[ROVERNET - UK] Crankcase venting

Steve Lawrence jaoa27 at verizon.net
Wed Feb 14 06:54:46 GMT 2007


That is some great info.  I have to chew on it, or better yet, send you a 
pic of the top of my engine and have you draw the connections on there for 
My car is a 1969, so when I replaced the HS8 carbs with the older HD8's 
(easier for me to adjust, although a bit less refined), I lost track of the 
best vent routing, as the setup was a bit different.


The purpose of the venting system is to draw air through the engine to
clear water vapor and other contaminants produced by combustion and

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.


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