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One other thing to consider here would be a blow off valve. Although
primarily intended to prevent overpressure when the throttle is slammed
closed, I believe that most blow by valves also work as a pop off valve at
high pressure, and would also work well for our purposes.
The blow by valve is simply a pop off valve that is referenced to the post
throttle body manifold pressure instead of referenced to the atmosphere.
This would make them altitude independent. Some are designed with a smaller
diaphragm area on the 'reference side' with a spring. Thus, the spring is
set for some absolute manifold pressure. I have one that I am considering
installing.
Dave Leonard.
Marvin Kaye wrote:
Hey guys,
We've been talking about these pop-off valves as safety devices for
some time, but the problem always seems to be that the available crop
of these things will only work at sea level. This is an obvious
shortcoming for airplane applications, as the MAP will always be
decreasing with altitude.
I've been kicking around some ideas for awhile and came up with a
little drawing of one of these things that, I think, addresses the
problem. See the attached picture.
My thinking is that if you create a chamber where you carry around
your sea level altitude with you, close it off with a piston, the
exposed side of which is open to the atmosphere, then as your altitude
increases the piston should move down an amount equal to the change in
atmospheric pressure. It then applies the required additional force
to our spring and keeps a steady 20psi of force on the valve at the
bottom. (This would be the equivalent of 40" hg MAP, but you could
make it be whatever you want by changing the spring rate.) As you can
see, the exposed sides of both the movable piston and the valve are
seeing atmospheric pressure... in this example drawing the piston and
valve's areas are both one square inch, just to keep things simple.
Now, for the $64,000 question... what's wrong with this? If it won't
work, why not? What do we change to make it work? This can't be that
hard. Suggestions????........
<Marv>
It looks to me (with my non-physics-trained eyes) like all this does is
shift the popoff pressure up by the pressure in the chamber. Total
pressure on the top of the disk would still vary with atmospheric,
wouldn't it? What would happen if you closed the 'open to atmosphere'
ports & only allowed internal pressure in the chamber to push on the disk?
A more 'universal' problem might be the size of the 'ports'. The turbo
is pumping a *lot* of air. As an analogy, consider your air compressor
when you use an air nozzle to blow dust off a surface. The pressure in
the nozzle drops pretty low, but it takes a relatively long time for the
tank pressure to drop noticeably. There has already been a lot of
discussion about the need for porting the exhaust side bypass port on
the turbine & it's already a lot bigger than the popoff ports being
discussed. Will these little ports have any appreciable effect on the
total pressure in the intake manifold? Will it happen quickly enough to
stop/prevent detonation?
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