Re: 2Bennett Ricirc setup...

["Bernard Littau" <bernardl@acumenassociates.com>]

Hi Lawrence,

The turbo recirculation setup provides for an alternate path for the turbo's
compressed air to flow, under the control of a valve.

The reason you would want this is to avoid stalling the turbo and dealing
with a high pressure spike in the air flow system.

An example is as follows:

You take off from a stop, redline in first, shift to second and redline in
second...

As you accelerate in first, more and more gas is pushed through the engine
and turbo, spinning the turbo faster and faster.  This causes an increase in
pressure in the air intake ducting beyond the turbo.  So far, this is good.
Now you shift.  Your foot comes off the gas, and the throttle body slams
shut.  The turbo is still spinning, but the air has nowhere to go.  More
pressure builds up in the ducting, and the back pressure causes the turbo
slow down.  The pressure can spike to around 3X the boost pressure.

As soon as you shift into second, you are back at full throttle, spinning up
the turbo again.

A few bad things happen during the shift.  The 3X pressure can blow out
hoses and even the intercooler.  The abrupt halt of the air flow and
increase in pressure also load the blades on the turbo, and can cause
erosion of the blades and extra wear on the bearings.  Slowing down the
turbo is also not what you want, since then you need to spin it back up once
you take off in second.

A blow off valve beyond the turbo can vent this extra pressure, preventing
the pressure spike and turbo slowdown.  A blow off valve vents to the
atmosphere, so the air is lost.  A recirculation setup ducts the air from
beyond the turbo to before the turbo.  This prevents the pressure spike, and
also keeps one from loosing measured air from the system.  I suspect it is
also more efficient to recirculate the air in terms of keeping the turbo
spinning -- you are in a sense using air that is already in motion rather
than accelerating new air.

Hope this helps.  I cc'ed the list, just in case someone else had the same
questions, or I got anything wrong.

Best,

Bernard Littau
Woodinville, WA
'88 5kcstq

----- Original Message -----
From: Lawrence C Leung <l.leung@juno.com>
To: <JShadzi@aol.com>; <quattro@audifans.com>
Sent: Monday, December 27, 1999 11:18 AM
Subject: Re: 2Bennett Ricirc setup...


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