WG connection, EM design

JShadzi at aol.com JShadzi at aol.com
Sun Dec 8 03:04:35 EST 2002

[ Picked text/plain from multipart/alternative ]

Oh boy, question blizzard ...  ;)

<<Yeah, I guess that is a sport quattro motor actually.  I look at some other
> and saw that it had the gear driven cam and two injectors per cylinder
> (crazy!).  I guess it's a long wheelbase SQ, or whatever iteration that
> would be called?  It looks like a regular ur-q, but has the single
> headlights.>>

Ya, after looking at the site, I'd gather its an Audi Sport Anit-lag system.

> <<So, the way the SQ system works is that the bypass valve output is
> directed into the exhaust manifold, instead of the turbo inlet or
> atmosphere? Or, does this system do something more
than just when the throttle is closed?>>

Yes, the pressure is routed into the exh. manifold using basically a
wastegate, from the intake tract, funtionally just like a blowoff valve.  The
pressure passes through the turbine causing it to spool up so that when the
gear is engaged and the throttle is reapplies, the turbo is already spinning,
but not just spinning, but actually spooled.

> <<Is it a requirement that there be no/little valve overlap for this system
> to work?  Seems like the flow would go backwards if there was overlap.  But
> that would be the case even without this system, right?  Since the exhaust
> has more pressure than the intake under boost, doesn't it?>>

In general, turbo cars should have lower overlap than their NA versions, but
realistically, air flows out of the combustion chamber, not the other way, so
a little overlap won't hurt anything.

> <<Is it possible that exhaust gas would go up backwards through that path?>>

No, air doesn't flow backwards through a motor, regardless of overlap.

<<  Like you shift at 15psi and the bypass valve opens, can exhaust to
> through the anti-lag system to the compressor side, or is the 15psi way
> higher than the pressure at the turbine?  (I thought turbine pressure was
> greater than compressor, as I described above.)
> Is it overkill that, if my understanding of the anti-lag system's function
> is correct, the bypass valve pressure is introduced at the individual
> cylinders?  Would it be possible/beneficial to have a single port on the
> turbine side of the EM that you could plumb bypass valve output pressure
> to?>>

Each cylinder is not pressurized, each runner is, which in turn passes
pressurized air through the turbo hot side.

> <<What effect would introducing bypass valve output into the turbine have?
> Would the extra oxygen ignite some unburnt fuel in the turbine?  Is it
> dangerous?>>

The effect would be the same, it is common to have fuel igniting inside the
turbo and collector, its not dangerous.  In fact, I've heard of some antilag
systems that actually inject raw fuel into the collector to cause an
explosion which in turn spools the turbo.

> <<So, in effect, the excess boost pressure upon shifting is recycled through
> the turbo just like in a regular bypass valve situation, but instead of
> going into the cold side intake, it is going into the hot side "intake"?>>

Yes, and helping to spool the turbine.


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