Date: Sun, 29 Sep 1996 21:38:19 -0500
From: David Paddock
Re crankcase breathing: the whole idea is to exhaust the space below the piston(s) from the products of combustion which blow by the piston rings. You can appreciate that after a couple of power strokes the pressure in this space will rise if it weren't vented. When you think about it, the pressure would eventually rise to the pressure of the combustion process taking place on the other side of the piston which would cause the engine to stop. (As a practical matter, the engine would be blowing oil out of every place it could and the cases would explode long before the piston pressures equalized.)
The ideal situation is to have a perfect vacuum in the cases so that the piston doesn't have to act as an air compressor in its downstroke. This is lost Work which doesn't appear at the output of the crankshaft. The perfect breather would expel the blowby gas while simutaneously providing this vacuum. A properly-timed breather is intended to do just that. A. Golland in his seminal work, "Goldie", describes the development of the breather system for the Gold Star which started out as a check valve a la PCV-style. The BSA factory found that crankcase vacuum steadily decreased from its maximum at 2000 rpm to zero (atmospheric pressure) at around 6000 rpm because the breather disc element oscillations couldn't keep up with increasing engine speeds. At engine revs greater than 6000 the pressure went positive, causing heavy oil loss from the breather pipe. BSA attempted to improve the situation by relocating the device from the crankcase wall to the timing cover; this helped a bit but even with further modification the matter was still unsatisfactory, and the scheme was discarded in 1954 in favor of a rotary, timed breather which was already in use on the BSA twins. And which is entirely satisfactory.
I don't doubt that opening the engine to the atmosphere will decrease oil loss due to internal pressure build-up but the horsepower wasted sucking in and blowing out the air is inherently unacceptable. If the breather isn't doing its job look for blockage, leaks, and/or improper breather timing.