[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 


>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 
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Vern Klukas                             I'm a little . . .
Inkspot Type & Design
vern at inkspotco.com

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