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Path: utzoo!linus!decvax!mcnc!ncsu!uvacs!edison!steinmetz!davidsen
From: davidsen@steinmetz.UUCP (Davidsen)
Newsgroups: net.auto.tech
Subject: Re: Theory of Turbos
Message-ID: <348@steinmetz.UUCP>
Date: Mon, 6-Jan-86 13:37:13 EST
Article-I.D.: steinmet.348
Posted: Mon Jan  6 13:37:13 1986
Date-Received: Thu, 9-Jan-86 05:57:25 EST
References: <2@emacs.UUCP> <4766@alice.UUCP> <7974@amdcad.UUCP>
Reply-To: davidsen@kbsvax.UUCP (Davidsen)
Organization: GE CRD, Schenectady, NY
Lines: 53
Keywords: turbo
Summary: 

In article <7974@amdcad.UUCP> mike@amdcad.UUCP (Mike Parker) writes:
>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.

>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.

Actually the turbo is not "pushing hard". The throttle plate is upstream
(airstream) of the turbo, while the manifold is downstream. When the throttle
plate is closed there is a vacuum on the input side of the turbo, resulting in
no air to be compressed into the manifold.

Also note that while turbo lag is caused by the time to get the turbo up to
rotational speed, there is also a lag caused by the distance from the throttle
plate to the input valves. An example of this could be seen when the Paxton
blowers used on mid-50's Studebakers (and a very few 57 Fords) were moved to
other cars.

Traditionally the carburator is placed before the blower, and a notable lack
of throttle response is caused. The Studebaker setup pressurized the whole
carb in an air box, keeping the throttle plates close to the valves. This
resulted is much better drivability around town. We also played with doing
this with a GMC 6-71 blower and handmade manifold once, and got the same
results.

NOTE: if you try this, all carb vents *MUST* be under pressure or they become
a sprinkler system. Also a "popoff" valve is needed between the blower and the
carb, so that closing the throttle at high rpm won't cause very high
pressures. The high pressures will blow not only the obvious things, but can
bend the throttle plates (calculate 75psi x area), which results in a
permanent half throttle effect. This can be a real thrill.

I have had a chance to drive a number of turbo'd cars in the last few years,
and have yet to find one that had the same responsiveness as a positive
displacement blower. In addition, having the turbo "come on" while cornering
at high speed and throttle can cause control problems. I don't deny that they
are cheaper to install and run than a blower, but if I had a choice I would
rather have a blower for street use.

-- 
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