[ROVERNET - UK] Crankcase venting
Vern Klukas
vern at inkspotco.com
Tue Feb 13 21:20:06 GMT 2007
My turn for a hats off, Fletcher. I suspect I'd better be pretty
careful about making sweeping technical claims and theories around
you.
Yours
Vern
>Steve-
>The purpose of the venting system is to draw air through the engine
>to clear water vapor and other contaminants produced by combustion
>and condensation.
>
>As such, there must be an air intake, and an outlet. Early systems
>used a vent somewhere near the top of the engine for the intake, and
>a road draft tube exiting near the ground to produce a slight vacuum
>by venturi effect as exit.
>
>Systems with the vent line to the SU between the vacuum chamber and
>throttle plate are called "constant depression" systems, since they
>always have a slight but constant vacuum to draw vapors out of the
>engine. (This is the same "constant depression" that is the basis of
>operation of carbs so designated - SU, Zenith-Stromberg, Bing, and
>others.) This is the exit for the vapors. The system causes a slight
>leaning of mixture, especially at idle, which is compensated in the
>mixture needle selection.
>
>Engines with "PCV" systems plumb the exit into the inlet manifold,
>where the vacuum varies excessively, and it upsets the mixture, so
>the PCV valve limits flow and/or max vacuum. This is usually a
>pretty crude arrangement, though the Smith's diaphragm type PCV
>valve is quite elegant and works very well if in good condition.
>
>Since it is not good to suck dirt into the engine, all vent systems
>use a filter on the air intake line. This was formerly usually in
>the oil fill cap, or was arranged to pull the air from inside the
>clean air stream after the main air filtration. The same holds
>today; the air is drawn from the oil fill vent/filter, the main air
>filter, or the charcoal cannister filter on cars with Evaporative
>control; modern systems are generally more restricted in airflow, so
>that fuel/air mixtures can be controlled within close limits
>.
>If you have connected all vents to the carb bodies, then you have
>only exits and no entrances. That gives no ventilation, and slightly
>high vacuum in the engine interior- good for stopping leaks, bad for
>oil consumption. and a slightly rich mixture with the OE needles.
>Reconnect the line that goes to the highest vent on the engine to
>the clean side of the air filter, or fit a vented oil filler cap,
>with a small restrictor hole, about 1/8" (this is a calibrated port,
>so it interferes with fuel/air ratios). The other vent line(s) from
>the engine should go to the carb constant depression ports. You feed
>the air into the engine in the cam cover because that is the coolest
>place, and thus is where water vapor condensation is the biggest
>problem.
>
>FRM
>
>
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--
Vern Klukas I'm a little . . .
Inkspot Type & Design
vern at inkspotco.com
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