> > etc.).  Thus at whether you're at sea level or 7,000 feet, when you first
> > turn on the ignition, the boost gauge will read 1.0; at full boost it will
> > read 1.4, but at 7,000 feet, atmospheric pressure is actually  0.77 bar, so
> > you're really operating at 1.17 bar.
> 
> is this really the case?  i always thought that the wastegate would
> simply open at a predetermined pressure regardless of what atmospheric
> pressure was.  i always thought that it was the reason why turbos
> are popular in places like denver.
> 
... I was thinking that too, and it all depends on how much of the force 
that holds the WG closed is provided by the spring vs the local atmospheric 
pressure.  I'm thinking that for the older cars where the ECU is out of the 
loop that it would be true that it would be somewhat independent of altitude.
For the newer cars where a good part of the spring is under computer control 
it will be determined by the pressure sensor.  If the sensor in the ECU does 
not measure absolute pressure (or have altitude correction) then I could see 
that the max boost pressure would be scaled to local atmospheric.  

Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)