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Message
Rusty
& John,
I have
a pop off valve on my installation which is there in the likely event that the
waste gate can't dump enough boost. I agree with Rusty that a POV is not
an acceptable method of boost control, but is a very good safety device. I
don't really think that it would unload the turbine when it opens, because of
two factors. One, mine pop off valve doesn't just jump open, it starts
opening at 8 lbs, but takes about 10-11 before opening all the way. Two -
even when open all the way, it is restricted by the outlet on just how much it
can dump. It certainly wouldn't unload the turbine
IMHO.
If you
weight the option of having the POV valve open in flight vs over boosting the
engine, it is a no brainer. I'll take the
POV.
Steve
Brooks
Hey
Rusty,
Do you still have that blow off
valve?
John
Never had one :-)
Technically, what I had was a "Pop off valve", or "relief
valve". A relief valve, is
just a spring loaded valve, that dumps pressure over a certain amount.
This is used as a final safety valve on some cars, but not RX-7's. A
blow off valve is something that's standard issue on turbocharged cars, but
probably not needed on airplanes. A BOV is vacuum actuated, and is used to dump the pressure
when slamming the throttle shut, such as between gear shifts. This saves
the turbo from sudden over-pressure.
I
still have the relief valve, and I will certainly sell it to you, but I have
to stress that this is NOT a fix to the problem. Opening the outlet side
of the turbo causes overspeed of the turbine, which is bad, and this is never
considered an acceptable method of turbo control. It also isn't
desirable for aircraft, because this is a differential device, meaning that it
looks at pressure compared to ambient. In other words, it adds a certain
amount of boost, which falls as you climb, just like a NA engine.
I
would urge you, and everyone else that's trying to use a stock turbo, to look
into modifications that reduce the boost level at low rpm. Ed has posted
links to a place in Oz that does this, and I'm sure there are folks in the US
that can do the same thing. Unless you can modify the turbo to be
controllable, it's not safe. We don't want the Subaru guys posting
your off-airport landing :-)
Cheers,
Rusty (I need a
plenum)
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