Compressor bypass valve (CBV) / Turbo Blow off valve (BOV) / Dump valve / Vent valve
written by Anthony Hyde, Australia - First written 1/2000, latest update 8/2010

Custom monster BOV
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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).

Quick note on Wastegate bleed valves - the CBV / BOV valves have nothing to do with the control (or fooling) of a turbo exhaust wastegate. The mechanical or electronic boost control of a wastegate is via a small air bleed and is a separate subject and application.
Described below are the two main types and the essential difference between them:
  • Compressor bypass valve (CBV)
    In the Bosch style CBV case for fuel injected vehicles, air is returned to just after the air-cleaner - for re-use into the turbo compressor air inlet - instead of being dumped to atmosphere. The bypass valve is open (kept open by engine vacuum (against an internal soft spring) under normal low-load (vacuum) engine running conditions, and closes firmly (overcoming spring resistance) when positive pressure (boost) is present in the inlet manifold (or plenum chamber).
    When on-boost and changing gear (throttle lift-off), if a sudden lower pressure (vacuum) condition is created in the inlet manifold and the valve pulls open again (spring assisted), venting pressurised air from your intercooler and pipework back to the turbo air inlet.
    Instead of venting the pressurised air into the engine bay for noise, your directing it back for the turbo inlet to use again and this method is quiet.
  • Blow-off valve (BOV) / Vent valve
    This valve type features an adjustable spring design (soft spring held in compression by preload) to keep the valve closed under idle, cruise and boost conditions (eg 0.5 bar). (If the valve was open, dust and dirt would be sucked in.)
    Only at sudden throttle lift-off (eg 0.8 bar) does the valve quickly open to vent / dump the pressurised air directly to atmosphere (into your engine bay) through a specially designed orifice (and wooshtssh).

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 Valve Comparison
by Anthony Hyde

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.
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

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 !
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