Racing manifolds usually have no provision for PCV valve, so racers vent the block to atmosphere. They use an oil-vapor separator.
Connect both breathers (rocker cover and block) to an oil-vapor separator. This "oil catch can" has its own 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 said:
i have had an engine on the dyno this week and ... picked up nearly 2hp atw.
Discussion: Rocker Cover breather