[BiturboS4] BOV in action

Todd Sager todd at awe-tuning.com
Sat Aug 24 13:10:14 EDT 2002

Except a blow off valve is *not* the same thing as a wastegate.

The picture on their site looks to be of a wastegate, which *regulates*
the amount of exhaust gasses going through the turbine of the turbo,
which in turn controls boost production by the turbo.

The video on their site shows a blow off valve in action, which is a
boost *release* device, which is designed to eliminate pre-throttle body
boost spikes.

When the throttle is partially closed or fully closed during load, the
boost cannot enter the engine and should be rerouted (diverter valve) or
vented (blow off valve), in order to minimize shock loads to the
compressor side of the turbo and also to keep turbo speeds up to
minimize lag.

In fact, my limited French tells me that they are in fact confusing both
the wastegate and blow off valve with a "pop-off" valve.  Pop-off valves
are plumbed into the intake to mechanically limit the amount of boost
going into the engine, such as to meet a specific racing class
regulation.  CART uses a spec pop-off valve that must be installed by
all teams in order to ensure all competitors are at the same boost

Either way, the video you are referring to is indeed a blow off valve in

Bear in mind that a blow off valve should *not* be used on a Mass Air
Flow based fuel injection system like the S4's, as it causes the ECU to
read false air when the valve vents to atmosphere.  The ECU thinks this
air is actually going to the engine, and begins to adjust fueling
accordingly, but incorrectly.  This leads to drivability issues and
potential check engine lights.  The only blow off valve kits we have
seen offered for the S4 that do not cause drivability and check engine
light issues achieve this by limiting boost release through the valve
via internal spring tension to a tiny amount, just enough to make the
whoosh sound.  However, this is at the expense of allowing the boost to
be properly released in order to minimize shock loads on the turbos.

My personal opinion is that the giddiness produced in some people by the
whoosh noise is certianly not worth the potential mechanical damage to
the turbos.

David wrote:
> Regarding an old topic of the forum (BOV and DV), I have finally found what
> a
> BOV (blow-off valve) looks like.
> Here is a small mpeg you can download (a the bottom of the page).
> http://www.forumauto.net/chroniques/Wastegate_Cyber-B.html
> The car is a Subaru Impreza.
> David.

A.W.E. Tuning
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