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Ed,
Great write up! In general I very much agree with you on all points but I
want to free flow some thoughts that may modify people's feeling about a
BOV. I am even less of a turbo expert than you, but here are some thoughts
to consider.
First,
I think we have to figure out exactly what is causing John's turbo's to
fry. IMHO we have 3 general candidates 1) overheating, 2) turbo
overspeading, or 3) compressors into the surge region. While it is
possible that there is a combined cause, and probable that more than one factor
plays a partial role, most likely ONE factor is primarily responsible for
JTD (John's Turbos' Demise).
IMHO
that one factor is overheating. We know that heat is a factor
while according you the map you posted we are probably not in
surge or overspeed region most of the time. Also, turbo overspeading and
compressor surge are likely to present with very different symptoms like surging
pressures or a disintegrated turbo. In contrast, overheating weakens the
metals involved and would behave much like what John Describes. In this
case it seems to be turbine axle and possibly the blades that are not up to the
task.
There
are 2 ways to improve the situation. Reduce the heat, of course, and reduce the
forces acting on the metals. The BOV can be
very instrumental in reducing the force acting on that axle, and possibly also
slightly reduce the heat transfer. My BOV (which I assume is fairly
standard) is not an on/off valve, but a spring loaded valve that will open more
or less depending on the pressure in the plenum, the ambient pressure, and the
pressure differential across the throttle body. There is no way it will
instantly drop the pressure significantly. In normal operation it would
never go from full closed to full open suddenly, but rather just start leaking
more and more air as the combination of those three pressures changes. The
only time it would suddenly open is when the throttle is suddenly
closed.
While
on one hand it does seem very inefficient to compress air only to have it bleed
out before entering the engine, I believe it is far more inefficient and
potentially harmful to have the compressor constantly fighting against a large
pressure differential. That pressure differential is the largest
determinant of the force on the axle.
Further, if the turbine wheel is free to turn a little easier (faster)
it places less back pressure on the exhaust and less heat will be
transfeffed. It is true that the faster RPM increases the radial stresses
on the blades, but radial stresses are tolerated much better than the axial
stresses caused by high back pressure. (Since I have decided that heating
is the problem and not overspeeding.)
There
is no doubt that we both agree on the BEST solution: a larger turbo with an even
larger waste gate placed perhaps a little further from the
engine. But until I change my set-up to fit such a turbo and waste
gate I think the BOV is a helpful addition.
Further, the BOV does provide some measure of protection against over
boost like what happened to Todd.
Dave
Leonard
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