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gearmesh, inc. · 06-18-2009, 10:01 PM

Under light throttle with a functional PCV system, your crankcase will be under a slight vacuum in which the vacuum is regulated by the PCV valve. This enables your engine to burn off a lot of the combustion byproducts that sneak by the rings (blowby). If these gasses were allowed to stay in the crankcase, your oil would get dirty very quickly. The PCV system is widely considered an evil requirement of the EPA, but it will keep a street car's oil cleaner between changes vs. being without the PCV system.

The PCV system is never designed to deal with a large volume of blowby gasses, such as a boosted or heavily nitroused mill. What happens under heavy engine load is that the PCV system can't get all the blowby gasses through it so the engine can reburn the gasses, so there will be a backflow up into the fresh air supply side of the PCV system. This is the flow you see coming out of your breather under heavy throttle.

On a stock engine, you will notice that the air supply side of the PCV system will be taken somewhere off the air intake aft of the mass air meter, so this air that is drawn through the crankcase to purge it of blowby will be metered air. When you go to the breather in your picture, unmetered air is getting in the engine through the PCV system, but only at lighter throttles when the crankcase is under a slight vacuum. Under heavy throttle conditions when the PCV system can't handle the extreme blowby, you won't have any unmetered air under this condition because of the excess volume of blowby gasses trying to escape the crankcase. Keep in mind that blowby gasses are originally created by burning fuel with metered air in the first place.

The moral of the story is that you only get the unmetered air through your aftermarket breather under light throttle conditions. If you want to get totally rid of unmetered air, design a catch can for the fresh air side of your PCV system like you have on your dirty side, but in which the top of your catch can has a line that hooks up to the stock location on your intake plumbing, rather than an open filter that sucks in unmetered air.

Another thing to keep in mind is that the older an engine gets, the more blowby you will have due to worn rings. A just broken in engine will have less blowby than a worn engine.

06-18-2009, 10:01 PM	#3
gearmesh, inc. creepy azz cracka Join Date: Nov 2008 Location: Summerville Posts: 1,587	Under light throttle with a functional PCV system, your crankcase will be under a slight vacuum in which the vacuum is regulated by the PCV valve. This enables your engine to burn off a lot of the combustion byproducts that sneak by the rings (blowby). If these gasses were allowed to stay in the crankcase, your oil would get dirty very quickly. The PCV system is widely considered an evil requirement of the EPA, but it will keep a street car's oil cleaner between changes vs. being without the PCV system. The PCV system is never designed to deal with a large volume of blowby gasses, such as a boosted or heavily nitroused mill. What happens under heavy engine load is that the PCV system can't get all the blowby gasses through it so the engine can reburn the gasses, so there will be a backflow up into the fresh air supply side of the PCV system. This is the flow you see coming out of your breather under heavy throttle. On a stock engine, you will notice that the air supply side of the PCV system will be taken somewhere off the air intake aft of the mass air meter, so this air that is drawn through the crankcase to purge it of blowby will be metered air. When you go to the breather in your picture, unmetered air is getting in the engine through the PCV system, but only at lighter throttles when the crankcase is under a slight vacuum. Under heavy throttle conditions when the PCV system can't handle the extreme blowby, you won't have any unmetered air under this condition because of the excess volume of blowby gasses trying to escape the crankcase. Keep in mind that blowby gasses are originally created by burning fuel with metered air in the first place. The moral of the story is that you only get the unmetered air through your aftermarket breather under light throttle conditions. If you want to get totally rid of unmetered air, design a catch can for the fresh air side of your PCV system like you have on your dirty side, but in which the top of your catch can has a line that hooks up to the stock location on your intake plumbing, rather than an open filter that sucks in unmetered air. Another thing to keep in mind is that the older an engine gets, the more blowby you will have due to worn rings. A just broken in engine will have less blowby than a worn engine. Last edited by gearmesh, inc.; 06-18-2009 at 10:04 PM.