Mike, just to add to what others have said. Keep in mind, in order to evacuate the engine you have to replace the air you are removing with the PCV with fresh filtered air. Ideally, a filtered breather on one side valve cover and the PCV on the other side. While Turbo said this only vents from "valve cover to valve cover", if you think about it, when you pull air through a valve cover, the only path it has to get through the motor and to the PCV on the other side is to go down through the pushrod slots and drain-back holes through the cylinder head, across the engine block and back up through the pushrod slots and drain-back holes in the other head finally exiting through the PCV to be passed through the combustion process once again.

Also, Dennis is right when he says as engine vacuum decreases (throttle is applied) and the spring in the PCV overcomes the decrease in engine vacuum, the system ceases to be effective - to an extent. Depends on how well your motor is sealing. If it has any detectable amount of blow-by (which all engines will have some) as throttle is applied the positive pressure in the crankcase will increase and thereby force the PCV open. So it is still somewhat effective at any RPM range.

I just purchased some Teague valve covers that don't have provisions for PCV - just the breathers that vent to atmosphere. I am having the covers machined to accept the factory grommets and breather tubes. I'm just a firm believer in evacuating the crankcase. (Plus I can't stand the smell of oil in my boat!)

Thanks to all that have posted on this thread, it has been very informative and quite helpfull.

Croozin2,
One might assume the path you described works and in many cases it does. But I spent 6 months tracing air flow through the crank case PCV system on Dino runs with 3 other engineers. Buy far the best was pulling it through the front engine cover and having dedicated PCV jacketing in the block. Air speed were you locate the valve is important too to high and it will pull a lot of oil into the intake.

The absolute best system was a hybrid I did on an ex race car were I "Y"ed an extractor system with a PCV system.

Okay, did we decide whether it was better to pull vapors from the motor from the back of the intake manifold valley? That sounds like a good one to me (assuming you use a splash pan to keep cam and lifter splash to a minimum.)

Turbo, I agree with you totally, but for a practical application for the average boater, the valve cover application should be more than adequate. Actually, as someone said earlier, a vacuum pump is the most efficient (and most cost prohibitive also). Just a matter of how efficient you want, or need, your crankcase ventilation system to be. You can spend thousands on the most minute detail if you choose (or need). Just my .02