Re: Changing the Bomb

[raf40@oes.amdahl.com (Richard Funnell)]

Alex,

--snip--

>BTW, what do you think on the subject of turbos,
>maximum boost, and altitude, regarding the response to the <Plethora of
>Questions...> post by Robert Houk?
>
>I still believe that whether you dynomometer a Turbo engine at Daytona
>Beach or the top of Pike's Peak, you'll eventually get the same final HP
>rating because the wastegate spring doesn't care about relative
>atmospheric pressure and instead opens only once it is pushed with
>sufficient force.  This is "maximum boost", as predetermined by the
>factory, and therefore entails a certain number of air molecules being
>forced through the intake, followed by fuel and such.  Unless, that is,
>you are so far out of the atmosphere (...your Audi is carried aloft
>beneath an SR-71 Blackbird and released because you have been Chosen to
>compete in the Autocross of the Audi Gods at 75,000 feet...Gasp!) that no
>matter how fast you spin the turbocharger there is virtually no air
>left to compress. The only difference altitude should make are turbo lag
>and low RPM/boost "driveability" because turbo motors often have a lower
>CR?

Like all things this complex, I think it depends ... In this case, on the
method used to control the wastegate.  On my '83 urQ, this control is a
spring and atmospheric pressure acting against the boost applied to the
other side of a diaphragm.  Under reduced atmospheric pressure, the boost
required to open the wastegate will be reduced; the ultimate boost and
resulting horsepower would also therefore be reduced.  Pardon the terible
ascii art, but maybe it helps.


RESISTING FORCES        atmospheric pressure + spring


ambient pressure------------> vvvvvvvvvvvvvv
                                ---------
                                |
                                ---------
spring---------------------->            |
                                ---------
                                |
                                ---------
DIAPHRAGM               ==========================

                              ^^^^^^^^^^^^^^^
OPENING FORCE--------------->  boost pressure

Later models use the ECU to apply back pressure to the wastegate to hold it
closed longer.  In theory, if the ECU sensed absolute pressure, it could
cause the ultimate power to always be the same by adjusting the back
pressure to cause the same final boost pressure.  I have no idea how these
systems actually work, though I'm sure many other listers do.


>
>Maybe secondarily they correlate the steering and brakes.  It has felt
>like that to me on every total-hydraulic system I've driven versus the
>other kind.  Could also be my own aesthetic judgment overreaching
>itself...

Your guess is as good as mine.

>
>Best Wishes,
>
>Alex Kowalski
>'84 4KQ

Richard Funnell,
San Jose, California
'83 urQ
'87 560 SL