PCV: The Basics
When the engine fires it creates what is known as "blow-by". This is when the gases from the combustion chamber "blow by" the sealing rings, and make their way into the oil pan. This is a problem, because the crankcase uses certain principals to effectively feed oil to the pump, and from there, the rest of the engine. With excess pressure, the oil does not flow correctly and the pressure can damage seals, and in extreme cases, the gas can build up to such a high pressure that the valve covers may be blown off, and other internal damage may occur.
80 years ago, this problem was solved by simply putting a hole in the crankcase or valve cover and feeding air in with a tube. The problem with this, was that gases from the combustion chamber were getting into the air without being burned, which was an emissions nightmare (If that sort of thing gives you nightmares).
The solution to this, was the PCV valve. A PCV valve is a one-way check valve that allows the crankcase gases to vent into the air intake when the pressure becomes high enough. Engine vacuum from the intake helps to pull these gases from the crankcase and into the engine to be burned. This causes several problems...
The first problem caused by recirculating gases from the crankcase into the intake is that it changes the Air/Fuel Ratio being fed into the engine. Devices, such as the Mass Air Flow sensor, record the amount of matter (air) passing through them to calculate the fuel needs of the engine. These sensors, however, have no way of telling if there is fuel, or any other foreign particles in the air that is being pulled in. Therefore, and crankcase gases being pulled into this stream are not calculated by the computer, and cause the engine to run rich, causing loss of power and fuel economy.
The second problem that the PCV system introduces is pretty obvious. When you have pressure somewhere, and you release it into a non-pressurized area (or worse yet, somewhere with vacuum), whatever is in the pressurized area tends to move to the lower pressure area. This moves the crankcase gases out of the oil pan, but can also pull oil out with it! Now, loss of such a minor amount of oil would be no big deal, if it weren't being routed back into the intake. Unfortunately the oil that leaves the pan into the PCV eventually ends up in the intake, your air filter, and even the combustion chamber. This causes numerous problems, but most of all, is just a mess.
The final problem with the PCV valve is that it introduces airflow. The pressure from the crankcase comes into the intake and causes turbulence in the air traveling through your intake. Now, a certain amount of turbulence is necessary for the engine to run correctly, but this turbulence must be tuned so that the most air is sent to the engine in the most efficient way possible. The air flow from the PCV comes in at a perpendicular angle to the air flow of the intake, and slows the air travel, and changes the direction of the flow. This problem can also be seen in the reverse at idle, and during cold-starts. At these times, the crankcase may not have enough pressure to for the PCV to allow pressure to flow into the intake, and therefore the air from the intake actually can attempt to flow into the PCV. Obviously, being a one-way valve, the PCV will not allow air to flow into the crankcase, but the tube leading to the PCV allows a secondary pathway for the air to take, and disturbs turbulence and flow.
Now, if you are concerned about horsepower, and don't really care about emissions, you can simply remove the PCV. If you are looking for performance on an N/A engine that is not running high compression, this would be my recommendation:
-Remove the PCV
-Add a tube (to keep oil from splattering all over your engine)
-Add a Catch Can (optional, will collect expelled oil and particulate matter)
-Add a breather (air filter, mesh, anything that will keep particles from getting into the crankcase)
Pros:
-Does not feed into intake
-Clean look after install
Cons:
-Won't pass emissions
-Not effective in high-performance applications
Removing the PCV, with no further modifications, is not recommended for Boosted, or High-Compression applications. The higher compression involved in these applications causes more blow-by to get through into the crankcase, and the atmospheric pressure outside the engine may not create enough flow to pull these gases out fast enough, causing seal damage, or worse. For turbo/supercharged applications, or for high-compression applications, many would recommend that you buy a catch can. This will collect the oil expelled from the engine, so it does not get recirculated back into the intake. Install the catch can between the PCV and the intake system. The reason you do not remove the PCV, and do not allow the gases to vent into the air, is because the higher pressure in the crankcase of boosted (high-compressions) vehicles will likely need a vacuum to be put on them in order to remove the gases at a high enough rate as to not cause pressure build-up.
Pros:
-Easy install (2 hose clamps)
-Keeps engine vacuum without complicated systems
Cons:
-Bulky looking (takes up a lot of space in the engine bay)
-Still feeds into the intake causing turbulence and adding gases to the airstream
-Does not offer much for performance benefits
More complex methods of crankcase evacuation would be vacuum pumps and exhaust evacs.
Vacuum Pumps can be either a mechanical, belt-driven pump, similar to a water pump, or power steering pump. The main purpose of these would be to create a vacuum on the crankcase, and pull the gases into the atmosphere. These pumps usually include a metering valve, which you will want to set between 12-15psi to create optimal vacuum to pull the gases from the crankcase, without causing so much vacuum as to hinder functionality of basic engine parts (such as the oil pump or crank bearings).
Usually the setup would go in this order: Crankcase, to Catch Can, to Metering Valve (If not internal to the pump), to Pump, and then to Atmosphere.
Pros:
-Does not feed back into intake
-Applies set vacuum to allow for perfect flow
Cons:
-Slight parasitic draw due to power needed to run pump
-May be bulky and have unappealing appearance
-Expensive!
-Likely won't pass emissions testing
The final system of crankcase evac is the exhaust evac. This is a style of crankcase venting that uses the venturi effect (also used in carburetors) to pull the gases out of the crankcase, and into the exhaust stream. A catch can may be placed between the crankcase and the exhaust, but is usually not used, as the oil will just burn up in the exhaust. This system is widely used on Top Fuel dragsters, who have massive, open headers, which do not use the principals of back-pressure. This largely varies from our street-car exhaust, which needs back-pressure to create low-pressure areas in the headers to pull exhaust gases from the engine during valve overlap. The reason the top fuel style does not work on street-cars, is that it is just a tube. No metering, no dampening, whatever vacuum is applied to the exhaust is applied to the crankcase. When you make 8000 horsepower that is all fine and dandy, but not so much on your daily driver... Such a device on a standard exhaust would cause exhaust back-pressure to flow up the tube, and into the crankcase, causing even more pressure!
The way this is corrected is by using a one-way check valve. During the cycle between pulses of the engine, the valve remains mostly closed, allowing only slight amount of air to be drawn one way or another. But, when the exhaust is creating a vacuum, it pull the valve open and allows the crankcase to vent into the exhaust.
Pros:
-Does not feed into intake
-Does not require catch can
-If your exhaust has working Catalytic converters, they will burn the crankcase gases and reduce emissions.
-Creates vacuum without causing parasitic draw
-Cheap to buy and replace
-Clean look after installed
Cons:
-Install. The install is somewhat complex, and you must know EXACTLY where to place the valve in the exhaust, or it will not work.
-Likely won't pass emissions testing
I will shortly be buying the Exhaust Evacuation setup for myself. They generally range from about $40-$60. I will do a write-up on the install, or update this thread when I get some pictures.
