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From: mike@amdcad.UUCP (Mike Parker)
Newsgroups: net.auto.tech
Subject: Re: Theory of Turbos
Message-ID: <7974@amdcad.UUCP>
Date: Sat, 4-Jan-86 21:07:13 EST
Article-I.D.: amdcad.7974
Posted: Sat Jan  4 21:07:13 1986
Date-Received: Sun, 5-Jan-86 04:49:57 EST
References: <2@emacs.UUCP> <4766@alice.UUCP>
Reply-To: mike@amdcad.UUCP (Mike Parker)
Organization: AMD, Sunnyvale, California
Lines: 27

In article <4766@alice.UUCP> ark@alice.UucP (Andrew Koenig) writes:
>> Take the case where you are charging down the road at 4,000 rpm and you
>> let up on the throttle without depressing the clutch.  Under the above
>> theory the turbo would continue spinning at tc(4000) since the engine is
>> still at 4,000 rpm.  However the boost guage indicated an IMMEDIATE drop
>> in boost.
>
>Right you are.  A turbocharger is driven by a turbine that, in turn,
>is driven by exhaust gases.  If you let up on the throttle, that
>decreases exhaust pressure, which decreases turbine torque, which
>decreases boost.
>

Maybe the decrease in exhaust gases is a contributor, but I believe it is
a small one. Turbo lag is mostly caused by rotational inertia of the 
impeller, and this inertia should keep it spinning a least for a second
when the throttle is let up.

In my car the "boost gauge" is not a boost gauge at all, it is a manifold
pressure gauge. When I lift the throttle, the turbo may still be pushing
hard, but with the throttle plate closed it doesn't get to the manifold.
I imagine that a gauge measuring the pressure differential between the
atmosphere and the turbo output would go up suddenly when the throttle 
closes. 

Mike