Re: turbo theories Part 7.32.d

[glen.powell@smc.com]

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  Not sure about your logic here, but it might just be a case of semantics.
  When the throttle is closed during boosted operation you create a *high
  pressure* situation between the turbo compressor and the throttle plate,
  not low pressure. What the bypass valve does is to allow the high
  pressure between the compressor and throttle plate to "feed back" into
  the compressor inlet, essentially equalizing the pressure on the inlet
  and outlet sides of the compressor. This reduces the drag on the
  compressor and allows the turbo to decelerate less Vs if the compressor
  were just pushing against the closed throttle with nowhere for the
  compressed air to go. Think of the bypass valve and a "recirculating"
  valve. It "bypasses" the engine, not the turbo, and feeds the compressor
  output back into the compressor input "bypassing" the engine. Also, to
  keep the turbo spinning you want to keep the WG closed as much as
  possible, so all the exhaust gas passes through the turbine to keep it
  spinning.


  ========== On Tues, 21 Feb 1995 - quattro-owner wrote: ==========


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Here's another idea:

I would think that even with a bypass valve that some of the turbine speed
loss is due to the turbo creating a low pressure area between it and the
engine when you lift off throttle (hence the possible use of extra
injectors in the headers of rally cars).  In this case the turbine wheel is
fighting atmospheric pressure on the exhaust side.  Maybe if you can get
the wastegate to open under both vacuum and boost you can better maintain
turbine speed (sort of like a compressor bypass for the exhaust end).
Might be as simple as drilling and welding a fitting to the opposite end of
the wastegate, attaching a manifold vacuum source and t-ing in a one way
valve so that it only sends vacuum.  Of course this does mean that the
wastegate will be open while idling which will introduce some lag off the
line.  Am I crazy or what?

Ray

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