[DML] Re: Turbo PRV.

["endotex23" <endotex23@xxxxxxxxx>]

Hmmm...a bit of confusion here, mostly about nomenclature. Skipping
wastegate theory (after all, most turbo setups have 'em) a few
corrections:

A pop off/dump valve a not technically a BOV (in the USA at least).
One is a simple spring loaded safety for overboost while the other is
used to avoid compressor surge. BOVs typically vent to the ambient
while the recirc type mentioned is correctly called a compressor
bypass valve (CBV) and dumps to the inlet side of the compressor. BOVs
are usually closed at less than atmospheric pressure and during boost
but open momentarily upon throttle lift, while a CBV is open below
atmospheric and closes at atmospheric and above. Put another way, a
CBV continuously returns some air back to the inlet when the engine is
not being boosted.

As stated by others, BOVs and CBVs are used to prevent compressor
surge and keep the turbo spooled on throttle lift, mainly for shifts.
I personally feel CBVs do a better job of this because they equalize
the delta P across the compressor in a smoother fashion.

There is another reason you'll see a CBV on some cars. Martin
mentioned a BOV shouldn't be used on a mass flow metered system while
a CBV can be used in either a mass flow or manifold absolute pressure
design. He's right. A CBV should be used in a mass airflow system
because any boost dumped to ambient by a BOV has already been metered
and will result in an enrichment spike by the PCU. On the other hand
MAP sensors are usually located in the intake plenum and don't care
about dumped boost, they meter based on pressure in the intake tract
downstream of the throttle plates.

If turbos are placed upstream of the throttle plates on a K Jet a CBV
should be used for the same reason: dumped boost has already been
metered by the airflow plate but never made it into the engine. The
result is an enrichment spike during throttle lift that goes right out
the taiplipe. This is not only wasteful and hurts performance but
if you have a leaky exhaust system can result in annoying afterfire.

G








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