Re: Turbos at Altitude

[tdonohue@xnet.com (Terry Donohue)]

ptimmerm@mashtun.JPL.NASA.GOV said:

>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.
>
>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.

   What I'm interested in is controlling the boost pressure in an absolute
sense.  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.  I know Porsche controls their turbos
in an absolute sense, so they can get full boost even at higher altitudes.
Is there any way to do this on Audis?  I'm particularly interested in this
since I'm about to move from Chicago (700 ft.) to Tucson (2400 ft on up to
9,000 ft).  Getting turbos to run at full boost absolute at higher
altitudes is one real advantage we have over aspros (look at results from
Pike's Peak for confirmation).

Terry Donohue
tdonohue@xnet.com
'71 BMW 2002Ti
'90 200TQ
'95 BMW M3