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Racing manifolds usually have no provision for PCV valve, so racers vent the block to atmosphere using an oil-vapor separator, also known as a "catch can". This prevents oil vapor from escaping with the crankcase vapor (for example, oil from worn rings). It allows oil to be caught when catastrophic (internal) engine damage occurs -- which a positive crankcase ventilation system won't do.
Overview
Connect both breathers (Rocker_cover#Breather and block breather tube) to an oil-vapor separator. This "oil catch can" system is vented to the atmosphere, sometimes using a separate air filter, and sometimes with no air filter.
If you don't properly vent the block, pressure builds up and want to push oil out. If you block it ... the blowby would likely blow the dipstick out, and squirt hot oil on the engine bay. In a CA18 oil pumps straight onto exhaust manifold. Makes a great engine bay fire on the dyno. But this only if you block all breathers. The A-series also have a breather on the back of the engine, so it would be unlikely to cause much problem under normal driving. If you connected the two breathers together, you would have dipstick action.
It also hurts horspower as the rotating pistons have to push the air in the crankcase around.
Scott 77: i have had an engine on the dyno this week and ... picked up nearly 2hp atw.
Discussion: Rocker Cover breather
Because racing engines have a tendency to produce more blowby than a PCV system can handle, many racing classes or track rules require an oil catch can.
