written by Anthony Hyde, Australia - First written 1/2000, latest update 8/2010 |
![]() Custom monster BOV - don't argue -
Bosch style CBV |
Introduction - there are numerous names for big relief Valves fitted to the pipework of a turbocharged engine. The valve is located before the inlet manifold. Nearly all cars with an intercooler will have one. Typical names are Compressor bypass valve (CBV) / Blow off valve (BOV) / Vent valve / Relief valve. The above relief valves are suited only for fuel injection engines, either mechanical (K-Jet) or electronic (EFI). Relief Valves must NOT be used with carburettor installations (a potential flame thrower).
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How do they
work? A small diameter
hose connects between the inlet manifold and the relief valve -
changing pressure conditions (vacuum or pressure) will exceed the
valves internal spring, and the valve will depending on type either
open or close accordingly.
A CBV
or BOV
relief valve is forced firmly closed under pressure conditions
(turbocharger producing boost ), until such time as a lower pressure
condition occurs with throttle lift-off or between gear
changes. An instant lower pressure condition or *
vacuum inside the inlet
manifold will then pull the valve open, momentarily venting
boost pressure to atmosphere BOV,
or recycled CBV.
*
Vacuum (pressure less than
atmospheric) is formed in the cylinder bore/s when a piston decends
(on intake stroke). Vacuum
transfers through the opening and closing inlet valves back into the
inlet manifold, and rises when the throttle plate is
fully closed.
Bosch style relief valves (reviewed below) are compressor bypass valves CBV, and are open most of the time under engine vacuum (idle, cruise, throttle lift-off), but close firmly under positive (boost) pressure. This CBV style returns (recycles) the air quietly back to the turbo compressor inlet or air cleaner area.
Cars and trucks without a vent valve emit a high shrill-whirr sound, as the compressor back cuts the air. Listen to a turbo truck between gearchanges.
Why vent the pressure
?
- Primary reason is to reduce
strain on the compressor turbine wheel due to compressor surge. Surge
occurs when flow is restricted and hence a risk of abrupt reversal of
airflow over the compressor wheel,. Between gearshifts and sudden
throttle lift-off, the turbine is still spinning very fast (but
slowing) and pumping air at the closed throttle plate, as well
as placing strain on the intercooler, hoses and fittings. -> This
is why its a good idea to momentarily vent the pressure. A
CBV /
BOV allows the turbine to
continue spinning freely. So when back on the throttle, boost
pressure quickly rises again with little lag. A suitable valve
location is just before the throttle plate, it acts as the
wall.
A compressor bypass CBV must be used with Bosch K-Jet injection, as being a closed system, any loss of air for which fuel has been metered through the airflow sensor plate, will result in an over-rich condition and possible backfire. A CBV is found on many "OEM" original engine manufactured EFI turbo systems.
A blow-off BOV is suited to an EFI system that uses a MAP (manifold absolute pressure) sensor. As the MAP sensor is connected to the inlet manifold, relieving air before the throttle body under closed throttle (throttle lift-off) will not affect running, as the air is not yet metered.
Whether you decide to vent/wooosh to
atmosphere with a
BOV,
or quietly back to the turbo
compressor with a
CBV
depends on
your injection system.
Magazine test results over the
years have shown little performance difference between the two, other
than an interesting aural sound. The reason for fitting one is
to reduce strain on the compressor turbine wheel, the intercooler,
hoses and fittings.
Bosch
style CBV Porsche
spec
Bosch vs
standard
Bosch
CBV
(compressor
bypass valve) : I compared the readily
available Bosch valve # 0 280 142 103 (recommended in
magazine tests) that myself and others have used for a
couple of years in turbos, against a similar Bosch valve
fitted to Porsches (and sold only through a
Porsche dealer # 993.110.337.50) (in reality Bosch #
0 280 142 108); described in Porsche speak as an
'Air Cut Off Valve' (reportably off a 933 twin
turbo). Both Bosch made valves are
plastic moulded, look the same, and have exactly the same
inside and outside diameters. The outside diameter is 1" or
25.4 mm, so you need 1" I.D. hose or tubing to
match The difference is in the
valve travel or lift. Under engine idle vacuum, the Porsche
spec valve opens approximately 3.5 mm (9/64"), being half
the distance of the ..103 at 7 mm (9/32"). As a result, the
'opening action' is short and sweet vs long and a bit
wobbly. The inside valve face on the
Porsche unit has a brass
dome that might
assist vent flow (the standard 103 has a flat face).
Otherwise I noted little difference between the two,
although on the bench the Porsche spring seemed to offer
just a little more resistance when trying to unseat the seal
with an index finger. Both units were road
tested under all conditions (boost to 15 psi, vented
back to turbo inlet), and I prefer the Porsche spec unit.
The valve opens and closes snapily, an action which is
more audible as well. Remember the valve won't give a
'noticeable' performance gain, instead it reduces strain on
the compressor turbine and inlet plumbing. The choice is
yours, price and availability are similar, both are
good. Note: Under higher boost
pressures, (eg above 16 psi), the valve can start to leak,
ie the diaghram begins to lift off the sealing face as the
internal springs' resistance is exceeded. - A higher
pressure/quality valve is required. Examination
of torn diaphragm
by
Anthony Hyde A fellows Bosch
Porsche version CBV
valve with a
torn
diaphragm was
examined (by the author) and the reason it failed is the
neoprene diagphram rubs against a dead sharp plastic
moulding edge. The edge is located where the side air exit
pipe moulding interesects with the main body pipe - proof
being the tear is visible as you look into the exit pipe. I
placed the suspect CBV in the lathe and machined off the
plastic welding that keeps the uint together. Examining the
rub area points exactly at the sharp moulding
edge. Before examination of
this damaged unit we thought a torn diagphragms was from
over-boost, not now, just a sharp plastic internal edge was
to blame. The edge is on the exit port, and when the
diaphragm passes it cuts. The diaphragm was not centrally
located during manufacture !
by
Anthony Hyde
For high flow turbos, dual (x2) vent valves have been
used.
Saab
Turbos use the Bosch
CBV
v # 0 280 142 102
or 103 or 104 or 105 0r 110. [103 tested above]
The recommended higher boost version
from a 9000T and 9.3 (Y.2000) is Bosch # 0 280 142 110 (Saab
dealer #4441895 - described as 'Bypass Valve').
Volvo
Turbo 740/760T owners have
fitted the low cost Bosch unit closer to the throttle body, replacing
the high cost (A$675) and oddly positioned
CBV
valve made by Pierburg. For Volvo owners fitted with the Mitsubishi
turbo, these already have a built-in compressor bypass valve. The
pnuematic actuator is mounted on the side of the compressor housing,
perhaps not the best position, but functional.
Citroen CX GTI
Turbo 2 uses Bosch
CBV
type # 280 142 103
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