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From: neal@weitek.UUCP (Neal Bedard)
Newsgroups: net.auto.tech
Subject: Re: Theory of Turbos
Message-ID: <369@weitek.UUCP>
Date: Wed, 8-Jan-86 05:37:46 EST
Article-I.D.: weitek.369
Posted: Wed Jan  8 05:37:46 1986
Date-Received: Thu, 9-Jan-86 06:17:54 EST
References: <2@emacs.UUCP> <2493@amdahl.UUCP>
Distribution: net
Organization: Weitek Corporation, Sunnyvale
Lines: 27

In article <2493@amdahl.UUCP>, ems@amdahl.UUCP (ems) writes:
> 
> The reason boost is not related only to engine rpm and
> time is that it is the VOLUME of exhaust (and hence
> it's velocity, temp and pressure - but I'm supposed
> to be simplifying here for illustration ...) that you
> care about.  Less throtle, less exhaust gas, less turbo boost;
> even if rpm is constant.

[First, remember that Saab uses the `blow-though' turbo setup.]

The original poster alluded to the fact that there is quite a bit energy
left in the rotating turbine/compressor immediately after the throttle is
closed. Thus, the pressure on the *upstream* side of the throttle butterfly
may remain quite high until the turbo has dissipated its stored rotational
energy by raising the pressure (and temperature) of the air *upstream* of
the throttle.

The boost gauge plunges to zero at drop-thottle because the boost pressure
is measured *downstream* of the throttle butterfly in the intake manifold
(the only meaningful place to measure boost pressure.) The intake manifold
will be in *vacuum* until the throttle is re-opened and the turbo spins
up again. 

-Neal
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