Re: Low turbo boost '86 5KCSQ

[Orin Eman <orin@WOLFENET.com>]

> In message <s4474719.074@wppost.depaul.edu> Alex Kowalski (audial@aol.com) (Law Guest Account) writes:

> > It's strange that your boost is increasing when you disconnect the WG
> > valve.  Like you, my belief was that the valve functions under boost to
> > keep the WG _closed_ longer than it would with the spring alone...maybe
> > your hoses are mixed up or my brain is, but this has been posited many
> > times by many different sources -- it would be a big surprise to me.

> I thought that - until I hooked a boost gauge up to the line between the 
> wastegate frequency valve and the top of the wastegate.

> That thing doesn't just _switch_ between supplying boost and vacuum to the 
> upper chamber - it postively _vibrates_ between the two.  Once the ECU takes 
> control from the spring, it buzzes.  Scott Mockry has a test lead that plugs 
> into the wastegate frequency valve - it shows just how intensive the ECU's 
> control efforts are.

There are a few things to note here.

Firstly, the wastegate spring is set to give a maximum boost of less
than the desired maximum, even when manufacturing tolerances are
taken into account.  Normally, (WOT switch open) the upper chamber
is connected to just before the turbo which will be a slight vacuum.

When the WOT switch is activated (and a few other safety considerations),
the computer adds boost pressure to the upper chamber in order
to raise/maintain the boost at a desired pressure.  This is done
by switching the upper chamber back and forth between boost and slight
vacuum as you note.  Vibration is a reasonable description; the signal
is pulse width modulated.

Now what happens if you disconnect the frequency valve (plug the line)
and leave the upper wastegate chamber open?  You'll get slightly
higher boost than normal would be without the WOT switch closed.
Depending on manufacturing tolerances and how much the spring
has aged, you will probably have less boost than if the computer is
active.

If you now seal the upper wastegate chamber, the air in the chamber will
be compressed as the wastegate opens, adding a little extra pressure
to the spring.  You may get a little higher than stock WOT boost
this way.

Orin.