Re: Dump valves (was Max boost)

["D.A.K." <dak-ignore@rochester.rr.com>]

OK, I've gone along with this idea for a long time, but now I'm thinking more
about it and I reach a different conclusion. 

With a bypass valve;
When throttle closes, intake pressure rises and stays high due to recycling
if air. Since air is already metered, and throttle is closed, rich condition
occurs.
When throttle opens again, pressure is allowed into engine, soon, more air
begins to get metered. Before that can happen, much air is put into the
engine without matching fuel, so we have a lean condition.

With blowoff valve;
When throttle closes, intake pressure rises and some pressure is vented to
the atmosphere. Since air is already metered, and throttle is closed, rich
condition occurs.
When throttle opens again, pressure is allowed into engine, soon, more air
begins to get metered. Since the amount of air that was damned up behind
the throttle plate is less than in the bypass case above, the lean condition
is not as severe as above.

So, I would conclude that a blowoff valve would do a better job of preventing
a lean condition (which is more dangerous, right?)
I have a feeling that the bypass valve does help improve throttle response
after a shift since more air pressure is present in the intake. One could also
argue that the lean condition is so brief that it hardly matters.

Thoughts?

	David


EICHSTEV@aol.com wrote:
> 
> Dan Hussey said in regards to a bypass valve:
> 
> >  THE THING I DON'T UNDERSTAND IS why it would create any more of a rich
> >  condition (off full throttle) than without one!???  I really can't figure
> out
> >  why it would!  I mean, that pressure (air) is not entering the intake
> anyways
> >  since the throttle plate is closed.  So, why does it matter what happens to
> >  this back-pressure.  It can just run upstream and cause your turbo to stall
> and
> >  create more problems.  And, even if it runs a little rich just off throttle
> >  (littleraly for only a second) would it creat any problems?  I have thought
> a
> >  lot about about it, and can't see why installing a bypass valve would
> create
> >  any problems, but I don't actually have one.  Anyone out there with a
> different
> >  view or any different thoughts....I'd like to hear them.
> 
> The bypass valve won't really cause a rich spike, although a blow-off valve
> will.  With a bypass valve, all metered air stays in the intake tract, not so
> with the blow-off valve.  With the blow-off valve, metered air is released
> directly into the atmosphere from the intake system through the blow-off
> valve.  The fuel injection system "thinks" more air has gone into the engine
> than really has, and meters an amount of fuel to match producing the overly
> rich air/fuel ratio.  This can cause premature wear on the cat. and O2 sensor
> (maybe cool flames out of the downpipe too).  How bad it actually is surely
> the subject of debate, but IMHO, the bypass system is the way to go.
> 
> Personally, I have never seen an Audi (street car anyway) with a blow-off
> valve, but plenty of 5ktqs, etc. with fabricated bypass systems that do work.
> 
> Steve Eiche

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