For What It's Worth

Positive Crankcase Ventilation

Don't leave the garage without out it

Why do you need a PCV system with a PCV valve?   Short answer, to keep your gaskets in place.   A running engine generates fumes from the oil and blow-by.   If these are not vented out of the crankcase and internal areas they can build up and become flammable.   With the right conditions, a little blow-by and the fumes can ignite.   How bad is it?   I have seen valve covers and oil pans turned into something round and blobby from the explosion.   Even the oil filters have blown off.

Before the 1960s, cars had a road draft PCV system.   The oil fill cap had a primitive filter screen that let air in to the crankcase.  A pipe at the back of the engine went down into air stream flowing under the car.   The open end on the pipe with the air rushing past created a slight vacuum and vented the crankcase.  Side effects were fumes in the air with oil deposits under the car and on the road.   You could often see the oil in the center of the lane on busy roads.  

Now come the 1960s when we are a little more concerned about the fumes and oil on the roads.   The PCV system was updated and crankcase fumes were sucked back into the intake to be burned.  A PCV valve was added to control the flow of air and fumes.    Also the air intake for the crankcase was moved to the air filter.  A flame arrestor was often used on the air intake in the air cleaner also.  A flame arrestor looks like a wire mesh filter

So what does the PCV valve do?   First it meters the amount of air that is pulled back into the intake. A controlled vacuum leak.  It needs to be matched to the engine size and vacuum it pulls.   That is why there are so many of them.  Secondly the PCV is an anti blow-back valve.   That means it only lets things go one way.  That is why it rattles when you shake it.  When it stops rattling, the valve no longer works correctly.  

So why do you need an anti-blow-back valve?  What happens if the car misses, sputters etc. and causes a backfire, burning the fuel in the intake?  We have all had this happen.  More so on cars with carbs.  Imagine that flame going down the PCV line and igniting the crankcase fumes when the condition is just right.  The PCV valve in this condition with backward pressure will close, blocking the back pressure and flame.

Why don't all the air intakes for a PCV system in the air filters have flame arrestors?  Best guess is some engineer figured that with just fresh air in the inlet tube, the flame wont go down the tube.  A flame arrestor on the inlet side is probably still a good idea on hot rods running carburetors.

Most people have never seen the result of a crankcase explosion.  They are rarer today than in the past.   The primary reason is the PCV system is simple and effective.   The PCV system is often misunderstood or dismissed as a "pollution control" only.  This is a mistake.

Many hot rod owners will say they don't run a PCV system and don't need it.   They have never had a problem.   This could be true, up to their level of experience.   It only takes one crankcase explosion to really see the need for a PCV system.

Now that you know why you need a PCV system, do the job correctly.  Get the correct size PCV valve.   PCV valve in one valve cover, filtered air inlet or breather in the other.  Also make sure you have the correct baffles in the valve covers under the PCV valve.  The baffles help to make sure only fumes are vented, not oil.   

Happy Motoring, VHubbard Mar 2011

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