Almost fired off a flame, then realized you're (probably) referring to the
fresh air source that typical PCV systems require (hence the word 'system').
These systems work by drawing in fresh air--usually through the main or a
secondary air filter--as you describe. This air is circulated through the
system by the vacuum pulled by the PCV valve (and, I believe, some systems
actually have a positive pressure air source). Without a positive pressure air
source it's probably more correct to refer to this as a 'negative crankcase
ventilation,' since the movement of air is driven by a vacuum. Anyway, there is
going to be positive air pressure in the crankcase due to blow-by which the PCV
valve dumps into the manifold, with or without a fresh air source. On a Healey
without a rear seal I think some air may be drawn through the rear 'seal'
(hence the lower oil consumption and smaller wet spot).
The original rear air filter for my BJ8 has a fitting on the backside for the
hose from the crankcase and valve cover. Inside the filter, the fitting makes a
right-angle turn and flattens out to a narrow, flat opening along the edge of
the carb inlet. This is clearly--IMO--done this way to facilitate drawing the
crankcase and valve cover gasses into the carburettor, at least partially due
to Venturi effect--NOT as a source for fresh air for the crankcase. I believe
the PCV is a more effective way to do this; the downside, if you can call it
that, is that the rear cylinders can run a little leaner, esp. at idle, but
that is easily enough remedied. Not familiar with other Brit cars, but if they
had a PCV valve fitted new they would have had a source for filtered air. Note
some 'merkin cars--usually V8s--have a small air filter on the oil filler hole
to provide clean air.
So, yes, a true PCV 'system' requires a fresh air source. However, adding a PCV
valve to a Big Healey engine just makes scavenging blow-by and valve cover
gasses more effective.
Bob
--------------------------------
Bob Spidell - San Jose, CA
----- Original Message -----
Flame away, Mark
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