[Diy_efi] Venturi effect crankcase breather

[Shannen at grolen.com]

Matt, you're right on the money here.  The Positive Crankcase
Ventilation system is designed to draw air through the crankcase during
high vacuum conditions by using engine vacuum sourced after the throttle
plates.  The PCV valve will seal as crankcase when the crankcase
pressure increases above manifold pressure, forcing crankcase vapors out
the breather tube.  The breather tube is generally connected on the
atmosphere side of the throttle plate.  The fact that crankcase "waste"
vapors are drawn into the manifold rather than released to atmosphere is
what caused many mechanics and pundits to fear reduced engine life when
the PCV system was introduced in the 60's.

The total vacuum in the crankcase is system dependent, however.  Many
stock GM vehicles show little to no vacuum as the breather side of the
PCV system offers little restriction to fresh air.  Look for a post in
the archives titled "wicked squeel" or somesuch to see what happens to a
GM engine when a substantial vacuum is introduced in a tpi equipped v8.

Shannen


DI>On 10/7/03 6:22 PM Adam Wade wrote

DI>> --- Matt Porritt <porrittm at anet.co.nz> wrote:
DI>>
DI>>> Not quite magic, but a vacuum gauge easily shows
DI>>> there is vacuum.
DI>>
DI>> I suggest you test this teory at some point.

DI>Like me to start one of the many vehicles here and photograph the vacuum
DI>shown on gauge?



DI>>
DI>>>> Measured at the plenum?  If it's properly designed,
DI>>>> it should barely register.
DI>>
DI>>> At idle it will register quite high actually
DI>>
DI>> Since a PCV line has to connect to the intake BEFORE
DI>> the throttle, no, it will eseentially be
DI>> indistinguishable from atmospheric.


DI>Hang on a second.. 1 minute you're taking about no vacuum at the plenum ,
DI>now PCV before the throttle?
DI>Does it not matter which side the throttle is on the plenum?
DI>Hmm.. I'm looking at a PCV line that connects AFTER the throttle here at the
DI>moment.
DI>On highly boosted cars, this is removed as the valves fail and 25psi+ into
DI>the crankcase is not a desired result.
DI>I'm also looking at a crankcase breather, BEFORE the throttle body, with no
DI>PCV valve on it.

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