Hi Bill, I'll take a crack at this one, as I got a great explanation from
Barney Gaylord (of Chicagoland MGA fame) when I asked him a similar question
off-line about setting up breathing on my modified later 'B motor. Here's
how I interpret B series engine breathing.
Think of it as a loop, whereby engine blowby gases and windage airflow
(which comes from piston/rod/crank motion) are pulled out and fresh air
pulled in, basically under vacuum (more on that in a bit). The purpose of
both the tubed valve cover and the oil cap with the small orifice (use one
or the other, not both) is to pull some fresh air into the system. The pull
to the entire system comes from porting a side cover (earlier cars from the
rear cover's draft tube, later ones from the front cover's upwardly curved
tube) to vacuum source. Early cars, as somebody already mentioned, pulled
vacuum from a "venturi" effect through the rear angled draft tube, and
consequently the nasty oily vapors went out this tube to the environment--
an "open" looped system. Later cars ported the draft tube into the engine's
intake system (which also creates the necessary vacuum) via a PCV system, so
that the vapors got reburned and the venting system was basically "closed"
loop. Barney explains that this type of system needs to be one-way, which
the stock PCV gizmo ensures. Without it, if there's some reversing of basic
engine breathing, a la intake backfiring, then the reverse pressure wave
goes right back into the crankcase, pressurizes it, and forces oil out where
it's not supposed to go, like the oil cap/valve cover tube, crank seals,
dipstick, oil pan, etc. Not good. The PCV valve only allows breathing
one-way, like a check valve, and thus stops this reverse flow. In worn
engines with loose bore/piston ring relationships, combustion blow-by
creates more pressure in the crankcase than mere windage, which in turn
clogs PCV valves or in systems without the valve (which mine used to be,
when the front vent was just ported to the Weber carb) dumped excessive oily
vapor into the carb's intake. Using the later 'B's front vent cover
arrangement with its wire mesh and room for oil to accumulate to drain back
into the sump is optimum. Also, the intake of fresh air in through the valve
cover needs to be restricted, otherwise under conditions of high vacuum
excess air being pulled into the intake will lean the combustion mixture.
Some PCV systems account for this by restricting air intake under high
vacuum conditions.
Whew, getting long-winded, but hope that covers it...
Larry Ruggiero '72 MGB roadster
Warner Robins, GA
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