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 A blow off
valve is an intricate part to any turbocharged system, it relieves the
excess pressure when the engine doesn't need it. Your turbo is
constantly spinning and producing pressure, even a tiny bit at idle.
This
pressure is great when you have your foot on the gas and want to
accelerate, but when you close that throttle plate the turbo is still
pumping pressurized air into those pipes at full blast which now has
nowhere to go but back where it came from.
The waves of
backpressure stall the spinning compressor wheel and cause it to rapidly
slow down, producing tremendous load on the turbo bearings in ways they
were not meant to handle. Eventually the bearings become worn out
and your turbo will need to be replaced.
This phenomenon is called
Compressor Surge, in many cases it is loud enough to hear it (search
youtube for sound clips), but it can
also be a silent turbo killer. The cure to compressor surge is to
install a high quality blow off or bypass valve. The majority of
gasoline powered turbocharged vehicles come from the factory with some
form of bypass valve, further proving that venting compressed air is a
measure worth taking. However, where the factories try to make
them as quiet as possible so that the driver is unaware of their
existence, gearheads like us want them to be loud, agressive and
jawdroppingly gorgeous.
The added turbo longevity and reliability
is a pleasant bonus. On top of those points, a blow-off or bypass
valve also serves another key feature, quicker shifts! When you're
experiencing compressor surge during a shift the turbo stalls and slows
down rapidly, so when you stand on the throttle again and want boost
right now, the turbo has to work hard to get spinning as fast as it
previously was.
This can cause a noticeable lag between fast
shifts. A high quailty blowoff valve will release the excess
pressure genereated when you close the throttle and allow the turbo to
essentially freewheel, keeping its speed, so that when you step on the
gas again and need boost right now, the blowoff valve piston
closes instantly and you have full pressure. Fascinating little
device, we have spent over a year developing, testing, and obsessing
over our blowoff valve before it was ready to be released to the market.
What's the
difference between a blowoff valve (BOV) and a compressor bypass valve
(CBV)? A blow off valve like ours releases this pressurized air
into the atmosphere, whereas a bypass valve re-routes that air back in
front of the turbo so it gets recycled. On some vehicles, such as
ones equipped with air mass meter (AMM) fuel injection systems, losing
this air after it has already been measured can cause a rich spike (lost
oxygen plus same amount of fuel = a ratio of too much fuel).
However with a properly set up blowoff valve using the right
spring for your application, this effect is minor and usually not even
noticeable. For cars with Air Mass Meters the trick is to use a
very hard spring to keep the piston closed at idle, otherwise the car
will die. Bypass valves are typically much smaller, move a lot
less air, and are quieter, great for a bone stock car at stock boost
levels. But for those of you with extensive modifications, running
more boost, seeking that distinctive sound, or looking to make the most
power possible while running the fastest track time, the Craving Boost blowoff valve is exactly what you need.
How does a
Craving Boost blow-off valve work? A tube is welded to your
intercooler piping, the blowoff valve is bolted to the tube, and a
vacuum port on top receives vacuum/pressure from the intake manifold
(after the throttle body).
There is a strong spring inside which keeps the piston closed, so at
idle the force of the spring overpowers the pulling vacuum force from
the intake manifold. In boost, lets say a meager 20psi, the
intercooler pipes are pressurized and want to push the piston
upwards, however since the intake manifold is also pressurized this puts
20psi to the top of the blowoff valve through the vacuum port, equaling
the force from the bottom, and the spring provides even more force to
keep the piston closed.
After a romp through the gear in boost
you take your foot off the loud pedal and close the throttle
plate, this now puts the intake manifold into vacuum which
will want to pull up on the piston. Coupled with 20psi
in the intercooler pipe plus the instant backpressure wave,
the piston easily overpowers the spring and moves upwards to
relieve the excess pressure with a glorious sound. The
pressures stabilize quickly and the piston is closed
again. There is a Viton o-ring at the bottom of the piston
ensuring that none of the charge air gets out until we want it to.
Our blowoff valves have a massive 1/4"
vacuum port on the top, ensuring the quickest response
possible. Most other brands have a much smaller 1/8"
port.
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