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TURBO FLUTTER and BLOW OFF VALVES

explained in DETAIL

let’s get started so here we have an
engine connected to the engine we have
an intake manifold
and an exhaust manifold connected to the
exhaust manifold we have
a turbocharger the hot side of the turbo
is obviously also connected to the rest
of the exhaust
while the cold side of the turbo sucks
in air
through the turbo inlet compresses it
and then
sends it out via the turbo output
through the intercooler
into the engine at the entrance into the
intake manifold we have a throttle plate
the throttle plate dictates how much air
is allowed into the engine
the more the throttle plate opens the
more air comes in
the more air comes in the more fuel we
can inject to make
more powerful combustion events the more
powerful the combustion vents the more
power we make and ultimately the harder
the vehicle accelerates
[Music]
so let’s imagine a concrete scenario
you’re flooring it
wide open throttle maximum opening of
the throttle plate maximum air coming
into the engine so the engine is working
very hard it’s trying to make the most
powerful combustion as it can
which means that it’s also producing a
lot of exhaust gases very rapidly
and this large amount of exhaust gases
is driving the turbine wheel very very
quickly
because the turbine wheel is connected
to the compressor wheel
via a common shaft the compressor wheel
is also spinning very very fast
and it’s trying to compress the air as
much as possible so in other words
the harder the engine is working the
harder the turbo is working and the
harder and the more it’s trying to
compress the air
that it’s stuffing into the engine so
when the throttle plate is wide open we
have both
high airflow and high air pressure we
have high airflow
because the throttle plate is wide open
because it’s wide open
there’s no restriction to airflow so we
have a high amount of air
coming very fast through the intake
manifold we also have
high air pressure because the turbo is
working very hard
it’s trying to compress the air as much
as possible in other words
it’s trying to stuff as much air as
possible into the same space
thus increasing its pressure now
following it
is really fun but as you know it can
never last forever eventually you’re
going to run out of rpms and you’re
going to have to shift gears
or maybe there’s a corner coming and you
have to start braking of course in such
a scenario you’re going to release the
throttle pedal
and the throttle plate is going to snap
shut so when the throttle plate snaps
shut
what does that mean for our engine well
it means that we have created a pretty
dramatic and
sudden restriction to airflow at a time
when airflow was previously very high
and the turbo was spinning very rapidly
trying to compress
and stuff as much air as possible into
the engine we can make an analogy
between this scenario
and a person drinking water so what
would happen if you suddenly closed your
mouth
while drinking water
now something kind of similar actually
happens inside our
engine when we suddenly release the
throttle after flooring it
what actually happens is that we’re
transitioning rapidly from a situation
of
high airflow and high air pressure into
a situation of
low airflow and even higher pressure
we are transitioning into all air flow
because the throttle plate suddenly
closes
the rapidly moving air hits a dead end
and it has
nowhere to go but on the other hand the
turbo still has a lot of momentum left
in it it’s still rapidly spinning
and it’s still trying to stuff air into
the engine which it can’t do
because the throttle plate is cold and
this sudden blockage
further increases the pressure inside
the intercooler piping
now the blades of a compressor wheel
inside a turbo are designed
to grab onto the air they grab the air
and
push it along into the turbo now in a
scenario where we have
high airflow and high air pressure
followed by a sudden closing of the
throttle
plate the dramatic drop in airflow and a
spike in air pressure that falls after
the throttle plate causes
can actually overpower the aerodynamic
capabilities
of the compressor wheel blades causing
them to be no longer capable
of grabbing the air and this then leads
to
compressor surge if you look at a
compressor map of
any turbocharger this line right here is
going to be your
surge line everything left of this line
is your surge zone when compressor surge
occurs it simply means that we have
too much air pressure and too little
airflow
for the compressor wheel off the turbo
to do its job
when this happens the air has nowhere
left to go and it carries so much
pressure behind it that it can actually
force its way past the compressor wheel
blades and
out the intake this wrong way out
actually becomes the only escape path
for the highly pressurized air
now compressor surge in a car engine is
almost never powerful enough to stop the
turbo and can certainly not
change the direction of the rotation of
the turbo but what what it can
definitely do is
it can slow the turbo down and shorten
its lifespan because it
exposes the turbo to increased stress
now when compressor surge aka turbo
flutter happens it makes a very
characteristic
[Music]
sound
[Music]
so this sound is actually telling us
what’s happening inside the engine
instead of grabbing onto the air and
pushing it into the engine
the compressor blades are actually
chopping up the air
this is happening due to increased air
pressure and reduced airflow
which is forcing the air against the
blades it’s forcing the air to actually
separate from the blades and eventually
a small amount of air is going to escape
past the compressor blades out through
the intake
when this happens when a bit of air
escapes it’s going to relieve
the local air pressure and this is going
to allow the compressor plates to again
grip onto the air and again try to push
it into the engine but the throttle
plate is still
closed and the turbo still has a lot of
momentum left it’s still spinning very
fast so it’s still trying to do its job
so when again when it grabs onto the air
and tries to push it
the air still has nowhere to go so air
pressure again
increases the turbo just managed to
briefly escape
the search zone but by doing what it
does it again
pushes itself back into the search zone
pressure again increases until
again it separates the air from the
blades and then again
another small chunk of air escapes out
the intake
the cycle actually keeps repeating
itself because the turbo is still
spinning very fast
but on the other hand the throttle plate
is still cold and every time the cycle
repeats
a small amount of air gets to escape
past the compressor blades out
the intake and every time a bit of air
escapes it relieves local air pressure
until all of the excess air pressure is
relieved in the intercooler piping
or until the throttle plate opens again
now the sound of compressor surge or
turbo fodder is often voiced as
now each of these stew or chew is
actually a
small amount of air separating from the
compressor voids and escaping
out the intake the first stew is going
to be the loudest
because it carries the most air pressure
behind it and each subsequent stew
is going to be less loud until all the
excess air pressure
is relieved it’s undeniable that the
sound of turbo fighter is
interesting but it can actually be bad
for your turbo because it exposes it
to some very strong pressure pulses
unless turbo flutter is of negligible
magnitude it’s going to
shorten the lifespan of your turbo and
your bearings and your compressor wheel
are going to bear the brunt
of the damage obviously most of us don’t
have the budget to replace turbo charges
very often so compressor surge
is best avoided so how do we avoid
compressor surge that occurs when you
release the throttle
well instead of letting the excess air
pressure force its way
past the compressor blades we simply
relieve all the excess air pressure at
some other point in the system
and that’s exactly what a bov or blah
valve like this one does
as its name suggests it blows off or
gets rid of the excess air pressure in
the system when you release the throttle
by blowing off the pressure at a
different location in the system we do
not
subject the turbo and its components to
the increased
stresses and shock codes associated with
turbo flutter now when it comes to
getting rid of the excess air pressure
you have two options
option one is to simply vent everything
into atmosphere
option two is to recirculate it back in
front of the turbo inlet
venting everything into atmosphere will
give you a loud
hissing sound like this
as you can see instead of letting the
excess air pressure force its way
past the compressor blades in small
chopped up chunks
we simply and efficiently release all of
it
at once your other option is to be
stealthy and do what the oems do and
that is to recirculate
everything in front of the turbo in it
by recirculating you’re going to lose
the hissing sound
but in reality recirculation is more
efficient
because when you vent everything into
atmosphere use
all the work previously done by the
turbo so when you’re back on the
throttle
the turbo has to start making boost from
zero again
which can result in noticeable lag
between gear shifts
but there’s a better option than being
forced to choose between 100
recirculation and 100 percent venting
and that’s to use a clever dual port
like this power valve from go fast bits
and it gives you a nearly infinite
number of choices
when it comes to the ratio between
venting and recirculation
as i said this ball valve has two ports
one for venting to atmosphere and one
for a circulation
which means that you can set it up to do
both at the same time
the beautiful thing is that it doesn’t
have just a few preset positions between
venting and circulation
instead you can choose any of the nearly
infinite number of positions between
100 venting and 100 recirculation
by simply turning this knob this is an
exceptionally well made
product and turning this knob is
honestly pure pleasure but other than
knob turning pleasure
it also means that you can set this
thing up to recirculate
most of the air and vent just enough the
atmosphere to give you a nice
sound that also means that this bov can
be set up to work with a turbocharged
engine
using a mass airflow sensor or math
usually venting to atmosphere on engines
that have a map
doesn’t work perfectly because a map has
already metered the air you’re venting
and is injecting fuel for the air that
you’re not actually letting into the
engine
and are dumping into the atmosphere
instead this means that you can get side
effects such as engine stumbling or even
stalling when you release throttle as
the engine runs rich for a brief moment
when the throttle plate closes sometimes
you can even see black smoke coming
out the exhaust but with the gfb
response you can
fine tune your setting to recirculate
enough air
until the engine is happy while also
still getting the ball off sound
okay so now we know what a ball valve
does but
how does it do it well it’s actually
pretty simple
okay as you can see we have multiple
connections on the ball valve
and this part the bottom part is what
gets connected to the intercooler piping
and as you can see on the underside we
have a piston
when this piston is in its bottommost
position as it is now
it prevents any air from escaping past
it
at the top we also have this connection
and it’s connected via a hose
to the intake manifold after the
throttle body
this means that when the throttle plate
is fully open the top of the piston
and the bottom of the piston see the
same air pressure
but inside there is also a spring acting
on top of the piston and when the
throttle plate is fully open
the spring is what keeps the piston down
now boost pressure cannot
overpower the spring because the same
boost pressure is also present
on top of the piston but what happens
when we close the throttle plate
high pressure remains in the intercooler
piping and below the piston
but the top of the piston is now seeing
much lower pressure or vacuum
the closed throttle plate prevents high
pressure air from entering
the intake manifold while at the same
time the downward motion of the piston
is actually quote unquote sucking air
into the engine creating a low pressure
zone
in the intake manifold past the throttle
plate
this means that the high air pressure in
the piping can now
overpower the spring and lift the piston
the high pressure air rapidly escapes
out from the piping relieving any excess
air pressure
by turning the knob we simply choose how
much each of the different ports
gets obstructed and thus choose how much
of the air is recirculated
and how much vented to atmosphere so as
you can see a very simple effective and
elegant solution
so now we know what the bov does we know
how it does it
the final question is where do we
install it well anywhere between the
turbo outlet
and the throttle plate now in most cases
it’s best to avoid installing a bov
very close to turbo output because this
part the piping near this part can have
unpredictable pressure fluctuations
which can confuse the ball of valve
causing it to work incorrectly in most
cases the best location is
after the intercooler and before the
throttle plate
so is there a scenario where a ball
valve cannot help you get rid of
compressor surge
aka turbo flutter well yes there is and
it is when turbo fighter occurs
during wide open throttle instead of
when you close
the throttle
when turbo photo occurs during wide open
throttle it means that your turbocharger
is trying to
stuff more air into the engine than the
engine can
ingest in other words you have a
mismatch between your turbocharger and
your engine and to fix it you’ll need to
either
modify your turbocharger or change some
components
or simply get a completely different
turbo

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